How to check if a string is a number?

Question

I want to check if a string is a number with this code. I must check that all the chars in the string are integer, but the while returns always isDigit = 1. I don\'t know wh

Answer 1

In this part of your code:

if(tmp[j] > 57 && tmp[j] < 48)
  isDigit = 0;
else
  isDigit = 1;

Your if condition will always be false, resulting in isDigit always being set to 1. You are probably wanting:

if(tmp[j] > '9' || tmp[j] < '0')
  isDigit = 0;
else
  isDigit = 1;

But. this can be simplified to:

isDigit = isdigit(tmp[j]);

However, the logic of your loop seems kind of misguided:

int isDigit = 0;
int j=0;
while(j<strlen(tmp) && isDigit == 0){
  isDigit = isdigit(tmp[j]);
  j++;
}

As tmp is not a constant, it is uncertain whether the compiler will optimize the length calculation out of each iteration.

As @andlrc suggests in a comment, you can instead just check for digits, since the terminating NUL will fail the check anyway.

while (isdigit(tmp[j])) ++j;

Answer 2

#include <stdio.h>
#include <string.h>
char isNumber(char *text)
{
    int j;
    j = strlen(text);
    while(j--)
    {
        if(text[j] > 47 && text[j] < 58)
            continue;

        return 0;
    }
    return 1;
}
int main(){
    char tmp[16];
    scanf("%s", tmp);

    if(isNumber(tmp))
        return printf("is a number\n");

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

You can also check its stringfied value, which could also work with non Ascii

char isNumber(char *text)
{
    int j;
    j = strlen(text);
    while(j--)
    {
        if(text[j] >= '0' && text[j] <= '9')
            continue;

        return 0;
    }
    return 1;
}

Answer 3

Forget about ASCII code checks, use isdigit or isnumber (see man isnumber). The first function checks whether the character is 0–9, the second one also accepts various other number characters depending on the current locale.

There may even be better functions to do the check – the important lesson is that this is a bit more complex than it looks, because the precise definition of a “number string” depends on the particular locale and the string encoding.

Answer 4

The problem is that the result of your code "isDigit" is reflecting only the last digit test. As far as I understand your qustion, you want to return isDigit = 0 whenever you have any character that is not a number in your string. Following your logic, you should code it like this:

char tmp[16];
scanf("%s", tmp);

int isDigit = 0;
int j=0;
isDigit = 1;  /* Initialised it here */
while(j<strlen(tmp) && isDigit == 0){
  if(tmp[j] > 57 || tmp[j] < 48) /* changed it to OR || */
    isDigit = 0;
  j++;
}

To get a more understandable code, I'd also change the test:

if(tmp[j] > 57 || tmp[j] < 48) 

to the following:

if(tmp[j] > '9' || tmp[j] < '0')

Answer 5

More obvious and simple, thread safe example:

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

int main(int argc, char **argv)
{
    if (argc < 2){
        printf ("Dont' forget to pass arguments!\n");
        return(-1);
    }

    printf ("You have executed the program : %s\n", argv[0]);

    for(int i = 1; i < argc; i++){
        if(strcmp(argv[i],"--some_definite_parameter") == 0){
            printf("You have passed some definite parameter as an argument. And it is \"%s\".\n",argv[i]);
        }
        else if(strspn(argv[i], "0123456789") == strlen(argv[i])) {
            size_t big_digit = 0;
            sscanf(argv[i], "%zu%*c",&big_digit);
            printf("Your %d'nd argument contains only digits, and it is a number \"%zu\".\n",i,big_digit);
        }
        else if(strspn(argv[i], "0123456789abcdefghijklmnopqrstuvwxyz./") == strlen(argv[i]))
        {
            printf("%s - this string might contain digits, small letters and path symbols. It could be used for passing a file name or a path, for example.\n",argv[i]);
        }
        else if(strspn(argv[i], "ABCDEFGHIJKLMNOPQRSTUVWXYZ") == strlen(argv[i]))
        {
            printf("The string \"%s\" contains only capital letters.\n",argv[i]);
        }
    }
}

Answer 6

  if(tmp[j] >= '0' && tmp[j] <= '9') // should do the trick