1. Lecturer: Ami Berler ami@cs.bgu.ac.il Semester A 2010-2011
מעבדת תכנות C++/C
Lecturer: Ami Berler
Semester A
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2. Info Page: http://www.cse.bgu.ac.il/Courses/course.asp?ID=484
Course requirements:
4 assignments
If the grade of the final exam is at least 56, the final grade is 30% final exam and 70% home assignment.
In case of having a grade lower than 56 in the final exam, the final grade is the grade of the final exam.
Literature:
C Programming Language, Brian W. Kernighan and Dennis M. Ritchie
The C++ Programming Language, Bjarne Stroustrup
C++ Complete reference, Herb Schildt
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3. Objectives
Acquire familiarity with the C and C++ programming languages.
Emphasis is placed on Object Oriented Programming (OOP).
Working under Unix(Linux) Operating System.
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4. Meetings
The first 4-6 meetings introduce the C language syntax and concepts: 
Types, operators and control flow
Functions and program structure
Pointers and arrays
Structures, dynamic memory and I/O
Linked lists
Files
 The remainder of the meetings addresses C++ programming:
 Object oriented programming concepts
Name spaces, statements
Classes
Operator overloading
Inheritance
Templates
Exceptions
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5. Announcements
You need look for announcements in the page of the course.
Office hours :
Lecturer :
Wednesday – 1300 – –Room 305- Building 34
(schedule by mail).
Assistant:
Monday – – Room 418 – Building 37
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6. Computer Structure CPU Internal Memory Output Input Mouse Keyboard
Microphone
Detectors
Monitor
Speakers
Motors
Hard Disk
CD / DVD
Tape
DiskOnKey
External
Memory
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7. Internal Memory Components
ROM – Read Only Memory
An integrated circuit chip containing programs and data that can be accessed and read but cannot be modified (only way specials programs).
RAM – Random Access Memory
The PC's primary storage area, used to write, store and retrieve information and program instructions which are then passed to the CPU for processing.
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8. Memory Unit : Byte
Byte is a unit of digital information. It is an ordered collection of bits (binary digit), in which each bit denotes the binary value of 1 or 0.
number of bits used to encode a character of text in the computer, which depended on computer hardware architecture; but today it almost always means eight bits.
A byte can represent 28 = 256 distinct values, such as the integers from 0 to 255
ASCII (American Standard Code for Information Interchange)
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9. Language types Computers understand only machine language.
Basically looks like a sequence of 1’s and 0’s.
The computer does not understand C.
Assembly – machine language with some text codes (still inconvenient).
Interpreted languages – Java, Perl.
The program is translated into machine language line by line during execution.
Compiled languages – C, Pascal, Fortran.
The program is translated into machine language before execution
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10. High level languages vs. machine languages.
Actually, binary instructions.
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11. Why teach C? C is small (only 32 keywords).
lots of C code are available (OS, editors, DB).
C doesn’t change much.
C is quick (nearly as fast as assembly).
C is the basis for many languages (Java, C++).
C is one of the easiest languages to learn.
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12. Software Development Process
Understand Problem Definition
Generalize & Decompose the problem definition
Develop Solution Algorithm
Write the Code
Test and Debug
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13. Programming Process Use any text editor to write a program a.c
Compile the program a.c->a.o
C compiler will print error messages and abort
Or produce an object file
Link the program a.o+b.o+c.o->prog.exe
linker will print error messages and abort
Or produce an executable program
Run the program prog
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14. Writing the code
In this course we use the Linux (Unix) operating system .
The gcc compiler is a native linux C/C++ compiler
We can use a simple text editor (gedit,vim,emacs) or a more evolved integrated development environment (IDE) like netbeans and eclipse, in order to edit the code and than use the compiler to compile the code.
Assume the code is edited in a file named - HelloWorld.c (.c is the common extension for C code)
gcc −Wall HelloWorld.c −o HelloWorld.out
would generate a HelloWorld.out (.out is the common extension for executable file for the Linux operating system)
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15. Our first C program Comments are good
/* HelloWorld – An example program */ #include <stdio.h> int main(void) { printf(“Hello, world!\n”); return 0; }
Preprocessor directive
Includes C standard library
main() means “start here”
Brackets define code blocks
Note that all C statements end with a semicolon (;).
