Check for Character instead of Integer

Question

my program adds numbers entered by the user. It runs and works great until a character is entered instead of an integer. Is there a simple way to make sure only integers are entered from the keyboard?

Here is my code.

#include<stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
   int n, sum = 0, i, TotalOfNumbers;

   printf("Enter the number of integers you want to add\n"); 
   scanf("%d", &n); 

   printf("Enter %d integers\n",n); 

   for (i = 1; i <= n; i++)
   {
      scanf("%d",&TotalOfNumbers);
      sum = sum + TotalOfNumbers;
   }

   printf("Sum of entered integers = %d\n",sum); 

   return 0;
}

Check this question http://stackoverflow.com/questions/2023079/check-if-a-value-from-scanf-is-a-number — C. E. Gesser, Feb 19 '14 at 17:52
read the value in as a character string instead of an int, and then validate it. Convert to int if valid. — OldProgrammer, Feb 19 '14 at 17:52
Check the results of `scanf()` It will tell you how many requested parameters were *successfully* parsed. And since you're only requesting one.. well... — WhozCraig, Feb 19 '14 at 18:12

score 2 · Accepted Answer · answered Feb 19 '14 at 18:13

You need to check the return value of scanf. If the input was a valid number, it will return 1. If the input was not a valid number, it will return something else. Here is your code modified to put the checks in.

#include<stdio.h>
#include <stdlib.h>

int get_number()
{
   int num;
   int ret;

   ret = scanf("%d", &num);

   if (ret != 1) {
      printf("bad number\n");
      exit(EXIT_FAILURE);
   }

   return num;
}

int main(int argc, char **argv)
{
   int n, sum = 0, i, TotalOfNumbers;

   printf("Enter the number of integers you want to add\n"); 
   n = get_number();

   printf("Enter %d integers\n",n); 

   for (i = 1; i <= n; i++)
   {
      TotalOfNumbers = get_number();
      sum = sum + TotalOfNumbers;
   }

   printf("Sum of entered integers = %d\n",sum); 

   return 0;
}

Thanks for taking time to answer my question. Greatly appreciated. — Derk, Feb 19 '14 at 18:34

Joatin · Answer 2 · 2014-02-19T18:12:14.747

1

Check the ferror state on the input stream

scanf("%d",&TotalOfNumbers);
if(!ferror(stdin)){
  sum = sum + TotalOfNumbers;
}

edited Feb 19 '14 at 18:12

answered Feb 19 '14 at 17:58

Joatin

106
3

I won't downvote, but there is no `try..catch` construct in C. – juan.facorro Feb 19 '14 at 18:00
1

Lol, i see now that i've messed up. Ill delete this and post a new solution for you ;) Your suposed to use ferror – Joatin Feb 19 '14 at 18:04

score 1 · Answer 3 · answered Feb 19 '14 at 19:31

In addition to posted answer, there options not general as posted, but quicker. First if you want to skip some final set of characters.In following example all letters,! and + will be skiped

int n;
scanf("%*[a-zA-Z!+]%d",&n);
printf("\n%d",n);

for input

 weweqewqQQWWW!!!!+++3332

the output is

Next option is to use buffer wich allowed to read everything untill number is met, and then read the number. The disadvantage is that buffer size is limited

char buf[25];
int n;

scanf("%[^0-9]%d",buf,&n);
printf("\n%d",n);

For input

fgfuf@$@^^@^@@4565

Output

Check for Character instead of Integer

3 Answers3