CP264 Notes: C Basics

• Basic syntax
• Data types, variables
• Operators and expressions
• Control flow
• Functions and program structure
• Pointers
• Arrays, string
• Structures, union, enumerate
• Input and output

Basic syntax

Comments

//    --  line comment, compiler ignores the line
/*
 *  …
 */    -- block comment, compiler ignores what in between

Keywords

C has 32 reserved words (keywords). Keywords are those words whose meaning is already defined by the compiler.

Keywords cannot be used as variable names, function names, or constant names.

Program Statement and Block

• Statement end with ;
• Block scoped by { } contains a sequence of statements

Program structure

• The statements in a C program are written in a logical sequence to perform a specific task.
• A C program contains one or more functions.
• Every program must contain one function named main.
• Execution of a C program begins at the main function.
• Functions - or their prototypes - must be declared before they can be called

C source code

[pre-processor statements]
[declare global variables and constants]
[declare function prototypes]

int main(int argc, char *args[]) {
    [program statements]
    return 0;
}
[implementation of functions]

Memory map in execution:

Data Types and Variables

Data Type defines how data is stored in memory

Primary data types are defined using keywords

char, int, float, double

Secondary data types

array, pointer, struct, union, enum, etc

Each data type has a size: the number bytes in memory

Each addressable memory cell holds 8 bits (1 byte) Primary data types (size of some data type is platform dependent): For example, int can be 2 bytes, 4 bytes, 8 bytes or larger

See C Data Type Sizes

The char type is stored as an integer using ASCII code. E.g. 'A' is coded as 65, 'a' as 97, '+' as 43.

See an ASCII Table

Variables

• A variable is a name used to refer to a memory location and its stored value of certain data type.
• a variable must have a type
• a variable has to be declared and initiated (or assigned) a value by assignment statement before it can be used
• variable declaration allows the compiler to assign memory space for the variable
• memory space of the variable is instanced when loading the program or at run time

Declaration

int a;
char c;
float f;

Assign values to variables:

a = 2;       // assignment statement, compiler assign the value 2 to memory space
c = 'a';     // compiler replaces 'a' by  97, then assigns it to the memory space
f = 1.41;

Declaration & initialization:

int a = 2;
int b =  3;
char c = 'a';
float f = 1.41;
int x = 9,  y = 3;

The sizeof Operator

• sizeof is a unary operator that calculates the sizes of data types
• Can be applied to all data types and variables
• Returns the number of bytes required to store a variable, data type, or expression

Syntax:

• size_t sizeof(type)
• size_t sizeof expression

(size_t is an unsigned integer)

Examples:

int n = 0;
sizeof(int): 4
sizeof n: 4
sizeof &n: 4

double d = 0;
sizeof(double): 8
sizeof d: 8
sizeof &d: 4

char c = 'A';
sizeof(char): 1
sizeof c: 1
sizeof &c: 4

int a[10];
sizeof(int [10]): 40
sizeof a: 40
sizeof &a: 4
sizeof a / sizeof a[0]: 10
sizeof a / sizeof *a: 10

typedef struct {
    int x;
    double y;
    char z;
    char *p;
} some_struct;
sizeof(some_struct): 24
sizeof q: 24
sizeof &q: 4

(What is 'strange' about the size of some_struct?)

Constants

Constant value: A fixed value used in program, like 3.1415926

#define PI 3.1415926
float r = 4;
float area = PI*r*r;
float cf = 2*PI*r;

The pre-processor replaces all occurrences of PI in the source code with 3.1415926

Constant variable: Variable with initialized value, but not allowed to change, i.e. read-only variable

const float pi = 3.1415926;
float r = 4;
float area = pi*r*r;
float cf = 2*pi*r;
pi = 3.14;   // this is not allowed by compiler

Type Conversion

• Variables cannot change types, but their values can.
• Conversion takes place when an expression has variables of different data types.
• When an expression is evaluated, the data type is promoted from lower to higher level type.
• The hierarchy of data types (high to low): double, float, long, int, short, char.

int b = 2;
float f1 = 1.5, f2 = 3.5;

f1 = b;          // implicit casting, f1 = 2.0,
f1 = (float) b;  // explicit casting
b = (int) f2;   //  b = 3

Operators and Expressions

The right size of an assignment statement must be an expression. Expressions consists of constants, variables, operators and parentheses. Example:

z = x + y
a = (b + 2) * (c + 4);

Be aware of operation issues

• Type issues

  • int / int gives int, e.g 5/2 gives 2
  • float / float gives float, e.g. 1.0/2.0 gives 0.5
  • float/int gives float, e.g. 1.0/2 gives 0.5

  • int i = 1;
    double x = 2;   // 2.0
    x = i / 2.0;    //  i  is casted to double before being multiplied, x = 0.5
    x = (double) i / 2.0;    // equivalent
    
    x = i / 2;             // i is not promoted to double because 2 is an int
    x = (double) (i / 2);  // so evaluates as this, x = 0.0
    

• Over-flow: values are out of range

  int a = INT_MAX;
  int b = a + 1;     // overflow
  

Precedence of Operators

See Operators in C

Flow Control

• Sequence
• Selection/ decision control
• Repetition

Sequence

Sequence: linear order. Lines are executed in the order given.

Selection

• if statement

  if (condition) {
      group1;
  }
  

• if-else statement

  if (condition) {
      block1;
  } else {
      block2;
  }
  

• if-else-if statement

  if (condition) {
      block1;
  } else if (condition) {
      block2;
  } else {
      block3;
  }
  

• switch statement

      char c;
      …
  
      switch (c) {
      case 'A':
          printf("Excellent");
          break;
      case 'B':
          printf("Good");
          break;
      case 'C':
          printf("OK");
          break;
      default:
          printf("What is your grade anyway?");
      }
  

  Another example:

  // Program to print the day of the week.
  
  #include <stdio.h>
  
  int main() {
      int day;
      printf("\n Enter any number from 1 to 7 : ");
      scanf("%d", &day);
  
      switch (day) {
      case 1: printf("\n SUNDAY"); break;
      case 2: printf("\n MONDAY"); break;
      case 3: printf("\n TUESDAY"); break;
      case 4: printf("\n WEDNESDAY"); break;
      case 5: printf("\n THURSDAY"); break;
      case 6: printf("\n FRIDAY"); break;
      case 7: printf("\n SATURDAY"); break;
      default: printf("\n Wrong Number");
      }
      return (0);
  }
  

Repetition (loop)

• for statement

  for (count = 1; count <=10; count++) {
      printf("%d",count);
  }
  

• while loop

  while (condition is true) {
      group;
  }
  

• do-while loop

  do {
      group;
  }  while (condition is true);
  

Example of prompt input

#include <stdio.h>

int main() {

    setbuf(stdout, NULL);

    char c;
    printf("Enter a character ('*' to quit): ");
    c = getchar();

    while (c != '*') {
        getchar(); // throw away enter key

        if (c >= 'a' && c <= 'z') {
            printf("%c %d %c\n", c, c, c - ' ');
        } else {
            printf("Not a lower case letter\n");
        }
        printf("Enter a character ('*' to quit): ");
        c = getchar();
    }
    return (0);
}

continue statement moves to the next iteration of a loop:

/* output the integers between 100 and 200, not divisible by 3*/
#include <stdio.h>

int main(void) {
    int n;
    for (n = 100; n <= 200; n++) {
        if (n % 3 == 0)
            continue;
        printf("%d ", n);
    }
    printf("\n");
}