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16. void main()
This tells the compiler we are about to define a function named main.
main() is a special function – it is where the program starts running.
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17. printf() This is a C statement.
This statement calls a function called printf, which causes text to be printed on the screen.
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18. #include <stdio.h>
This is an instruction to the compiler to insert the contents of the file stdio.h to the program prior to compilation.
This file contains information about the printf fuction and others functions.
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19. Identifiers
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20. Identifiers naming rules
Letters, digits, underscores i
CSE_8a
a_very_long_name_that_isnt_very_useful
fahrenheit
First character cannot be a digit
8a_CSE is not valid!
Case sensitive
CSE_8a is different from cse_58
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21. Naming Using Identifiers
Variables.
Reserved Words
Functions.
Constants .
Structures.
Unions.
Type of variables.
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22. Variables
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23. Declaring variables in C
Before using a variable, one must declare it.
The declaration first introduces the variable type, then its name.
Many variables of the same type can be separated by ,
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24. Example Variable Declarations
int i;
char c;
float f1, f2;
unsigned int ui ;
double t1,t2,t3;
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25. Data types in C char – a single byte character ‘A’.
short int (or just short) – an integer number, usually 2 bytes (rarely used) 15 .
int - an integer number – usually 4 bytes
long int (or just long) – an integer number, 4 or 8 bytes (rarely used)
float – a single precision real number – usually 4 bytes
double – a double precision real number – usually 8 bytes
long double - a double precision real number – usually 8 bytes (rarely used)
Signed vs. unsigned
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26. Initialize variables
When a variable is declared, its value is undefined.
In order to use variable it must be valuable.
There are three ways:
Assignment
Input from user
Initialization
Examples of initialization:
int e = 7 , r = 34, t;
double ab = 9.7;
char c = `k`;
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27. Assignments
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28. Memory types Heap .&x .&y Stack x y Loads program in Code memory
Loader
start
Loads program in
Code memory
CPU
execute
Takes each instruction
and executes it storing
new data values in
Stack or Heap
Code memory
Heap
.&x .&y
Stack x y
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29. Stack vs. Heap Both are used for memory management.
The stack is used to store your code, and runtime values : parameters, return address, links, return value and local variables.
Heaps are used to contain all your dynamically allocated variables.
A heap maintains a freelist (list of all the free space available) and as and when you allocate and deallocate memory it updates this free list
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30. Stack vs. Heap (cont.)
The stack is local to your program space and is therefore usually faster to access.
The heap is memory that resides outside of your programs direct control and as such must be acquired and allocated via the operating systems functions or by special interrupt calls.
The management of a heap is relatively complex.
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31. Example
/* Get a length in cm and convert to inches */ #include <stdio.h> int main() { double cm, inches; printf("Please enter length in centimeters: "); scanf("%lf",&cm); inches = cm / 2.54; printf(“%g centimeters are equal to %g inches\n", cm, inches); return 0; }
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32. printf and scanf
printf – prints to the standard output (for example screen).
Can also accept variables and print their values.
scanf – gets values from the standard input (for example keyboard), and assigns them to variables.
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33. printf can print variable values
printf ("z=%d\n", z );
The sequence %d is a special sequence and is not printed!
It indicates to printf to print the value of an integer variable written after the printed string.
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34. scanf gets input from the user
scanf("%lf", &var);
This statement waits for the user to type in a double value, and stores it in the variable named ‘var’.
The meaning of ‘&var’ is the address of the variable var in stack.
To get 2 doubles from the user, use – scanf("%lf%lf", &var1, &var2);
var1 and var2 were defined as double.
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35. prinft/scanf - Conversion codes - %?
? is replaced by a respective variable.
%c – a character
%d – an integer, %u – an unsigned integer.
%ld – a long integer.
%f – a float.
%lf – a double.
%g – a nicer way to show a double (in printf)
%x – an hexadecimal.
%s – a string .
%% - the ‘%’ character (in printf)
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36. Operator \? \n – New line. \t – Tabulator space. \\ - Symbol \
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37. ASCII (American Standard Code for Information Interchange)
Every character is assigned a numeric code.
