LAB 6: While and do-while Loops

Transcription

1 Name: Joe Bloggs LAB 6: While and do-while Loops Exercises 1 while loop /* Program name: lab6ex1.c * Purpose : This program demonstrates use of while loop int num = 0; while( num!= -1 ) printf("please enter your number (or -1 to quit): "); scanf("%d", & num); printf("the number your entered was %d\n", num); Comments: The while loop iterates or repeats while the condition is true. When the condition becomes false, program control passes to the line after the loop code. If the condition is initially false the statement in the loop will never execute. So in this example, if the user enters -1 as his/her first input, then the statements within the loop are never executed. The loop is a conditional loop in that the program will continue looping until the user enters -1, i.e. the condition number not equal to -1 becomes false.

2 Exercises 2 do-while loop /* Program name: lab6ex2.c * Purpose : This program demonstrates use of do-while loop int num = 0; do printf("please enter your number (or -1 to quit): "); scanf("%d", & num); printf("the number your entered was %d\n", num); while( num!= -1 ); /* Program name: lab6ex2b.c * Purpose : This program demonstrates use of do-while loop int num = 0; do printf("please enter your number (or -1 to quit): "); scanf("%d", & num); if( number!= -1 ) printf("the number your entered was %d\n", num); while( num!= -1 ); Comments: The do-while loop iterates or repeats also while the condition is true. However, unlike the while loop, the do - while loop tests the loop condition at the bottom of the loop. This means that a do - while loop is always executed at least once. In the first sample solution example, the user is asked to input a group of numbers, then the number is input and then displayed even if the user enters -1. In the second example, there is an additional if statement placed within the loop so that when the user enters -1 this if not displayed.

3 Exercises 3 Select a looping construct /* Program name: lab6ex3a.c * Date: 27 th October 2005 * Purpose : This program demonstrates use of while loop int radius = 0; /* radius of the circle float area = 0; /* area of the circle const float PI = f; /* Declare constant PI /* Get User Input printf("please enter the radius of the circle as a\n" "whole number in the range 1 to 10 metres: "); scanf("%d", & radius); /* Validate input data while( radius < 1 radius > 10) printf("error: INVALID RADIUS VALUE \n"); printf("please try again using a whole number in the range 1 to 10 metres: "); scanf("%d", & radius); /* Calculate the area of the circle area = PI * radius * radius; /* Display the result printf("the area of a circle with radius %d metres is %.2f metres squared\n", radius, area); /* Program name: lab6ex3b.c * Purpose : This program demonstrates use of do-while loop int radius = 0; /* radius of the circle float area = 0; /* area of the circle const float PI = f; /* Declare constant PI /* Get User Input do printf("please enter the radius of the circle as a\n" "whole number in the range 1 to 10 metres: "); scanf("%d", & radius); /* Validate input data if( radius < 1 radius > 10) printf("\nerror: INVALID RADIUS VALUE \n"); dt089-2-soln06.doc Page 6

4 while( radius < 1 radius > 10); area = PI * radius * radius; printf("\nthe area of a circle with radius %d metres is %.2f metres squared\n", radius, area); Test you program as follows: TEST ÈXPECTED CASE RESULT 1. radius < Display ERROR 1m message 0_ Ask user to reenter the correct 2. radius > 10m _11_ 3. radius between 1m and 10m 5 data Display ERROR message Ask user to reenter the correct data Display the area of the circle ACTUAL RESULT please enter the radius of the circle as a whole number in the range 1m to 10m: 0 ERROR: INVALID RADIUS VALUE Please try again using a whole number in the range 1 to 10 metres: please enter the radius of the circle as a whole number in the range 1m to 10m: 11 ERROR: INVALID RADIUS VALUE Please try again using a whole number in the range 1 to 10 metres: please enter the radius of the circle as a whole number in the range 1m to 10m: 5 The area of a circle with radius 5 metres is metres squared Press any key to continue Comments: Exercise 3 could be implemented using a while loop or a do-while loop. However if a do-while loop is used, an additional if statement was required within the loop to make the error message conditionally displayed, this is not as efficient as the while loop implementation. dt089-2-soln06.doc Page 6

