1. Administrative details
Project #1 due Tuesday
Lab #6 on Tuesday
Midterm exam next week
Content for exam:
General computer knowledge
Unix commands
C language
Chapters 1-5 Afzal
Chapters 1-4 Deitel
Class slides
Lab work
Homework
CISC 105 – Topic 1

2. Topic 1: Introduction to Computers and Programming
“A journey of a thousand miles must begin with a single step.” - Lao Tzu

3. Binary Math
A binary digit or bit for short is the smallest unit of computer information. Just as our familiar decimal number system has 10 digits,(0,1,2,3,4,5,6,7,8,9) the binary number system has only 2 digits (0,1). This is the perfect number system for computers since we can store a binary digit by making an electrical or magnetic field either on or off, positive or negative, 1 or 0. In the decimal system we can count 10 (we start counting with 0) items with one decimal place, 100 with two decimal places, 1000 with three decimal places and so on.
CISC 105 – Topic 1

4. Binary Math
The binary number system works the same way except since we only have 0s and1s our base is 2.
So we can count 2 permutations of 1 bit: 0 1
4 permutations of 2 bits:
8 permutations of 3 bits:
16 permutations of 4 bits:
...and so on.
CISC 105 – Topic 1

5. Binary Math
So in an eight bit byte, the numbers are represented this way:
Bit:
Value:
Example:
Values: = 182
The minimum number is 0 (all 0s) and the maximum number is 255 (all 1s), so you can represent 256 numbers (0-255) with one byte.
CISC 105 – Topic 1

6. Hardware Components Hardware consists of many types of components:
Primary Storage
Secondary Storage
The Central Processing Unit (CPU)
Input Devices
Output Devices
CISC 105 – Topic 1

7. Primary Storage and Secondary Storage
There are two primary types of storage in a computer:
Primary Storage – often referred to as main memory, or just memory. This storage is very quickly accessed by the various hardware components.
Secondary Storage – includes such devices as a hard disk, floppy disk, CD-ROM, DVD-ROM, etc…
CISC 105 – Topic 1

8. Computer Software All major computers run an operating system.
An operating system is a special type of program which controls the interaction between all hardware components and the user.
CISC 105 – Topic 1

9. Operating Systems Tasks
Responsibilities of the OS include:
Communicating with the user(s)
Managing resources including memory and processor time among programs
Collecting data from input devices
Providing data to output devices
Accessing data from secondary storage
Writing data to secondary storage
CISC 105 – Topic 1

10. Computer Programming - Machine Language
Computer programs were originally written in machine language. Machine language is a sequence of binary numbers, each number being an instruction.
Each instruction is computer-understandable.
CISC 105 – Topic 1

11. Computer Programming - Assembly Language
To make machine language more abstract, assembly language was introduced.
Assembly language provides a one-to-one mapping between a machine instruction and a simple human-readable instruction.
Assembly language is converted to computer-understandable machine language by an assembler.
CLR A
ADD A
ADD B
CISC 105 – Topic 1

12. Computer Programming – High-Level Languages
In order to get around these obstacles, high-level languages were introduced.
High-level languages provide a one-to-many mapping between one high-level statement and multiple assembly language instructions.
High-level language is converted to assembly instructions by a compiler, which creates object files. Modern compilers perform the assembler function as well.
CISC 105 – Topic 1

13. Modern Software Development
Programmers often use library functions, which are pre-written functions provided as part of the compiler/development toolset.
A library is an object file.
After the source code is compiled into machine code, a linker resolves cross-references among these object files, including libraries. The object files are linked into an executable file.
CISC 105 – Topic 1

14. Modern Software Development
Compiler
Source Code File
Object File
Other Object Files
(perhaps libraries)
Linker
Executable File
Loader
CISC 105 – Topic 1

15. The Software Development Method
Specify the problem.
Analyze the problem.
Design an algorithm to solve the problem.
Implement the algorithm.
Test and verify the program.
Maintain and update the program.
CISC 105 – Topic 1

16. Review Name the principle components of a computer.
What are three responsibilities of the operating system?
What advantages does assembly language have over machine language?
What advantages do high-level languages have over assembly language?
CISC 105 – Topic 1

17. Review
What computer program translates a source code file into an object file?
What program combines object files and produces an executable file?
What part of the operating system is responsible for the loading and scheduling of programs?
What are the steps in the software development method?
CISC 105 – Topic 1

