 All C programs are made up of functions that perform operations on variables.  In this lecture we examine variables  Variables are the basic building blocks of a program  Variables have: types, names, and constants  Operations performed on variables using operators  Each operation forms an expression  A statement is a collection of expressions terminated by ; 2

 C is a “typed” language.  We must explicitly define variables with a specific type.  Different types have different characteristics.  Size: how many bytes of memory a variable occupies.  Representation: what does the bit-pattern mean.  Usage: what operations may be performed a variable.  By specifying types, we enable the compiler to perform a number of tasks for us:  Compiler allocates fixed amount of memory for a given type.  A variable’s type determines what value it can represent and what operations may be performed on it.  Compiler can detect type-mismatch errors. 3

 Names are composed of alphanumeric characters (letters and numbers), and _ (underscore)  Names may not start with a number  Names are case sensitive; eg, step and Step are different variables  Good Style : By convention, variable names begin with a lower-case letter. Short names are given to local variables, while longer more descriptive names to distant variables. Local: step, top, i, j, x, fahr, flag Distant: lookup_table_index, nameListHead 4

 C has three basic data types 1. Integer types: char, int 2. Floating point types: float, double 3. No type (an empty set): void  Different types have different properties:  Values they can represent  Operations that can be performed on them.  These types have finite precision and range.  There is a limit to the size of a number (min, max)  Floating point values have a limit to the number of significant figures.  Size of int may be qualified by long or short 5

 Basic building blocks of digital systems.  1 Bit = 1 B inary Dig it  Stores 1/0, True/False  1 Byte = 8 Bits 6

 The standard does not specify exact sizes. The amount of memory storage allocated for a particular type will be different on different systems.  ISO C standard requires only that  char is at least 8 bits  short is at least 16 bits  long is at least 32 bits  short <= int <= long  Range of permissible values of integers is defined in limits.h 7

 ISO C does not specify the size of floating point types, or even that their sizes are different.  Simply says: float <= double <= long double  Range of permissible values of integers is defined in float.h 8

signed  explicitly tells the compiler that the quantity is a signed integer (ie, the type may represent a negative number). unsigned  explicitly tells the compiler that the quantity is an unsigned integer (ie, cannot represent a negative number). This doubles the size of the max representable number. 9

 An integer value is represented by a sequence of bits: base 2 numbers.  For a signed integer, the most-significant-bit (MSB) is the sign bit.  If MSB is 0, the number is positive. If 1, the number is negative (using a 2’s complement binary representation).  An unsigned integer does not have a sign bit, and the MSB is part of the actual number – extending the maximum possible value by a factor of 2. Examples for 8-bit integers: > 7 (signed or unsigned) > -121 (signed, 2’s complement) > 135 (unsigned) 10

 signed char, -128 to 127, (-2 7 to 2 7 – 1)  unsigned char, 0 to 255, (2 8 – 1)  signed short, to 32767, (-2 15 to 2 15 – 1)  unsigned short, 0 to 65535, (2 16 – 1)  int, -2,147,483,648 to 2,147,483,647, (-2 31 to )  unsigned, 0 to 4,294,967,296, ( )  float, 1.2e-38 to 3.4e+38, (6 decimal digits precision)  double, 2.2e-308 to 1.8e+308, (15 decimal digits precision)  All are available in limit.h and float.h 11

const  indicates that a variable is intended to remain constant and should not be changed. const int DoesNotChange = 5; DoesNotChange = 6; /* will not compile */ 12

 There are many ways to represent the same integer value  Decimal, 12  Octal, 014  Hexadecimal, 0x0C  All represent the same binary value,  Consider the following 8-bit binary number binary: decimal: = 106 hex: 6 and 10 = 0x6A  a long constant is written like L or l  for an unsigned integer, add the suffix U or u  for an unsigned long integer, add the suffix UL or ul 13

 Specify by a decimal point and/or e or E e-3  Are type double by default  Can specify type float by appending an f 12.5f 7.F 14

 All data is stored as sets of bits.  2 main questions:  How do we represent letters?  Convert to a number: e.g. ‘A’ = 1; ‘B’ = 2 etc.  How do we communicate between different systems?  Use a standard conversion table. 15

 Standard method for representing characters as integers. 16

 Character constants are simply integers  To represent characters, C uses a lookup table  Each character has a unique integer code  ASCII table is most common code  Eg., Letter “R” we can represent by  Its ASCII integer code: 82 or 0122 or 0x52  Or, use character constant: ‘R’  The type of ‘R’ is an int not a char  Some ASCII constants are defined as escape sequences (for characters that are difficult or impossible to type) \n new line \t tab \b backspace \0 NUL character 17