5 Exercises 4 Select a looping construct /* Program name: lab6ex3.c * Purpose : This program demonstrates use of do-while loop float litres = 0; /* litres used float distance = 0; /* distance travelled float km_per_litre = 0; /* distance per litre int number_of_tankfuls = 0; /* number of tankfuls float sum = 0; /* sum of distance per litre values float average = 0; /* average distance per litre /* Get User Input printf("\nfor tank%d", number_of_tankfuls+1); printf("\nplease enter the litres used (-1 to end): "); scanf("%f", & litres); while( litres!= -1) printf("please enter the distance travelled in km: "); scanf("%f", & distance); /* Calculate the km / litre if( litres > 0 && distance > 0 ) /* protect against divide by zero error km_per_litre = distance / litres; /* sum the distance per litre values sum += km_per_litre; /* increment number of tanks value number_of_tankfuls ++; /* Display the result printf("\nthe kilometre/litre for this tank was %.2f km/l\n", km_per_litre); else printf("error invalid input - values must be a positive\ntry again\n"); /* Next Tank - Start Get Input printf("\nfor tank%d", number_of_tankfuls+1); printf("\nplease enter the litres used (-1 to end): "); scanf("%f", & litres); if( number_of_tankfuls > 0 ) /* Calculate average average = sum / number_of_tankfuls; /* Display the overall result printf("\nthe average kilometre/litre was %.2f km/l\n\n", average);

6 Log your work Software Development - Laboratory Print out your code and submit with this sheet when all questions are completed. Questions 1) What is the purpose of a loop in C programming? Repetition/Loop -> A series of steps is repeated, i.e. loops allow a programmer to execute one or more program statements repeatedly. In a conditional loop, the steps are repeated as longs as a certain condition is true. In a counting loop the steps are repeated for a predetermined number of times 2) Describe the main differences in the while and do-while loops The while loop (and the for loop), check the loop condition at the top of the loop, the do - while loop tests the loop condition at the bottom of the loop. This means that a do - while loop is always executed at least once whereas the while loop is executed zero or more times, i.e. if the condition is initially false the statement in the while loop will never execute. 3) Complete the C code for the following flowchart: BEGIN OUTPUT<- "Input a non-negative number to quit" int x; true Input-> x x = -1 is X<0? false printf( "Input a non-negative number to quit: "); x = -1; while( x < 0 ) scanf("%d", &x); OUTPUT<- "Thank you and goodbye!! END printf("thank you and goodbye!!\n");

7 4) State what is displayed on the console screen when the following C code segment is executed: int i=0, sum=0; while( i!= 10 ) sum = sum + i; i = i + 2; printf("sum = %d\n", sum); this code sums all the even numbers between 0 and 8 (10 not included) so the output displayed is 20 Start of loop i=0, sum=0 1 st loop -> sum = sum+i =0+0 = 0 i = i + 2; = 0+2 = 2 2 nd loop -> sum = sum+i =0+2 = 2 i = i + 2; = 2+2 = 4 3 rd loop -> sum = sum+i =4+2=6 i = i + 2; = 4+2 = 6 4 th loop -> sum = sum+i =6+6 = 12 i = i + 2; = 6+2 = 8 5 th loop -> sum = sum+i =12+8 = 20 i = i + 2; = 8+2 = 10 now i=10 -> condition ( i!= 10 ) is FALSE -> exit loop and display sum as 20 5) State what is displayed on the console screen when the following C code segment is executed: int i=0; do i = i + 2; printf("%d,", i ); while( i <= 10 ); The output displayed is 2,4,6,8,10, Start of loop i=0 1 st loop -> i = i + 2; = 0+2 = 2 display 2 2 nd loop -> i = i + 2; = 2+2 = 4 display 4 3 rd loop -> i = i + 2; = 4+2 = 6 display 6 4 th loop -> i = i + 2; = 6+2 = 8 display 8 5 th loop -> i = i + 2; = 8+2 = 10 display 10 note: loop condition still true 6 th loop -> i = i + 2; = 10+2 =12 display 12 now loop condition false note: i=10 -> condition ( i <= 10 ) is TRUE -> loop again so 12 also displayed!

8 6) State what is displayed on the console screen when the following C code segment is executed and the user enters 1234: int number; printf( "Please enter a number "); scanf("%d", & number); do printf("%d", number % 10 ); number = number / 10; while( number!= 0 ); this code perform modulus operator on number each iteration iteration1 number MOD 10 = 1234 MOD 10 = 4 number divided by 10 => 123 iteration2 number MOD 10 = 123 MOD 10 = 3 number divided by 10 => 12 iteration3 number MOD 10 = 12 MOD 10 = 2 number divided by 10 => 1 iteration4 number MOD 10 = 1 MOD 10 = 1 number divided by 10 => 0 -> exit loop Therefore the output is 4321 dt089-2-soln06.doc Page 8