18. Topic 2 – Introduction to the C Programming Language

19. Preprocessor Directives
A preprocessor directive is a command given to the C preprocessor, which is a part of the compilation process that modifies a C source code file before it is compiled.
Preprocessor directives always begin with a “#” character.
In the example program, there are two preprocessor directives used, #include and #define.
CISC 105 – Topic 1

20. #include
The #include directive tells the C preprocessor to give a program access to a library. By putting a #include in a program, the preprocessor loads a header file, which tells the compiler what functions and other information is provided in the library.
In this program, #include <stdio.h> indicates that this program uses the stdio library which contains functions such as printf(). Page 542 of Deitel lists standard libraries.
CISC 105 – Topic 1

21. #define The #define directive specifies a constant macro.
This tells the preprocessor that every time it encounters the first text, it should replace it with the second text.
In this program, #define KMS_PER_MILE tells the preprocessor to replace KMS_PER_MILE with every place KMS_PER_MILE appears in the program.
CISC 105 – Topic 1

22. Preprocessor Directives
/* Converts distances from miles to kilometers */
#include <stdio.h>
#define KMS_PER_MILE 1.609
int main(void) {
double miles, /* distance in miles */
kms; /* equiv. distance in kms */
/* Get the distance in miles */
printf(“Enter the distance in miles>”);
scanf (“%lf”, &miles);
/* Convert the distance to kilometers */
kms = KMS_PER_MILE * miles;
/* Display the distance in kilometers */
printf (“That equals %f kilometers.\n”,kms);
return(0); }
CISC 105 – Topic 1

23. Preprocessor Directives
#include directive –
Tells the preprocessor to
include the stdio.h header file.
This file describes the functions
and other information included
in the stdio library.
/* Converts distances from miles to kilometers */
#include <stdio.h>
#define KMS_PER_MILE 1.609
int main(void) {
double miles, /* distance in miles */
kms; /* equiv. distance in kms */
/* Get the distance in miles */
printf(“Enter the distance in miles>”);
scanf (“%lf”, &miles);
/* Convert the distance to kilometers */
kms = KMS_PER_MILE * miles;
/* Display the distance in kilometers */
printf (“That equals %f kilometers.\n”,kms);
return(0); }
CISC 105 – Topic 1

24. Preprocessor Directives
#include directive –
Tells the preprocessor to
include the stdio.h header file.
This file describes the functions
and other information included
in the stdio library.
/* Converts distances from miles to kilometers */
#include <stdio.h>
#define KMS_PER_MILE 1.609
int main(void) {
double miles, /* distance in miles */
kms; /* equiv. distance in kms */
/* Get the distance in miles */
printf(“Enter the distance in miles>”);
scanf (“%lf”, &miles);
/* Convert the distance to kilometers */
kms = KMS_PER_MILE * miles;
/* Display the distance in kilometers */
printf (“That equals %f kilometers.\n”,kms);
return(0); }
#define directive –
Tells the preprocessor to
replace every occurrence of
KMS_PER_MILE in the program
with
CISC 105 – Topic 1

25. What is a valid identifier?
Identifiers can only be composed of letters, digits, and underscores.
Identifiers cannot begin with a digit.
Reserved words cannot be identifiers.
Identifiers can be as long as you want.
Upper and lower case letters are different (i.e. kms and Kms are not considered to be the same identifier).
CISC 105 – Topic 1

26. Review
Which of the following are valid identifiers? For each that is not valid, why is it not?
This_is_a_long_identifier_but_is_it_valid?
8timesTheRadius
miles
Phil’s variable
kilometers_per_hour x
“radius”
CISC 105 – Topic 1

27. Variables
A variable is a memory cell that is used to hold data acted upon by the program.
A variable declaration tells the C compiler the name and type of a variable used in a program.
A variable declaration consists of a data type and an identifier which is the name of that variable.
Every variable that will be used in a program must be declared.
CISC 105 – Topic 1

28. Variable Declarations
double mile;
int counter;
The first line declares a variable named mile of the double data type.
The second line declares a variable named counter of the int data type.
CISC 105 – Topic 1

29. Data Types
There are a large number of data types. These are some of the most popular ones.
void – this keyword means “no data type”.
int – An integer (a whole number). This data type can represent an integer in a specific range, at least –32767 through
CISC 105 – Topic 1

