how to check if the input is a number or not in C?

Question

In the main function of C:

void main(int argc, char **argv)
{
   // do something here
}

In the command line, we will type any number for example 1 or 2 as input, but it will be treated as char array for the parameter of argv, but how to make sure the input is a number, in case people typed hello or c?

Answer 1

Another way of doing it is by using isdigit function. Below is the code for it:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#define MAXINPUT 100
int main()
{
    char input[MAXINPUT] = "";
    int length,i; 

    scanf ("%s", input);
    length = strlen (input);
    for (i=0;i<length; i++)
        if (!isdigit(input[i]))
        {
            printf ("Entered input is not a number\n");
            exit(1);
        }
    printf ("Given input is a number\n");
}

Answer 2

You can use a function like strtol() which will convert a character array to a long.

It has a parameter which is a way to detect the first character that didn't convert properly. If this is anything other than the end of the string, then you have a problem.

See the following program for an example:

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

int main( int argc, char *argv[]) {
    int i;
    long val;
    char *next;

    // Process each argument given.

    for (i = 1; i < argc; i++) {
        // Get value with failure detection.

        val = strtol (argv[i], &next, 10);

        // Check for empty string and characters left after conversion.

        if ((next == argv[i]) || (*next != '\0')) {
            printf ("'%s' is not valid\n", argv[i]);
        } else {
            printf ("'%s' gives %ld\n", argv[i], val);
        }
    }

    return 0;
}

Running this, you can see it in operation:

pax> testprog hello "" 42 12.2 77x

'hello' is not valid
'' is not valid
'42' gives 42
'12.2' is not valid
'77x' is not valid

Answer 3

Using scanf is very easy, this is an example :

if (scanf("%d", &val_a_tester) == 1) {
    ... // it's an integer
}

Answer 4

A self-made solution:

bool isNumeric(const char *str) 
{
    while(*str != '\0')
    {
        if(*str < '0' || *str > '9')
            return false;
        str++;
    }
    return true;
}

Note that this solution should not be used in production-code, because it has severe limitations. But I like it for understanding C-Strings and ASCII.

Answer 5

Using fairly simple code:

int i;
int value;
int n;
char ch;

/* Skip i==0 because that will be the program name */
for (i=1; i<argc; i++) {
    n = sscanf(argv[i], "%d%c", &value, &ch);

    if (n != 1) {
        /* sscanf didn't find a number to convert, so it wasn't a number */
    }
    else {
        /* It was */
    }
}