Unable to confirm user input is a number using isdigit in c

Question

My code is attempting to;

• take user input from the command line;
• check if the input is a whole number;
• confirm positively if so;
• confirm negatively if not;
• however all numbers return a negative message - what's going wrong?

int main (int argc, string argv[])

{
    int k = atoi(argv[1]);

    if (isdigit(k))

    {
        printf("is a number\n");

    }

    else
    {
        printf("is not a number\n");
    }


}

Answer 1

This is a common newbie mistake. The isdigit() function, like all of it's issomething functions take a single char as input. But, argv[1] is a string, so feeding a string to this function will cause it to choke and cough up a seg fault.

Further, in this case, k is an int. The isdigit() function simply won't operate on an int.

There's also the problem that if there's a non-digit in the string argv[1], then it will not convert the string to an int. (I don't remember if it throws an error or just puts 0 in the int. You can look it up.)

The right way to do this is to apply isdigit() to each char in the string argv[1] string, say, by using a for loop. Only after confirming that each char really is a digit should you then use atoi to store the actual number in an int.

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