30. Data Types
char – A character. One letter, digit, or symbol. This is enclosed in single quotes.
float – A real number (an integer part and a decimal part).
double – A real number. Note that this data type is a memory cell double the size of a float data type. This allows a bigger number to be represented, or a specific number to be represented more precisely. This is referred to as a double-precision floating point number.
CISC 105 – Topic 1

31. Review of Variables
Write a #define preprocessor declaration for a constant macro of STUDENTS_PER_SECTION to 22 and variable declarations of num_students as an integer, GPA and class_GPA as double-precision floating point numbers, and letter_grade as a character data type.
#define STUDENTS_PER_SECTION 22
int num_students;
double GPA, class_GPA;
char letter_grade;
CISC 105 – Topic 1

32. Assignment Statements
An assignment statement is one type of executable statement.
An assignment statement uses the “=“ operator, and follows the form:
variable = expression;
This statement first evaluates the expression on the right and stores the result in the variable on the left.
CISC 105 – Topic 1

33. Assignment Statements
Here are some examples of assignment statements:
x = 12;
negative_x = -x;
x = y z * 5;
result = y;
Note that any variables in the right-side expression are not modified by an assignment statement.
CISC 105 – Topic 1

34. Function Calls
A function is a piece of code which performs a specific task.
Functions can be created by programmers or supplied as part of the C compiler toolset.
A function is called, which causes it to execute.
A function call is composed of the function name, an open paren, a set of function arguments separated by commas, and a close paren.
CISC 105 – Topic 1

35. Function Calls A function call looks like this: function name
function_name(argument1, argument2, argument3);
function name
open paren
function arguments
close paren
CISC 105 – Topic 1

36. The printf Function
The C function for displaying output on the screen to the user is printf.
printf is called in the following manner:
printf(“The final GPA of student number %d is %f.\n”,student_num, GPA);
format string
print list
function arguments
CISC 105 – Topic 1

37. The Format String and Print List
The format string is the text that is to be displayed on the screen.
The “%” characters are called placeholders. They indicate the display position for variables whose values are to be displayed.
The variable names to be displayed are specified in the print list and appear in the same order as their placeholders.
CISC 105 – Topic 1

38. Placeholders This pair specifies the location in the print
printf(“The final GPA of student number %d is %f.\n”,student_num, GPA);
This pair specifies the location in the print
string to display the student_num value.
This pair specifies the location in the print
string to display the GPA value.
CISC 105 – Topic 1

39. Placeholders All placeholders begin with a “%”.
The text after the “%” symbol indicates how to format the output, i.e. what kind of variable it is.
%d – decimal number (int)
%f – floating-point number (float or double)
%c – character (char)
CISC 105 – Topic 1

40. Escape Sequences All escape-sequences begin with a backslash, “\”.
A letter after the “\” character denotes an escape sequence, which has special meaning.
The “\n” sequence indicates a new-line character, which cause any following text to appear on the next line on the display.
In order to display a “\”, the format string must contain a “\\”.
CISC 105 – Topic 1

41. Placeholders and the Newline Escape Sequence
printf(“The final GPA of student number %d is %f.\n”,student_num, GPA);
Specifies to display student_num,
which is an integer.
Specifies to display GPA,
which is an floating-point number.
CISC 105 – Topic 1

42. Placeholders and the Newline Escape Sequence
printf(“The final GPA of student number %d is %f.\n”,student_num, GPA);
Specifies to display student_num,
which is an integer.
Specifies to display GPA,
which is an floating-point number.
Therefore, if the student_num variable was set to
10 and the GPA variable was set to 3.03
this printf function call would cause:
The final GPA of student number 10 is 3.03.
to be displayed on the screen and any
subsequent output to begin on the next line.
CISC 105 – Topic 1

43. The scanf Function
The C function for reading input from the user is scanf.
scanf(“%d %f”,&student_num, &GPA);
format string
input list
function arguments
CISC 105 – Topic 1

44. The Format String
The format string is the set of placeholders which specify what type of data is being input.
The same placeholders are used as for printf, except for when inputting a floating-point number. A float type still uses the “%f”, however the double type uses the “%lf” placeholder.
CISC 105 – Topic 1

45. The Input List
The variables to store the inputted data are specified in the input list. They must be in the same order as their corresponding placeholders.
Notice that each variable name is preceded by a “&”.
The “&” is an operator which means “the address of”.
Therefore, “&student_num” tells the scanf function to store what it reads from the user at the memory address of student_num.
CISC 105 – Topic 1

