UMBC CS 201, Fall 05

The while loop

    while (condition)
    {
        ...statements...
    }

not(condition) is the "termination condition"
...statements... is the "body"

Example problem

Write a program to add a list of numbers entered at the keyboard. The end of the input is indicated by entering 0 as a sentinel value.

Program

/***************************************** ** File: addlist.c ** Author: Sue Bogar ** Date: 2/2/98 ** Section: 101 ** E-Mail: bogar@cs.umbc.edu ** ** This program adds a list of numbers. The end of ** the input is indicated by entering 0 as a ** sentinel value. A priming read is necessary to ** gain entry into the loop. *****************************************/ #include <stdio.h> int main() { int value, total; /* Give directions to user */ printf("This program adds a list of numbers.\n"); printf("Signal end of list with a 0.\n"); /* Initialize total */ total = 0; /* Get the first integer from the user ** This is known as a "priming read" */ printf("Enter an integer, 0 to end : "); scanf("%d", &value); /* Continue to get integers from the user ** until a 0 is entered, accumulating into total */ while (value != 0) { total = total + value; printf("Enter an integer, 0 to end : "); scanf("%d", &value); } /* Print the total */ printf("The total is %d\n", total); return 0; }

Output

linux1[88] % a.out This program adds a list of numbers. Signal end of list with a 0. Enter an integer, 0 to end : 1 Enter an integer, 0 to end : 2 Enter an integer, 0 to end : 3 Enter an integer, 0 to end : 4 Enter an integer, 0 to end : 5 Enter an integer, 0 to end : 6 Enter an integer, 0 to end : 7 Enter an integer, 0 to end : 8 Enter an integer, 0 to end : 9 Enter an integer, 0 to end : 10 Enter an integer, 0 to end : 0 The total is 55 linux1[89] %

Sum of Digits

The Task

Read in a number from the keyboard and add up the digits it contains.

The Program

/***************************************** * File: digitsum.c * Author: S. Bogar * Date: 3/9/97 * Section 101 * E-Mail:bogar@cs.umbc.edu * * This program counts the number of digits in * a positive integer. The program depends on * the fact that the last digit of a integer n * is given by n % 10 and the number consisting * of all but the last digit is given by the * expression n / 10. *****************************************/ #include <stdio.h> int main() { int n, dsum; /* Print messages to and get input from user */ printf("Sum the digits in an integer.\n"); printf("Enter a positive integer: "); scanf("%d", &n); /* Initialize dsum to 0 */ dsum = 0; /* This loop repeatedly strips the last ** digit off of n and adds it to dsum until ** there are no more digits */ while (n > 0) { /* Add one's digit to dsum */ dsum += n % 10; /* strip off the one's digit */ n /= 10; } /* Print the sum of the digits */ printf("The sum of the digits is %d\n\n", dsum); return 0; }

The Sample Run

linux1[90] % a.out Sum the digits in an integer. Enter a positive integer: 342 The sum of the digits is 9 linux1[91] % a.out Sum the digits in an integer. Enter a positive integer: -12 The sum of the digits is 0 linux1[92] %

A More Robust Digitsum

The Task

Read in a number from the keyboard, making sure that it is a positive integer, and add up the digits it contains.

The Program

/***************************************** Program: robust.c Author: S. Bogar Date: 3/9/97 Section: 101 EMail: bogar@cs.umbc.edu A more robust version of digitsum.c that verifies that the user has input a POSTIVE integer *****************************************/ #include <stdio.h> int main() { /* Declare variables */ int n, dsum; /* Print purpose of program */ printf("Sum the digits in an integer.\n"); /* Get input from the user */ printf("Enter a positive integer: "); scanf("%d", &n); /* Error checking of user input : ** This loop assures that a positive ** integer was input by printing an error ** message and asking for new input. */ while (n <= 0) { printf("Positive integers only, please.\n"); printf("Enter a positive integer: "); scanf("%d", &n); } /* Initialize dsum to 0 */ dsum = 0; /* This loop adds the last digit to dsum ** and then strips away the last digit until ** no digits remain */ while (n > 0) { dsum += n % 10; n /= 10; } /* Print the sum of the digits */ printf("The sum of the digits is %d\n\n", dsum); return 0; }

The Sample Run

linux1[93] % a.out Sum the digits in an integer. Enter a positive integer: -12 Positive integers only, please. Enter a positive integer: 42 The sum of the digits is 6 linux1[94] % a.out Sum the digits in an integer. Enter a positive integer: -32 Positive integers only, please. Enter a positive integer: -0 Positive integers only, please. Enter a positive integer: -20 Positive integers only, please. Enter a positive integer: 21 The sum of the digits is 3 linux1[95] %

The Lesson

Robustness is a property of programs
Think about the assumptions you are making about your inputs.
Check your inputs to make sure they meet the assumptions.
Your projects WILL be tested for and graded on robustness!

Sunday, 21-Aug-2005 09:53:22 EDT