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 ex

Answer 1

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 
#include 

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