46. Placeholders and the Input List
scanf(“%d %f”,&student_num, &GPA);
This pair specifies to read
in an integer and store it
at the memory address of
student_num, thus setting
the student_num variable
to the inputted value.
CISC 105 – Topic 1

47. Placeholders and the Input List
scanf(“%d %f”,&student_num, &GPA);
This pair specifies to read
in a single-precision floating
point number and store it
at the memory address of
GPA, thus setting the GPA
variable to the inputted value.
CISC 105 – Topic 1

48. Placeholders and the Input List
scanf(“%d %f”,&student_num, &GPA);
Therefore, if the input into this
scanf function call was:
<ENTER>
the student_num variable would be
set to 9 and the GPA variable would be
set to 3.560
CISC 105 – Topic 1

49. Review of printf and scanf
What is the displayed output when the following code fragment is run and the inputs are 8 and 12?
int x, y;
printf(“My name is”);
printf(“ Phil Viscito”);
printf(“\nEnter two integers> ”);
scanf(“%d%d”,&x, &y);
x = x + 2;
y = x + y;
printf(“Thanks! The answer is %d.\nBye now!”,y);
My name is Phil Viscito.
Enter two integers> 8 12
Thanks! The answer is 22.
Bye now!
CISC 105 – Topic 1

50. Customizing Integer Output
The “%d” placeholder, used to display an integer variable, can be altered to format how the number is displayed.
Instead of “%d”, use a “%Xd” where the X is an integer that is the field width, the number of digits to display.
For example, “%4d” displays four digits of the result. The negative sign (for negative integers) is also considered a digit here.
CISC 105 – Topic 1

51. Customizing Integer Output
When there are more places (the field width) than digits to be displayed, the output is right-justified.
When there are more digits than places, the field width is ignored, and the entire integer is displayed.
CISC 105 – Topic 1

52. Customizing Integer Output
As an example:
Value
Placeholder
Output
643
%1d
%4d
%5d
-643
%2d
%6d
CISC 105 – Topic 1

53. Customizing Floating-Point Output
Floating point output (float and double) can be formatted in the same manner, using “%X.Yf”).
Here, X is the total number of digits to display (the field width) and Y is the number of digits to display to the right of the decimal point.
The same rules for field width apply as for integer formatting.
The specified number of decimal digits is always displayed.
CISC 105 – Topic 1

54. Customizing Floating-Point Output
As an example:
Value
Placeholder
Output
%5.2f
3.14
%3.2f
%5.3f
3.142
0.1234
%4.2f
0.12
-0.006
%8.5f
CISC 105 – Topic 1

55. Arithmetic Expressions
Arithmetic expressions are executable statements that manipulate data.
Arithmetic expressions operate on both integer (int) and floating-point (float and double) numbers.
Arithmetic operators can operate on mixed types (i.e. one int and one float). The resulting type of such an expression is the “highest” data type present.
CISC 105 – Topic 1

56. Resulting Data Types
The “highest” data type is always considered to be a floating point number, with double-precision floating point numbers taking precedence over single-precision floating point numbers.
int + int = int
int + float = float
int + double = double
float + double = double
CISC 105 – Topic 1

57. Arithmetic Expressions
All of the common arithmetic operators are present in C:
+ (addition)
- (subtraction)
* (multiplication)
/ (division)
% (modulus – or “remainder”)
CISC 105 – Topic 1

58. Note: Integer Division
If two integer values are divided, the resulting data type is also an integer, as previously described.
Therefore, only the integer portion of the actual result will be the returned result.
For example,
9 / 4 = 2
9 / 10 = 0
CISC 105 – Topic 1

59. The Modulus Operator
The “%” operator is a modulus operator, which also means the remainder of the division.
For example,
9 % 3 = 0
10 % 6 = 4
90 % 8 = 2
CISC 105 – Topic 1

60. Review of Basic Arithmetic Expressions
What is the resulting output of this program segment?
int x, y, z;
float a;
x = 9 * 0.5;
a = 9 * 0.5;
y = 15 % 15;
z = 15 % 2;
printf(“x=%d\n, a=%f\n, y=%d\n, z=%d\n”,x,a,y,z)
x=4
a=4.5
y=0
z=1
CISC 105 – Topic 1

61. More Complex Arithmetic Expressions
C evaluates arithmetic expressions in the same order rules as normal mathematics.
Parentheses take priority over everything else.
Then, multiplication, division, and modulus operations from left to right.
Then, addition and subtraction from left to right.
CISC 105 – Topic 1

