Lecture 4 - Loops UniMAP EKT120 Sem 1 08/09

Outline Introduction While loops Do-while loops For loops Nested loops Three types of while loops Do-while loops For loops Nested loops UniMAP EKT120 Sem 1 08/09

Why Need Loops ? Suppose we want to add five numbers and find the average. From what you have learned so far, you could proceed as follows scanf(“%d %d %d %d %d”, &num1, &num2, &num3, &num4, &num5); sum = num1 + num2 + num3 + num4 + num5; average = sum / 5; If 100 numbers, 1000 numbers? UniMAP EKT120 Sem 1 08/09

Repetition (Loop) Used to control the flow of a program Loops are basically repetitions or iterations used to repeat a segment of code Three statements can be used to implement loops in C while statement do-while statement for statement while and for statements are similar in implementation, but have different syntax The do-while statement is different - the following code is executed at least once UniMAP EKT120 Sem 1 08/09

The while loop structure The general form of the while statement is: while (expression) statement; The example of pseudocode statement for while statement: While there are more items on my shopping list Purchase next item To avoid an infinite loop, make sure that the loop’s body contains statement (s) that assure that the exit condition i.e. the expression in the while statement will eventually be false. while loop repeated until condition becomes false UniMAP EKT120 Sem 1 08/09

Flowchart of a while statement UniMAP EKT120 Sem 1 08/09

The while loop structure-cont There are basically three types of while loops: Counter-controlled while loop Sentinel-controlled while loop Flag-controlled while loop UniMAP EKT120 Sem 1 08/09

Counter-Controlled while Loops Definite repetition: know how many times loop will execute Control variable used to count repetitions Example: int counter = 1; // declare and initialize while ( counter <= 10 ) // test condition { printf( "%d\n", counter ); ++counter; // update } Requirement: 1. Declare and initialize control variable value (or loop counter) 2. A condition that tests for the final value of the control variable (i.e., whether looping should continue) 3. Update control variable (incr/decr) UniMAP EKT120 Sem 1 08/09

Counter-Controlled while Loops Another example: int product = 2; while ( product <= 1000 ) product = 2 * product; declare and initialize test condition increment product <= 1000 product = 2 * product true false UniMAP EKT120 Sem 1 08/09

Sentinel-Controlled while Loops Indefinite repetition Used when number of repetitions not known and loop needs to input value repeatedly for each iteration Sentinel value indicates "end of data“ UniMAP EKT120 Sem 1 08/09

Sentinel-Controlled while Loops-cont Example: int number, count, sum; printf(“To stop enter -999. Enter positive numbers : " ); scanf(“%d”, &number); while (number != -999) { sum = sum + number; //find sum of numbers entered count++; //count how many numbers entered } printf(“\nThe sum of %d numbers is %d“, count, sum); Requirement: 1. Read control variable value before enter loop 2. A condition that tests control variable’s validity (i.e., whether looping should continue) 3. Read again control variable before end of loop Sentinel value UniMAP EKT120 Sem 1 08/09

Sentinel-Controlled while Loops-cont Another example: char choice; printf(“Continue?y-yes n-no :”); scanf(“%c”, &choice); while ( choice == ‘y’) { printf(“\nEnter grade point:”); scanf(“%f”, &grade_pt); if(grade_pt > 2.0) printf(“\nPass); } read control variable test condition read again UniMAP EKT120 Sem 1 08/09

Flag-Controlled while Loops Uses a boolean variable to control the loop. Loop exit when expression is evaluated to false. Set to false bool found = false; while (!found) { printf(“Enter number between 1 and 200:”); scanf(%d”, &guess); if ((guess>= 88) &&(guess <=128)) {found = true; printf(“\nBINGO!”);} } Requirement: 1. Set control variable to false before loop 2. A condition that tests control variable. If expr evaluated to true, loop continue 3. A decision structure to test value validity 4. Set control variable to true to indicate found test condition decision structure Set to true UniMAP EKT120 Sem 1 08/09

Example (while statement) (i) Fix any error you may find: while (c < 5) { product * = c; ++c; (ii) What is wrong with the statements which is supposed to calculate the sum of integers from 1 to 100? while (z > = 0) { sum + = z; UniMAP EKT120 Sem 1 08/09