 A string is a sequence of char s terminated by a NUL character, ‘\0’.  A string constant is written as e.g. “this is a string”  A string constant is automatically terminated by a NUL e.g. |t|h|i|s| |i|s| |a| |s|t|r|i|n|g|\0| WARNING!  Don’t confuse a character constant ‘X’ which is an int with a (null-terminated) string constant “X” which is an array of two char s: |X|\0|  Thus, sizeof(‘X’) is 4 (for Win32) and sizeof(“X”) is 2 18

 In printf(), use conversion specifiers, prefixed by % %c character - char %d decimal int %x hexadecimal int %o octal int %f %e %gfloat %f %e %gdouble  Conversion specifiers MUST match the type of the variable it corresponds to, else the program will be incorrect. 19

 See Text Section 2.5 and 13.1 for more information.  Refer to any C textbook for more complete discussion on printf() and its conversion specifiers. 20

 Extremely bad practice to have “magic numbers” in code. It may be difficult to see what the number stands for, and code changes become error-prone.  Use #define to define named constants, all in the one place #define ARRAY_LENGTH2500 #define BLOCK_SIZE4096 #define TRACK_SIZE 16*BLOCK_SIZE #define STRING“Hello World!\n”  Symbolic constants mean making changes of constants is easy and safe.  Example: tempf2.c 21

 Arithmetic operators are + plus - minus * multiply / divide = assignment % modulus (remainder after division)  The first 5 are valid for integer and floating-point types.  The % is valid only for integer types (including char ). 22

3.0 / 5.0 – equals / 5 – integer division truncates, equals 0 17 / 6 – equals 2 18 % 7 - equals 4 2*7 + 5*9 – equals : 59  Show prime.c 23

 Precedence and order of evaluation. eg, a + b * c  Order of evaluation from left to right.  *, / and % take precedence over + and -, so that a + b * c is the same as a + (b * c)  Precedence table exists, but use brackets () instead for safety!! 24

 Overflow (integers have finite range) y = x + 1; z = x * y;  Overflow and signed/unsigned  unsigned: modulo wrap around  signed: undefined behaviour  Divide by zero (integers or floats) z = x / y; 25

 Increment ++ and decrement -- ++a is equivalent to a = a + 1  Valid operators on integer or floating-point numbers.  Two forms: preincrement and postincrement int a=2, b, c; b = ++a; /* a=3 and b=3 */ c = a++; /* a=4 and c=3 */ 26

 Relational operators are > greater-than < less-than >= greater-than-or-equal-to <= less-than-or-equal-to == equal-to != not-equal-to  These operators are valid for integer and floating-point types.  Evaluate to 1 if TRUE, and 0 if FALSE 3.2 < 7 equals 1, and x != x equals 0 27

 Logical operators are && AND || OR ! NOT  && and || connect multiple conditional expressions.  ! negates a conditional expression (non-zero becomes 0, zero becomes 1). 28

int a=1, b=2, c=3, d=3; a < b && b < c && c < d /* FALSE */ a < b && b < c && c <= d /* TRUE */ (a < b && b < c) || c < d /* TRUE */ a && !b /* FALSE */  Show logical.c and leapyear.c  && and || are evaluated left-to-right and, once the result of TRUE or FALSE is known, evaluation stops – leaving the remaining expressions unevaluated. This is a useful feature, and leads to several common C idioms. i = 0; while(i < SIZE && array[i] != val) ++i; 29

 Bitwise operators are & bitwise AND | bitwise OR ^ bitwise XOR << left shift >> right shift ~ one’s complement (bitwise NOT)  Used to manipulate individual bits.  Details in later lecture, mention here to avoid confusion with logical operators. They cannot be used interchangeably.  & is not &&, | is not ||, >> is not “much-greater-than”. 30

 Assignment operators - for example, a += b; is equivalent to a = a + b; x *= y+1; is equivalent to x = x * (y+1);  Assignment also with other arithmetic operators : +,-,*,/,%  Show factorial.c 31

 One can write expressions with variables and constants of different types  The compiler performs implicit conversions on terms so each binary expression has same (larger) type.  (see Section 2.11 of lecture notes for more details) int x = 2, y = 1; float z = 5.4; double a; a = y + x*z;  Type promotion occurs stepwise for each binary expression 1. (tmp1) = x*z, the variable x is promoted to float; and result stored as a float 2. (tmp2) = y+(tmp1), the variable y is promoted to float 3. a = (tmp2), the implicit temporary is promoted to double and result stored in a 32