62. More Complex Arithmetic Expressions
x2 + 2xy + 4y2
(x * x) + (2 * x * y) + (4 * y * y)
a + b
c2 * 4d
(a + b) / (c * c * 4 * d)
a * (4x % y2)
a * (4 * x % (y * y))
x + y(x2 + y)
x + y * (x * x + y)
CISC 105 – Topic 1

63. Review What is the resulting output of this program segment?
#define PI
int x, y, z;
float a, b;
x = 5;
y = 10;
z = x + (4 * y * y * y) + PI;
a = x + (4 * y * y * y) + PI;
b = (x / y) + a;
printf(“x=%d, y=%d, z=%d\na=%7.2f, b=%f\n”,x,y,z,a,b);
x=5, y=10, z=4008
a= , b=
Since x and y are both integers, this evaluates to 0!
CISC 105 – Topic 1

64. Another Special Case: Increment and Decrement
Incrementing (adding one) and decrementing (subtracting one) are two very common operations.
C has a special syntax for increment and decrement operations that follows this form:
x++; OR
++x;
x--; OR
--x;
Increment
Decrement
CISC 105 – Topic 1

65. Another Special Case: Increment and Decrement
Notice that the increment operator expression x++ is the same as saying: x += 1;
So, why are there two forms of each operator?
The answer lies in when the increment (or decrement) operation is actually performed.
This distinction only occurs when the ++ (or --) operator is used on a variable in the same expression in which the value of the variable is used.
CISC 105 – Topic 1

66. Another Special Case: Increment and Decrement
For example,
int x = 20, y;
y = x++;
printf(“x=%d, y=%d\n”,x,y);
int x = 20, y;
y = ++x;
printf(“x=%d, y=%d\n”,x,y);
x=21, y=20
x=21, y=21
y = x;
x += 1;
printf(“x=%d, y=%d\n”,x,y);
x += 1;
y = x;
printf(“x=%d, y=%d\n”,x,y);
CISC 105 – Topic 1

67. Another Special Case: Increment and Decrement
The difference lies in when the expression (x++ or ++x) gets evaluated in relation to when the operator (++) gets performed.
The x++ expression is equal to the value of x and the ++ operator is performed after the evaluation is over.
The ++x expression indicates that the ++ operator is performed first (before ++x is evaluated) and thus, is equal to the value of x + 1.
CISC 105 – Topic 1

68. Review What is the output of the following program fragment?
int x, y, z;
x = 5;
y = 10;
z = x++ * y – 2;
z += ++x;
printf(“x=%d, y=%d, z=%d\n”,x,y,z);
x=7, y=10, z=55
CISC 105 – Topic 1

69. Topic 2A – Library Functions and Casting

70. Casting
Although each of the functions above take (and return) double data types, they can be used with float data types.
This is done with a cast.
A cast is the conversion of one data type to another data type.
This can be done explicitly or implicitly.
CISC 105 – Topic 1

71. Implicit Casting
When evaluating mixed type expressions, an implicit cast is performed, as mixed type expression cannot be directly evaluated.
For instance, in the code fragment:
float x = 1.0, y;
int z = 2;
y = x + z;
The variable z is casted to a float (2.0) in order to add it to the float-type x.
CISC 105 – Topic 1

72. Review What is the result of the following code fragment:
int w, z, q;
float x = 4.0, y;
y = sqrt(x);
w = y;
z = x – w;
q = sqrt(3);
printf(“y=%f,w=%d,z=%d,q=%d”,y,w,z,q);
y=2.0, w=2, z=2, q=1
CISC 105 – Topic 1

73. Explicit Casting
Casting can also be performed explicitly. This is done with the following form. What is the result of the following code fragment?
int a=9, b=4;
float c,d;
c = a/b;
d = (float)a / (float)b;
printf(“c=%.2f,d=%.2f”,c,d);
c=2.00, d=2.25
CISC 105 – Topic 1

74. Topic 3 – The General Form of a C Program

75. The main Function Header
Thus, a general C program looks like:
Preprocessor directives
int main() {
variable declarations
executable statements
return(0); }
CISC 105 – Topic 1

76. Review (Problem #1)
Write a complete C program that asks the user for a floating-point number, multiplies the number by 4 * PI (with PI set to in a constant macro) and then outputs the result.
CISC 105 – Topic 1

