Control Flow and Functions

Programming Paradigms


  • You can use arrays to store multiple values of the same type
int a[6] = {2,4,7,1,2,4}

True/False Comparison

  • Traditionally, C did not have boolean types and just used ints

  • Comparisons will evaluate to 1 if they hold and 0 if they don’t

  • C99 introduced bool, which is defined in stdbool.h

Statements and compound statements

  • A statement in C is a single instruction terminated with a semicolon

        printf("Hello World!\n");
  • A compound statement is a set of statements surrounded by a pair of curly brackets {}

        printf("Hello ");
  • You can always replace a statement with a compound statement

  • C doesn’t care about formatting - but we need it!

Some Style Conventions

Compound statement:

  • Curly brackets on own lines

  • Indent body with 2/4 characters or a tab

Variable names:

  • Constants - all capitals: MAX, PI

  • #defines - all capitals: DEBUG

  • Normal variables

    • camel case: myAge

    • or snake cake: my_age


/* This is a comment
that can go on multiple lines */
// This is a single line comment

Iteration Statements

  • C’s iteration statements are used to create loops

  • A loop is a statement whose job is to repeatedly execute other statements: the loop body

  • In C, every loop has a controlling expression

  • Each time the loop body is executes, the expression is evaluated

  • If the expression is true (has a non zero value), the loop continues to execute

C provides three iteration statements

  • The while statement is used for loops whose controlling expression is tested before the loop body is executed

    while (a> 100) {...}
  • The do statement is used if the expression is tested after the loop body is executed

    do {...} while (a>100);
  • The for statement is convenient for loops that increment or decrement a counting variable

    for (a = 199; a>100; a = a-1) {...}

The break and continue statements

  • The break; statement causes the innermost enclosing loop (or switch) to be exited immediately

    for(n = 10; n<=10; n++){
        break; // or continue;
  • continue; causes the next iteration for the loop to begin (it does not apply to switch)

  • In the case of a while or do loop, the test part is executed immediately; in the case of a for loop, control first passes to the increment step

  • In a do while, continue will evaluate before looping round

The if-else statement

if (expr1){

Cascaded if statements

Allows testing of a series of conditions

if(boolean_expression1) {
/* Executes when the boolean expression 1 is true */
} else if( boolean_expression2) {
/* Executes when the boolean expression 2 is true */
} else if( boolean_expression3) {
/* Executes when the boolean expression 3 is true */
} else {
/* executes when the none of the above condition is true */

Static program checking

Using the wall (all warnings) flag on the compiler will make static checks e.g. for

#include <stdio.h>
int main(){
    int x = 0;
    if(x=0){ printf("x is 0\n");}
    return 0;

This doesn’t give the correct output as there is an assignment in the if statement, which equates to zero, which is equivalent to false, and so the if statement passes

The switch statement

This has the form

    case const-expr: statements
    case const-expr: statements
    default: statements

Warning: if there is no break statement, execution falls through - all the cases will be executed

Incrementing etc

C has many methods of incrementing and decrementing


x++: Evaluates to x, then adds one

++x: Adds one, then evaluates to x (so +1)

In a for loop it doesn’t matter which one you use



  • Functions encapsulate code in a convenient way

  • Analogous to methods in an O-O language

  • Functions can be declared before they are defined, as a function declaration:

    return-type function-name (paramenters);
  • E.g. to calculate base raise to the power n

    int power(int base, int n);
  • Often we put these in a header file (.h)


Functions can be defined anywhere in a program wile, if the declaration precedes use of the function

int power (int base, int n) {
    int p;
    for ( p=1; n>0; n--)
    return p;

Call by value

  • Function parameters in C are passed using a call by value semantic

    result = power(x,y);
  • Here when x and y are passed through to power(), the values of x & y are copied to the base and n variables in the function

  • A function cannot affect the value of its arguments

  • swap (x,y) example