`` operator in C returns this code, `A`=65
ASCII code 0 (NULL character) is important.
Note continuous sets of numbers, upper case and lower case characters.
Generally, you don't care about the numbers.
The ASCII table.
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38. ASCII table 12 ספטמבר 18 ~ Communication Systems Engineering - BGU -
HEX
DEC
CHR
CTRL 00
NUL 20 32 SP 40 64 @ 60 96 ` 01 1
SOH ^A 21 33 ! 41 65 A 61 97 a 02 2
STX ^B 22 34
" 42 66 B 62 98 b 03 3
ETX ^C 23 35 # 43 67 C 63 99 c 04 4
EOT ^D 24 36 $ 44 68 D
100 d 05 5
ENQ ^E 25 37 % 45 69 E
101 e 06 6
ACK ^F 26 38
& 46 70 F
102 f 07 7
BEL ^G 27 39 ' 47 71 G
103 g 08 8 BS ^H 28 ( 48 72 H
104 h 09 9 HT ^I 29 ) 49 73 I
105 i 0A 10 LF ^J 2A * 4A 74 J 6A
106 j 0B 11 VT ^K 2B + 4B 75 K 6B
107 k 0C 12 FF ^L 2C , 4C 76 L 6C
108 l 0D 13 CR ^M 2D - 4D 77 M 6D
109 m 0E 14 SO ^N 2E . 4E 78 N 6E
110 n 0F 15 SI ^O 2F / 4F 79 O 6F
111 o 16
DLE ^P 30 50 80 P
112 p 17
DC1 ^Q 31 51 81 Q
113 q 18
DC2 ^R 52 82 R
114 r 19
DC3 ^S 53 83 S
115 s
DC4 ^T 54 84 T
116 t
NAK ^U 55 85 U
117 u
SYN ^V 56 86 V
118 v
ETB ^W 57 87 W
119 w
CAN ^X 58 88 X
120 x EM ^Y 59 89 Y
121 y 1A
SUB ^Z 3A : 5A 90 Z 7A
122 z 1B
ESC 3B ; 5B 91 [ 7B
123 { 1C FS 3C
< 5C 92 \ 7C
124 | 1D GS 3D = 5D 93 ] 7D
125 } 1E RS 3E
> 5E 94 ^ 7E
126 ~ 1F US 3F ? 5F 95 _ 7F
127
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39. Example of char as both a character and a small number
#include <stdio.h> int main() { char i = 'b'; printf("i as a character is %c\n", i); printf("i as an integer is %d\n", i); printf("The character after %c is %c\n", i, i + 1); return 0; }
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40. Another example
/* Get the position of a letter in the abc */ #include <stdio.h> int main() { char letter; printf("Please enter a lowercase letter\n"); scanf("%c", &letter); printf("The position of this letter in the abc is %d\n", letter-'a'+1); return 0; }
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41. Exercise Write a program that accepts as input – and outputs –
A lowercase letter
and outputs –
The same letter in uppercase
(e.g., if the input is ‘g’, the output should be ‘G’)
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42. Solution
/* Convert a letter to uppercase */ #include <stdio.h> int main() { char letter; printf("Please enter a lowercase letter\n"); scanf("%c", &letter); printf("This letter in uppercase is %c\n", letter-'a'+’A’); return 0; }
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43. Arithmetic operators
An operator is an action (in our case, usually a mathematical one) performed on something (e.g. constants, variables).
That “something” is called an operand.
Common operators:
Braces ()
Assignment =
Addition +
Subtraction -
Multiplication *
Division /
Modulo %
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44. Riddle me this
Suppose a program’s execution reaches the following line – scanf(“%d%c”, &i, &ch);
And suppose the user input is – 100 b
What will be the contents of i and ch?
Answer
i == 100
ch == ‘ ‘ (there’s a space in there)
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45. Spaces in scanf
One way to overcome this problem is to introduce a space before the %c – scanf(“%d %c”, &i, &ch);
The space tells scanf to ignore all whitespace characters that come after the %d is read
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46. Operations with different types
For example -
3 + 4 = 7
= 7.0
3 / 4 = 0
3.0 / 4 = 0.75
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47. Casting variables
Sometimes it is desirable for a variable of one type to be considered as belonging to another in an operation
We say the variable is cast to the new type.