77. Review (Problem #1)
#include <stdio.h>
#define PI
int main() {
float number, answer;
printf(“Number?”);
scanf(“%f”,&number);
answer = number * 4 * PI;
printf(“The answer is:%f\n”,answer);
return (0); }
Write a complete C program that asks the user for a floating-point number, multiplies the number by 4 * PI (with PI set to in a constant macro) and then outputs the result.
CISC 105 – Topic 1

78. Topic 5 – Control Structures

79. The if statement
In order to use a condition to make a decision about program control flow, C uses an if statement.
The if statement first evaluates the specified condition.
If the condition is true, the statement immediately following the if statement executes. If the condition is false, the statement immediately following the if statement is skipped.
CISC 105 – Topic 1

80. The if/else Statement
In addition, C supports the use of an else statement.
This statement can follow the optional statement (the one that may or may not execute depending on the condition).
If there is an else statement, the statement immediately following the else statement will execute if the original condition (in the if statement) is false.
CISC 105 – Topic 1

81. The if/else Statement
Therefore, if there is an else statement, either the statement immediately following the if statement is run (if the condition is true) OR the statement immediately following the else statement is run (if the condition is false).
CISC 105 – Topic 1

82. The if/else Statement
statement1;
if (condition)
statement2;
else
statement3;
statement4;
In this program fragment, statement1 executes. Then, either statement2 or statement3 executes, depending on the condition. Then, statement4 executes. Note that only 3 statements execute.
CISC 105 – Topic 1

83. Compound Statements
Sometimes, we wish to perform more than one thing as the result of an if statement.
In this case, we can make use of a compound statement.
A compound statement is a group of C language statements that run sequentially. A compound statement is enclosed with curly braces (“{” and “}”), in the same way a function body is.
CISC 105 – Topic 1

84. Compound Statements
When used as the statement immediately following an if or else statement, a compound statement is treated like one statement (all of the statements enclosed by the braces are run, sequentially.)
CISC 105 – Topic 1

85. Compound Statements
In this program fragment, statement1 executes. Then, either statement2 and statement3 execute or statement4 and statement5 execute, depending on the condition. Then, statement6 executes. Note that only 4 statements execute.
statement1;
if (condition) {
statement2;
statement3; }
else
statement4;
statement5;
statement6;
CISC 105 – Topic 1

86. Cascading if/else Statements
We can also cascade if/else statements. Here, condition1 is first evaluated. If it is true, statements 2 & 3 run and then statement7. If condition1 is false and condition2 is true, statements 4 and 5 run and then statement 7. If both conditions are false, statement 6 runs and then statement 7.
Note that if condition1 is true, condition2 is never evaluated!
statement1;
if (condition1) {
statement2;
statement3; }
else if(condition2)
statement4;
statement5;
else
statement6;
statement7;
CISC 105 – Topic 1

87. Conditions
So, now the structure of if/else statements are known. So, how do we write the conditions?
When an if statement is encountered, the program will first evaluate the condition in the if statement. Anything that evaluates to a nonzero value is true. Anything that evaluates to zero is false.
CISC 105 – Topic 1

88. Conditions Thus, an if statement that was written as:
if (x)
statement1;
would cause statement1 to run if x was
equal to any value other than zero. If
x was zero, statement1 would be
skipped.
CISC 105 – Topic 1

89. Relational and Equality Operators
In order to write conditions effectively, C provides a number of relational and equality operators.
For these operators, if the condition they specify is true, they evaluate to 1. If the condition they specify is false, they evaluate to 0.
CISC 105 – Topic 1

90. Operators The following relational and equality operators are used:
< less than
> greater than
<= less than or equal to
>= greater than or equal to
== equal to
!= not equal to
CISC 105 – Topic 1

91. Conditions & Operators
Some examples of the use of these operators include:
x <= 0
number > MAX_NUMBER_ALLOWED
male_or_female == ‘M’
num != flag_number
y < 12.0
q <= p
CISC 105 – Topic 1

92. A VERY Common Error
An assignment statement evaluates to the value being assigned. For example,
x = 6;
evaluates to 6. So…what would happen
if we wrote
if (x = 6) { … }
CISC 105 – Topic 1

93. A VERY Common Error
An assignment statement evaluates to the value being assigned. For example,
First, 6 would be assigned to x!
This overwrites whatever was previously
stored in x. Also, x=6 evaluates to 6.
As anything nonzero is true, this condition
is ALWAYS true.
x = 6;
evaluates to 6. So…what would happen
if we wrote
BOTTOM LINE:
Remember to use == for conditions!
if (x = 6) { … }
CISC 105 – Topic 1

