1. CS1061: C Programming Lecture 8: Repetition A. O’Riordan, 2004

2. Iteration The ability to iterate, i.e. to execute a block of statements repeatedly, is crucial to prgramming languages like C. C provides three statements for “looping”: While statement (sentinel repetition) used when the number of iterations is not known Do..while statement variant of while loop For loop (counter-controlled repitition) used when you know the number of iterations in advance

3. While Loop Repetition structure Programmer specifies an action to be repeated while some Boolean condition remains true - example: While there are more items on my shopping list purchase next item and cross it off my list. while loop repeated until condition becomes false The general form is: while (expression) The is executed repeatedly as long as expression evaluates to true. The expression is evaluated before is executed for the first time.

4. Note on syntax The form of a statement is it’s syntax. When giving the syntax of a statement in C we use a simple consistent notation. A statement is represented as. This may be a single statement such as square = number * 2; or a compound statement such as { count = count + 1; printf(“Incrementing count\n”); } An expression evaluates to a Boolean, either true (non-zero) or false (zero).

5. while loop example /* while loop example */ #include int main(){ char letter; printf("Enter a letter (Z to terminate):"); letter = getchar(); getchar(); while( letter != 'Z'){ printf("Next letter is %c\n", letter); printf("Enter a letter (Z to terminate):"); letter = getchar(); getchar(); }

6. Factorial Example The factorial function can be computing with a loop Aide Memoire: The factorial of an n = nx(n-1)x(n-2)...x1, for example 5! = 5x4x3x2x1 = 120 /* compute factorial of integer n (i.e. n!)*/ int i = 0; int factorial = 1; /* 0! =1 */ while (i < n) { i++; factorial *= i; }

7. do...while loop The do/while repetition structure - similar to the while loop Condition for repetition tested after the body of the loop is performed All actions are performed at least once Format: do { } while ( );

8. do..while Example Here is an example code fragment: counter = 1; do { printf( "%d ", counter ); } while (++counter <= 10); Prints the integers from 1 to 10 Note: counter is (pre-)incremented inside the while expression - this is an important side-effect because otherwise the loop won’t terminate.

9. Infinite Loops A loop needs to have a terminating condition - either a counter reaches a certain value or a Boolean expression becomes true. If a loop never terminates it is called an infinite loop. Infinite loops will crash a program. Examples: while (true){ /* do something */ } do{ /* etc. etc. */ }while (true);

10. for loop A for loop is an iterative control structure that we can use when we want to execute some statement or statements a fixed number of times. The syntax of the if statement is now given: for (expression1; expression2; expression3) The for loop can be understood as a two step process. Step 1) Evaluate expression1 Step 2) Evaluate expression2 if expression2 is true the is executed and then expression3 is evaluated. Repeat step 2. if expression2 is false, jump to next statement after for loop.

11. Factorial Example Here is an example where we use a for loop to calculate the factorial of a number. (Recall that the factorial of a number (written n! for a number n) is the product of all number less than or equal to it, e.g. 4! = 4*3*2*1 = 24. In general, n! = n*(n-1)!, n> 1 and 1!=1.) /* calculate factorial of integer num */ int factorial = 1, i; for (i =1; i <= num; i++) factorial = factorial * i; /* result is factorial */

12. for Loop Initialisation An operator that is commonly used in for statements is the comma operator, which specifies that two expressions are executed one after the other. E.g.: int i,j; for (i=0, j=20; i <= 20; i++, j--) printf(“%d + %d = %d\n”,i, j, i+j); This will print the sum 20 twenty one times in a row. Note that j is decremented each time. Why? Note: The comma operator is used in the initialisation part and in expression2.