The casting operator is of the form – (type)
For example, (float)i casts the variable i to a float.
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48. Casting variables
#include <stdio.h> void main() { int a=1, b=2; printf("%d / %d = %d\n", a, b, a/b); printf("%d / %d = %g\n", a, b, (float)a / b); }
Windows format
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49. Example – find what’s wrong
#include <stdio.h> void main() { int a = 10; int b = 20; printf("The average of %d and %d is %d\n", a, b, (a + b) * (1 / 2)); }
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50. Will this work?
#include <stdio.h> int main() { int a = 10; int b = 20; printf ("The average of %d and %d is %d\n", a,b, (a + b)*(1.0 / 2)); return 0; }
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51. This works !!
#include <stdio.h> int main(void) { int a = 10; int b = 20; printf ("The average of %d and %d is %f\n", a,b, (a + b)*(1.0 / 2)); return 0; }
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52. Incremental operators
Used as a short-hand for incrementing (or decrementing) variables.
i++ or ++i == i = i + 1
i-- or --I == i = i – 1
i += a == i = i + a
i -= a == i = i - a
i *= a == i = i * a
i /= a == i = i / a
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53. Prefix ++i and postfix i++
#include <stdio.h> int main(void) { int i=5; printf("i++=%d\n",i++); //i++=5 printf(“++i=%d\n",++i); //++i=7 return 0; }
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54. Selection statements 12 ספטמבר 18
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55. True or false In C, every expression has a numeric value
An expression is ‘true’ when its non-zero
If it is zero, it is ‘false’
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56. Relational operators They are –
A == B (Note the difference from A = B)
A != B
A < B
A > B
A <= B
A >= B
The value of logical expression is non-zero if it’s true, zero if it’s false
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57. Logical operators Allows to evaluate two or more expressions -
A && B – ‘and’ - True when both A and B are true
A || B – ‘or’ (inclusive or) - True when either A or B (or both) are true
!A – ‘not’ - True when A is not, and vice versa.
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58. AND - && A && B B A F T 12 ספטמבר 18
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59. OR - ||
A || B B A F T
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60. NOT - !
! A A T F
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61. Logical Expressions
There are logical order in the expressions(similar to the algebraic order)
&& - similar to *
|| - similar to +
! - similar t0 negative
(A <= B) && (N != M) || (Y > Z) is different from
(A <= B) && ((N != M) || (Y > Z))
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62. Bitwise Bitwise AND - & Bitwise OR - | Bitwise XOR - ^
Assignment by bitwise left and right shift:
<<= >>=
Assignment by bitwise AND, OR and XOR
&= |= ^=
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63. Bitwise
#include <stdio.h> int main ( ) { unsigned char Byte = 1 ; // Byte variables printf( "Byte i s : %d\n" , Byte ) ; // 1 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; //2 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 4 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 8 printf ( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 16 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 32 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 64 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 128 printf( "Byte<<1 i s : %d\n" , Byte<<=1) ; // 0 return 0; }
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64. Bitwise
#include <stdio.h> int main ( ) { unsigned char Byte = 1 ; // Byte variables char FindBit = 1; Byte = 89 ; // Set number : printf( " Bit 0 : %d\n" , Byte & FindBit ) ; // 1 FindBit<<=1; printf ( " Bit 1 : %d\n" , Byte & FindBit ) ; // 0 printf( " Bit 2 : %d\n" , Byte & FindBit ) ; // 0 printf( " Bit 3 : %d\n" , Byte & FindBit ) ; // 8
printf( " Bit 4 : %d\n" , Byte & FindBit ) ; // 16 FindBit<<=1; printf( " Bit 5 : %d\n" , Byte & FindBit ) ; // 0 printf( " Bit 6 : %d\n" , Byte & FindBit ) ; // 64 printf( " Bit 7 : %d\n" , Byte & FindBit ) ; // 0 return 0 ; {
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65. Selection Statements 12 ספטמבר 18
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66. Example Structure ? 12 ספטמבר 18
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67. Selection Statements
Selects statements to execute based on the value of an expression
The expression is sometimes called the controlling expression
Selection statements:
if statement
switch statement
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68. Selection statements: if
used to execute conditionally a statement or block of code.