94. Compound Conditions
More than one condition can be combined to create a compound condition.
A logical operator is used to combine conditions. These include:
&& AND
|| OR (shift-\)
CISC 105 – Topic 1

95. Logical AND (&&) The logical AND operator works as follows:
Condition 1
Condition 2
Overall Result
TRUE (nonzero)
TRUE (1)
FALSE (0)
CISC 105 – Topic 1

96. Logical OR (||) The logical OR operator works as follows: Condition 1
Overall Result
TRUE (nonzero)
TRUE (1)
FALSE (0)
CISC 105 – Topic 1

97. Logical NOT (!)
The last logical operator is the NOT operator. It behaves as follows:
Condition
!(Condition)
TRUE (nonzero)
FALSE (0)
TRUE (1)
CISC 105 – Topic 1

98. (X > 19.75 && number == 3) || big_letter != ‘Q’
Examples
Write a condition that is true if both x is greater than and number is equal to 3 OR big_letter is not equal to ‘Q’.
(X > && number == 3) || big_letter != ‘Q’
CISC 105 – Topic 1

99. The switch Statement
When one variable (type int or char ONLY) is used to make a control decision, where different statements are run if the variable is equal to different values, the switch statement can be used.
Note that this statement does not allow less than, greater than, etc. ONLY the equality operator (==) is used with a switch statement.
The switch statement is composed of a control variable and a series of case clauses.
CISC 105 – Topic 1

100. The switch Statement The switch statement takes this form:
switch(control variable) {
case (value1):
. . .
break;
case (value2):
default: }
The switch statement takes this form:
CISC 105 – Topic 1

101. The switch Statement
switch(control variable) {
case (value1):
. . .
break;
case (value2):
default: }
When the switch statement is encountered, the control variable is evaluated. Then, if that evaluated value is equal to any of the values specified in a case clause, the statements immediately following the colon (“:”) begin to run.
CISC 105 – Topic 1

102. The switch Statement
switch(control variable) {
case (value1):
. . .
break;
case (value2):
default: }
These statements then continue to run until a break statement is encountered. Control then flows to the statement immediately following the closing brace (“}”).
CISC 105 – Topic 1

103. The switch Statement
switch(control variable) {
case (value1):
. . .
break;
case (value2):
default: }
It is important to remember that control will pass into the next case clause if a break statement is not encountered.
CISC 105 – Topic 1

104. The switch Statement
switch(control variable) {
case (value1):
. . .
break;
case (value2):
default: }
So, what happens if the control variable is not equal to any of the values specified in the case clauses?
The default case clause runs.
CISC 105 – Topic 1

105. switch Statement Example
switch(x) {
case (3):
statement2;
statement3;
case (27):
case (1):
statement4;
case (2):
statement5;
break;
default:
statement6:
statement7: }
statement8;
Which statements run if:
x = 1?
x = 2?
x = 3?
x = 10?
x = 27?
CISC 105 – Topic 1

106. Topic 6 – Repetition and Loops

107. The while Statement
In order to create loops in the C language, the while statement can be used. This statement follows the form:
while (condition) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

108. The while Statement
When the while statement is first encountered, the condition is evaluated. If it is true (nonzero), the loop body executes sequentially. If the condition is false, the loop body is skipped. Notice that this behavior is exactly the same as a simple if statement.
while (condition) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

109. The while Statement
At the end of the loop body (when the closing brace “}” is encountered) the condition is evaluated again. If it is true, the loop body begins to execute again, beginning with the first statement in the body (immediately following the open brace “{”) and continuing to the end sequentially.
while (condition) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

110. Counting Loops and while Statements
As we have seen, counting loops have three principle components when created with a while statement:
Initialization – set the initial value of the loop control variable (usually to zero)
Testing – Test the value of the loop control variable according to the condition
Updating – Update the loop control variable as the last statement in the loop
CISC 105 – Topic 1

111. A Common Loop Error
What happens if, when writing a counting loop, the programmer does not update the loop control variable?
The condition will always be true
(assuming it starts off as true). Thus,
the loop with execute forever. This
is referred to as an infinite loop.
CISC 105 – Topic 1

