Read in a number from the keyboard and
add up the digits it contains.
The Program
/*
File: digitsum.c
----------------
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>
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)
{
dsum += n % 10; /*Add last digit to dsum */
n /= 10; /*Strip away last digit */
}
/* Print the sum of the digits */
printf("The sum of the digits is %d\n", dsum);
}
The Sample Run
lassie% a.out
Sum the digits in an integer.
Enter a positive integer: 342
The sum of the digits is 9
lassie% a.out
Sum the digits in an integer.
Enter a positive integer: -12
The sum of the digits is 0
lassie%
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
Query the user for a positive integer.
*/
#include <stdio.h>
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", dsum);
}
The Sample Run
lassie% 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
lassie% 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: 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
lassie%
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.