if (expression) statement
If expression is true, statement is executed (what is true?).
statement can be replaced by a block of statements, enclosed in curly braces.
if (expression) {
statements … }
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69. An example
/* This program displays the absolute value of a number given by the user */ #include <stdio.h> int main() { double num; printf("Please enter a real number: "); scanf("%lf", &num); if (num<0) num = -num; printf("The absolute value is %g\n", num); return 0; }
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70. If-else statement if (expression) statement1 (or block of statements)
if expression is true, statement1 is executed.
if expression is false, statement2 is executed
both statements can be (and very often are) replaced by blocks of statements (“compound statements”)
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71. An example (fragment)
int first, second, min; /* … */ if (first < second) { min = first; printf ("The first number is smaller than the second.\n"); } else min = second; printf ("The second number is smaller than the first\n"); printf("The smaller number is equal to %d\n", min);
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72. An example
int a, b; printf("Enter two Numbers\n"); scanf("%d%d", &a, &b); if (a == b) { printf("The numbers equal %d\n", a); printf("The expression a == b is %d\n", a == b); } else printf("The numbers are not equal\n");
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73. The assignment operator =
The assignment operator is also an operator. Hence, expressions involving it have a numeric value
This value equals to whatever appears on the right of the assignment operator
For example –
(x = 4) equals 4
(y = 0) equals 0
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74. A very common mistake
Very often a programmer might confuse between the equality operator and the assignment operator -
if (x==4) …
if (x=4) …
The second is usually a mistake, but legal in C so the compiler doesn’t call it!
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75. Else-if
if statements distinguish between exactly 2 cases and execute different code in each case
The else-if construction allows for a multi-way decision
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76. Else-if if (expression) statement else if (expression) else
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77. An example
if (grade >= 90) printf ("A\n"); else if (grade >= 80) printf ("B\n"); else if (grade >= 70) printf ("C\n"); else if (grade >= 60) printf ("D\n"); else printf ("F\n");
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78. The ?: operator expr1 ? expr2 : expr3
If expr1 is true (non-zero), expr2 is evaluated. Otherwise, expr3 is evaluated
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79. The ?: operator
#include <stdio.h> int main(void) { int i, j, min; printf("Please enter two numbers: "); scanf("%d%d", &i, &j); min = i<j ? i : j; printf("The minimum between %d and %d is %d\n", i, j, min); return 0; }
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80. getchar getchar() gets a single character from the user.
Requires including stdio.h
Returns a non-positive number on failure.
Similar to scanf.
char c;
c = getchar();
char c;
scanf(“%c”, &c);
====
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81. putchar putchar(‘char’) prints out the character inside the brackets.
Requires including stdio.h
Similar to printf.
char c;
putchar(c);
char c;
printf(“%c”, c);
====
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82. The switch statement a multiway conditional statement
similar to the if-else if-else … "statement"
allows the selection of an arbitrary number of choices based on an integer value
switch (expression) {   case const-expr: statements   case const-expr: statements   …   default: statements }
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83. That grade example again
switch (grade/10) { case 10: case 9: printf ("A\n"); break; case 8: printf ("B\n"); case 7: printf ("C\n"); case 6: printf ("D\n"); default: printf ("F\n"); }
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84. Give me a break
when the switch transfers to the chosen case, it starts executing statements at that point
it will “fall through” to the next case unless you “break” out
it causes the program gpout from the switch statement, that is , to immediately jump to the next statement after the switch statement
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85. Loops
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86. Desinnig a loop ? ?
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87. Loops for, while, do-while loops.
Used to repeat the same instruction(s) over and over again.
C provides some flexible ways of deciding how many times to loop, or when to exit a loop.
for, while, do-while loops.
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88. While loops while (condition) { statement(s); }
The statements are executed as long as condition is true
When the condition is no longer true, the loop is exited.
