CODEWITHSUNDEEP
c
Cody.L
Cody.L

Reputation: 31

Input validation on int

I'm having trouble with the following code, when I enter a letter isdigit won't detect it and I get the error message "Nothing entered"

    int age;
    char buffer[100] = "";
    printf("Enter your age: ");
    fgets(buffer, 100, stdin);
    age = atoi(buffer);
    printf("%d\n",age);


    if (age == NULL)
    {
        printf("Nothing entered\n");
    }

    else if (isdigit(age) == 0)
    {
        printf("Not a number\n");
    }

    else if (age <1 || age > 110)
    {
        printf("Out of range\n");
    }

    else
    {
        printf("Thank you\n");
    }

Upvotes: 0

Views: 180

Answers (3)

R Sahu
R Sahu

Reputation: 206567

Instead of

age = atoi(buffer);
printf("%d\n",age);


if (age == NULL)
{
    printf("Nothing entered\n");
}

Use

if ( sscanf(buffer, "%d", &age) != 1 )
{
    printf("Nothing entered\n");
}
else
{
    printf("%d entered\n", age);
}

Upvotes: 2

pm100
pm100

Reputation: 50110

explanation of the behavior

atoi("x")

will return 0. This is becuase atoi stops on the first non digit

if (age == NULL)

is then true because NULL is defined as 0 (basically)

You should check buffer first and then do atoi

Upvotes: 0

The Paramagnetic Croissant
The Paramagnetic Croissant

Reputation: 9343

atoi is the wrong function to use when you have untrusted/potentially erroneous input (i. e. almost always).

Use strtol() (man page) instead, which allows for extensive error checking.

Also, do check the return value of fgets().

char buf[100];
if (fgets(buf, sizeof buf, stdin)) {
    char *end;
    errno = 0;
    long n = strtol(buf, &end, 10);

    // check for errors
    if (n == 0 || n == LONG_MIN || n == LONG_MAX) {
        if (errno != 0) {
            printf("Erroneous input entered\n");
        } else {
            // process valid input
        }
    } else {
        // process valid input
    }
} else {
    printf("nothing was entered\n");
}

By the way, it doesn't make sense to compare an int with NULL (which is a pointer). If this doesn't trigger at least a compiler warning (better yet, a hard error), you need to boost your compiler warning level or change to a modern compiler.

Upvotes: 2

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