Example (while statement) What does the following program print? UniMAP EKT120 Sem 1 08/09

Example (while statement) Write a program that calculates the sum of integer from 1 to 10. Use the while statement. UniMAP EKT120 Sem 1 08/09

UniMAP EKT120 Sem 1 08/09

The do-while Repetition Structure Condition for repetition tested after the body of the loop is performed All actions are performed at least once Expression can be written as count-controlled or sentinel-controlled Format: do { statement; } while ( condition );   UniMAP EKT120 Sem 1 08/09

Flowchart of a do-while structure true false action(s) condition UniMAP EKT120 Sem 1 08/09

The do-while Repetition Structure Example : prints the integers from 1 to 10 int counter = 1; do { printf( "%d ", counter ); } while (++counter <= 10); Another example: do{ printf(“\nEnter grade point:”); scanf(“%f”, &grade_pt); if(grade_pt > 2.0) printf(“\nPass); printf(“Continue?y-yes n-no :”); scanf(“%c”, &choice); }while ( choice == ‘y’) count-controlled   sentinel-controlled UniMAP EKT120 Sem 1 08/09

The do-while Repetition Structure To avoid an infinite loop, make sure that the loop body contains a statement that ultimately makes the expression false and assures that it exits Another example: int i = 0; do { printf("%d “, i); i = i + 5; } while (i <= 20); The output is: 0 5 10 15 20 UniMAP EKT120 Sem 1 08/09

The do-while Looping Structure (Example) Example: Consider the following two loops printf("%d “, i); printf("%d “, i); UniMAP EKT120 Sem 1 08/09

The do-while Looping Structure (Example) In (a), the while loop, produces nothing In (b) the do...while loop, outputs the number 11 UniMAP EKT120 Sem 1 08/09

The for Repetition Structure Format when using for loops for ( initialization; loopContinuationTest; increment statement) Use for loops when already know how many times to repeat Example: for(counter = 1; counter <= 10;counter++ ) printf( "%d\n", counter ); Prints the integers from one to ten   UniMAP EKT120 Sem 1 08/09

The for Repetition Structure Example: For loops and while loops are equivalent: expression1; while (expression2 ) { statement; expression3; } for ( expression1; expression2 ; expression3) UniMAP EKT120 Sem 1 08/09

The for Repetition Structure Initialization and increment Can be comma-separated lists Example: for (int i = 0, j = 0; j + i <= 10; j++, i++) printf( "%d\n", j + i ); UniMAP EKT120 Sem 1 08/09

Flowchart of a for statement UniMAP EKT120 Sem 1 08/09

Nested loop Loop within a loop Inner loop is performed first E.g. for(i=1;i<4;i++){ for(j=1;j<5;j++) printf(“%d”, j); printf(“\n”);} Output 1234 UniMAP EKT120 Sem 1 08/09

Nested loop Another example: Program find and print avg of three test scores, then asks whether want to continue do{ for(count=0; count <3;count++) { printf(“\nEnter test marks:”); scanf(“%d”, marks); total=total+marks; } avg=total/count; printf(“\nAverage:%5.2f”, avg); printf(“\nContinue?”); scanf(“%c”, &choice); }while(choice == ‘y’); UniMAP EKT120 Sem 1 08/09

Example (for statement) What is the program output for the following: UniMAP EKT120 Sem 1 08/09

Example (for statement) Write C codes for the following: Sum the odd integers between 1 and 99 using for statement. Assume the integer variable and sum have been defined. Write for statement that prints the following values in sequence: a) 1 2 3 4 5 6 7 b) 1 2 3 4 5 6 7 UniMAP EKT120 Sem 1 08/09

Example (for statement) What is the program output if number 3 and 4 are entered by user? UniMAP EKT120 Sem 1 08/09

Example (for statement) (i) Write a program that calculates and prints the sum of the even integers from 2 to 30 UniMAP EKT120 Sem 1 08/09

Example (for statement) (ii) Write a program that reads five integers. For each integer read, your program should print a line containing the amount of symbol *. UniMAP EKT120 Sem 1 08/09

End Lecture 4 - Loops Q & A! UniMAP EKT120 Sem 1 08/09