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89. Example - factorial
#include <stdio.h> int main() { int i,n,fact = 1; printf("Enter a number\n"); scanf("%d", &n); i=1; while (i<=n) fact = fact*i; i++; } printf("the factorial is %d\n", fact); return 0;
This is a counter
Every iteration i is incremented by 1. (Equivalent to i=i+1)
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90. Exercise Input – Output – Note – Two integers – A and B
How many times A contains B
This is the result of the integer division A/B
Note –
Do not use the division operator!
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91. Solution
#include <stdio.h> int main() { int a, b, res; printf("Please enter two numbers.\n"); scanf("%d%d", &a, &b); res = 0; while ( (res+1) * b <= a) res = res + 1; printf("%d / %d = %d", a, b, res); return 0; }
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92. Additional Exercise Input – Output – Two integers – A and B
Only if A and B are Digits
Otherwise return to input.
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93. Solution
#include <stdio.h> void main() { int a, b, flag = 1; while ( flag){ flag = 0; printf("Please enter two numbers.\n"); scanf("%d%d", &a, &b); if(a > 9 || a < 0) flag = 1; else if (b > 9 || b < 0 ) } // executing statements using a and b
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94. Compact Solution
#include <stdio.h> int main() { int a, b, flag = 1; while ( flag){ flag = 0; printf("Please enter two numbers.\n"); scanf("%d%d", &a, &b); if(a > 9 || a < 0 || b > 9 || b < 0 ) flag = 1; } // executing statements using a and b return 0;
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95. do - while loops do { statement(s) } while (expression);
Similar to while loops
Except the condition is evaluated after the loop body
The loop body is always executed at least once, even if the expression is never true (equals zero)
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96. Example – waiting for legal input
#include <stdio.h> int main() { int i; printf("Please enter a positive number.\n"); do { scanf("%d", &i); if (i<=0) printf("That's not a positive number! Try again.\n"); } while (i<=0); /* The program continues.... */ return 0; }
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97. Loop for - format Program before the for loop for( ; ; ) { loop body;}
Continue the program after for loop
initialize
condition
increment
False
True
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98. For loops (cont.)
Equivalent to while… Any for loop can be converted to while loop and vice versa.
If we want to perform something for a predefined number of times, better use for.
If we just wait for something to happen (not after a certain number or iterations), better use while.
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99. Break in a loop
When break is encountered, the loop is exited regardless of whether the condition is still true.
The program then continues to run from the first line after the while loop.
If called within a nested loop, break breaks out of the inner loop only.
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100. Using break void main () {.
loop1(expression1) {
loop2(expression2) {
break;
} // end loop2
// continue with loop1
} // end loop1 }
void main () { . loop(expression) { break; } // continue with the program
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101. Continue in the loop
When continue is encountered, the rest of the loop is ignored.
The program then continues to run from the beginning of the loop.
Rarely used.
Can usually be replaced by an appropriate if-else statement.
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102. Using continue
void main () { . loop(expression) { continue; } // continue with the program
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103. Write a program that accepts a number from the user,
Exercise
Write a program that accepts
a number from the user,
and checks whether it is prime.
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104. Solution (isprime.c)
#include <stdio.h> int main() { int i, num; printf("enter a number\n"); scanf("%d", &num); for(i = 2 ; i < num; i++) if (num % i == 0) // Then we know it's not a prime break; if (i < num) printf("%d is not a prime number!\n", num); else printf("%d is indeed a prime number!\n", num); return 0; }
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105. Exercise Extend the former program so
it accepts a number from the user,
and prints out all of the prime numbers
up to that number
(Hint – gotta use nested loops here...)
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106. Solution (listprimes.c)
#include <stdio.h>
void main() {
int i, j, last;
printf("enter a number\n");
scanf("%d", &last);
for(i = 2; i <= last; i++) {
for(j = 2 ; j < i; j++)
if (i % j == 0)
break;
if (j == i)
printf("the number %d is prime\n", i);
} // end for }
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107. Exercise
Write a program that prints an upside-down half triangle of *. The height of the pyramid is the input.
*****
****
*** ** *
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108. Solution
#include<stdio.h> int main() { int i, j, size; printf(“Please enter a size:\n”); scanf(“%d”,&size); for (i = 1; i <= size; i++) { for(j = i; j <= size; j++) printf("*"); printf("\n"); } return 0;
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109. Exercise Change the former prime-listing program,
so that is displays only
the largest prime number
which is smaller than or equal
to the user’s input.