112. The for Statement
As we have seen, many loops have three components in addition to the loop body:
Initialization – set the initial value of the loop control variable
Testing – Test the value of the loop control variable according to the condition
Updating – Update the loop control variable
CISC 105 – Topic 1

113. The for Statement
The for statement offers a designed place for each of these three components. It follows the form:
for (counter = 0;
counter < high_value;
counter++) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

114. The for Statement
Thus, the for statement consists of an initialization, a semicolon, the condition, a semicolon, and the an update statement.
for (counter = 0;
counter < high_value;
counter++) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

115. The for Statement
Notice that the update statement does NOT have a semicolon.
Also, the initialization and update statements can actually be composed of more than one statement.
for (counter = 0;
counter < high_value;
counter++) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

116. The for Statement
If more than one statement is to be used for initialization or update, these statements are separated by commas.
for (counter = 0, x = 0;
counter < high_value;
counter++, x += 2) {
statement1;
statement2;
. . . }
CISC 105 – Topic 1

117. for / while Loop Equivalence
Notice that any for loop can be rewritten into a while loop.
The for loop is simply a special case of the while loop, with provisions for initialization and updating build into the loop statement itself.
CISC 105 – Topic 1

118. for / while Loop Equivalence
To convert from a for loop to a while loop, simply move the initialization statement(s) before the loop statement and move the update statement(s) inside the loop body.
We can rewrite the payroll example using a for loop instead of a while loop.
CISC 105 – Topic 1

119. The Payroll Example with a for Loop
printf(“How many employees do you have?”);
scanf(“%d”,&number_employees);
total_pay = 0.0;
for (counter = 0; counter < number_employees;
counter++) {
printf(“Hours?”);
scanf(“%d”,&hours);
printf(“Pay rate?”);
scanf(“%lf”,&rate);
pay = hours * rate;
printf(“The pay is %f.\n”,pay);
total_pay += pay; }
printf(“Total pay = %f.\n”,total_pay);
CISC 105 – Topic 1

120. The do-while Statement
Both of the loop statements we have seen evaluate the condition before the first loop iteration.
Sometimes, we wish to check the condition at the end of the loop iteration, instead of at the beginning of the loop iteration.
This has the effect of ALWAYS executing the loop the first time, then testing the condition at the end of the loop iteration.
CISC 105 – Topic 1

121. The do-while Statement
In order to create such loops, C offers the do-while statement.
This loop follows the format: do {
statement1;
statement2;
. . .
} while (condition);
CISC 105 – Topic 1

122. The do-while Statement
When the do statement is first encountered, the loop body begins to execute immediately.
When the loop body completes, the while statement is reached.
The condition is then evaluated.
statement1; do {
statement2;
statement3;
. . .
} while (condition);
statement4;
CISC 105 – Topic 1

123. The do-while Statement
Notice that the while statement has a semicolon at the end of the condition.
statement1; do {
statement2;
statement3;
. . .
} while (condition);
statement4;
CISC 105 – Topic 1

124. The do-while Statement
So…the do-while loop statement is the same as the while statement except the condition is tested at the end of the loop iteration rather than the beginning.
Thus, a do-while loop ALWAYS executes at least once.
statement1; do {
statement2;
statement3;
. . .
} while (condition);
statement4;
CISC 105 – Topic 1

125. do-while and while Comparison
Do the following loops do the same thing?
Loop #1
scanf(“%d”, &num);
while (num != SENTINEL) {
/* do something
with num */ }
Loop #2
do {
scanf(“%d”, &num);
if (num != SENTINEL) {
/* do something
with num */ }
} while (num != SENTINEL)
Yes, the loops do the same thing.
Is one better than the other? Why?
CISC 105 – Topic 1

126. Common Loop Errors Find the error(s) in the following code:
#include <stdio.h>
/* This program asks for a number and displays that
many lines of stars on the screen. */
int main() {
int count, num_lines;
printf (“How many lines would you like?”);
scanf(“%d”,&num_lines);
for (count = 0; count <= num_lines; count++)
printf (“********************\n”);
return 0; }
CISC 105 – Topic 1

127. Common Loop Errors Find the error(s) in the following code:
#include <stdio.h>
/* This program asks for numbers and keeps a running
sum. It terminates when –99 is entered. */
int main() {
int sum, number;
do {
printf(“Enter a number (-99 to quit)>”);
scanf(“%d”,&number);
sum += number;
} while (number != -99);
printf (“The sum is %d.\n”,sum);
return 0; }
CISC 105 – Topic 1