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110. Solution 1
#include <stdio.h> int main() { int i, j, last; int found = 0; // This indicates whether we found the largest prime printf("enter a number\n"); scanf("%d", &last); i = last; while (!found) { /* Loop until we find our guy */ for(j = 2 ; j < i; j++) if (i % j == 0) break; if (j == i) /* If this is true then i is prime */ found = 1; else i--; } printf("The largest prime not larger than %d is %d.\n", last, i); return 0;
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111. Solution 2 (with break)
#include <stdio.h> void main() { int i, j, last; printf("enter a number\n"); scanf("%d", &last); for(i=last ; i>1 ; i--) { for(j = 2 ; j < i; j++) if (i % j == 0) break; // break the inner for loop only !! if (j == i) // i is prime. We found our guy break; } printf("The largest prime not larger than %d is %d.\n", last, i);
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112. Arrays
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113. Problems with simple variables
Hard to give up values high number of variables.
Complex to sort a high number of variables by value.
Impossible to use loops .
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114. Arrays A block of many variables of the same type.
Array can be declared for any type.
E.g. int A[10] is an array of 10 integers.
Examples:
list of students’ marks
series of numbers entered by user
vectors
matrices
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115. Arrays in Memory Sequence of variables of specified type
The array variable itself holds the address in memory of beginning of sequence
Example:
double S[10];
The k-th element of array A is specified by A[k-1] (0 based) 1 2 3 4 5 6 7 8 9 S …
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116. Example - reverse
#include <stdio.h> void main() { int i, A[10]; printf("please enter 10 numbers:\n"); for(i=0; i<10; i++) scanf("%d",&A[i]); printf("numbers in reversed order:\n"); for(i=9; i>=0; i--) printf("%d\n",A[i]); }
Constant integer type
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117. #define #define defines a symbolic name.
During preprocessing phase, symbolic names are replaced by the replacement text.
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118. Reverse with #define
/* get 10 integers from the user and printing them in reversed order*/ #include <stdio.h> #define NUM 10 void main() { int i; int A[NUM]; printf(“Please enter %d numbers:\n",NUM); for(i=0; i<NUM; i++) scanf("%d",&A[i]); printf("numbers in reversed order:\n"); for(i=NUM-1; i>=0; i--) printf("%d\n",A[i]); }
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119. Will it work ? #define NUM 10 void main() { int i = 10; int A[i];
Example :
#include <stdio.h>
#define NUM 10
void main() {
int i = 10;
int A[i]; …
NO !! Need a constant integer type , no variable !
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120. Initialization
Like in the case of regular variables, we can initialize the array during declaration.
The number of initializers cannot be more than the number of elements in the array
But it can be lessin which case, the remaining elements are initialized to 0
The next declarations:
int array1[5] = {0};
int array2[8] = {2, 4, 6, 8};
meaning:
int array1[5] = {0,0,0,0,0};
int array2[] = {2, 4, 6, 8,0,0,0,0};
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121. Initialization (cont.)
If you like, the array size can be inferred from the number of initializers.
Leaving the square brackets empty .
So these are identical declarations :
int array1 [8] = {2, 4, 6, 8, 10, 12, 14, 16};
int array2 [] = {2, 4, 6, 8, 10, 12, 14, 16};
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122. Bubble Sort
#include <stdio.h> void main() { int array[] = {78,73,63,62,58,45,34,11}; int i,temp, n = 8 , flag; do { flag = 0; for ( i = 0 ; i < n-1 ; i++) { if ( array[i] > array[i+1] ) { temp = array[i]; array[i] = array[i+1]; array[i+1] = temp; flag = 1; } } while (flag);
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123. Insertion Sort
#include <stdio.h> int main() { int array[]= {12 ,3,34,14,98,33,9,17}; int temp,loops = 0 ,j, i, n = 8; for ( i = 1 ; i < n ; i++) { temp = array[i]; j = i-1; loops++; while (( j >=0) && ( temp < array[j])) { array[j+1] = array[j]; j--; } array[j+1] = temp; }
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124. Selection Sort
void selection_sort(int list[], int n) { int i, j, min; for (i = 0; i < n - 1; i++) { min = i; for (j = i+1; j < n; j++) { if (list[j] < list[min]) { min = j; } } temp = list[i]; list[i] = list[min]); list[min] = list[i]; }
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125. Two Dimensional Arrays
We sometimes want to keep an inherent Two-Dimensional structure of data.
Example:
We can define a two-dimensional 3x3 matrix by
double A[3][3];
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126. Two Dimensional Arrays
Array of arrays:
int A[2][3] = { {1, 2, 3},
{4, 5, 6} };
Means an array of 2 integer arrays, each of length 3.
Access: j-th element of the i-th array is
A[i][j]
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127. Initialization Two Dimensional Arrays
When initializing the array, it is necessary to indicate the size of the second dimension:
double B[][2] = {{1,2}, {2,3}, {3,4}};
Filling with zeros (0) is similar to one dimension array.
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128. Exercise
Write a program that defines 3 matrices A,B,C of size 3x3 with float elements; initialize the first two matrices (A and B)
Compute the matrix multiplication of A and B and store it in C (i.e. C = A*B)
Matrix Multiplication:
Print all the matrices on the screen
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129. Strings in C
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130. String Problem : It may be much shorter A sequence of characters.
Stored, as might be expected, in an array of chars.
Another way to initialize:
char A[]=“blabla”;
Problem :
It may be much shorter
than the array where it’s stored
How can we know where a string end ?!
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131. The Terminator
Strings terminate with NULL character, signed by ‘\0’ (ASCII code 0).
This is a convention used to know where the string ends.
It means that in order to hold a string of 7 chars we need an array of length at least 8
So the previous initialization :
char A[]=“blabla”;
is equivalent to
char A[] = {‘b’, ‘l’, ‘a’, ‘b’, ‘l’, ‘a’, ‘\0’};
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132. Printing strings
printf allows printing whole strings at once using %s – char str[200]; /* … */ printf(“%s\n”, str);
This will print the contents of str ,cell by cell,
until a ‘\0’ is encountered
this may be less than 200, but also more
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133. The fool on the null
#include <stdio.h> void main() { char str[]="I'm a full string"; printf("%s\n",str); str[7]='o'; str[8]='o'; str[11]='\0'; str[11] = ‘s’; printf("%s\n", str); }
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134. Reading-in strings
There are several ways of accepting strings as input from the user.
The obvious way to go is read character by character using getchar()
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135. Reading-in strings - scanf
A simpler way is to use scanf
To read in a string to a variable str we can use:
scanf(“%s”, str);
Note there’s no ‘&’ sign!!!
Scanf reads-in letters until a space or newline is encountered
The maximum length can be stated in the parentheses –
scanf(“%10s”, str);
This will read in 10 letters, plus the ‘\0’ sign (so str should have place for 11 characters)
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136. Reading-in strings – gets()
An other simpler way is to use gets()
To read in a string to a variable str we can use:
gets( str);
Note there’s no ‘&’ sign too!!!
gets reads-in letters until a newline (pressing enter on the keyboard) is encountered.
The maximum length can be stated for the array of characters declaration minus 1.
gets change the new line for the `\0` sign as last character in the string.
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137. Writing out strings – puts()
A simpler way to write out a string is to use puts()
To write out a string that is into variable str we can use:
puts( str);
puts write out letters until a `\0` sign is encountered.
puts change the `\0` sign for new line as the last character in the string.
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138. Comparing strings We cannot just compare strings’ contents by
char A[7]=“Hello”;
char B[7]=“Hello”;
if(A==B) { … }
Because A and B are addresses of A[0] and B[0]
A==B only if A and B are the same string in memory
In order to compare the contents we must scan char by char
‘H’
‘e’
‘l’
‘o’
‘\0’ …. B A
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139. String library
Like in the case of stdio.h and math.h, we have a special library for handling strings
We should #include <string.h>
Functions:
strlen(s) – returns the length of s
strcmp(s1, s2) – compares s1 with s2
strcpy(s1, s2) – copies to contents of s2 to s1
strcat(s1 , s2) – add the s1 at the end of s2
and more…
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