How to check a given string contains only number or not in C

Question

How to check a given string contains only number or not?

@H2CO3: That was an answer not a comment - you might post it as such. — Clifford, Jan 20 '13 at 08:38
@Clifford OK, if the community won't feel I'm repwhoring (I do, anyway... :P) — , Jan 20 '13 at 08:38
Just check manual of `ctype.h`. There are many more `is*` function — Shiplu Mokaddim, Jan 20 '13 at 08:39
Do you want to check that the string comprises only digits 0-9, or that it parses to produce a number with no whitespace etc? — Potatoswatter, Jan 20 '13 at 08:52
@Potatoswatter: What's the difference between those two choices? I agree that the question may not be clear, but I am not sure your counter-question is any clearer. — Clifford, Jan 20 '13 at 10:40
@Clifford `" -123.99e+22 "` can be parsed as a number, and contains only a number, but includes whitespace, punctuation, and an alphabetic character. — Potatoswatter, Jan 20 '13 at 12:43
@Potatoswatter: Good point. A clarification is indeed needed. — Clifford, Jan 20 '13 at 14:09
Possible duplicate of [Check if a string has only numbers in C?](https://stackoverflow.com/q/41776955/2908724) — bishop, Oct 05 '17 at 19:28

score 24 · Answer 1 · edited Feb 04 '19 at 20:13

24

You can use the isdigit() macro to check if a character is a number. Using this, you can easily write a function that checks a string for containing numbers only.

#include <ctype.h>

int digits_only(const char *s)
{
    while (*s) {
        if (isdigit(*s++) == 0) return 0;
    }

    return 1;
}

Subminiature stylistic sidenote: returns true for the empty string. You might or might not want this behavior.

edited Feb 04 '19 at 20:13

aioobe

413,195
112
811
826

answered Jan 20 '13 at 08:40

2

Considering the other answer so far, that was well worth posting after all ;-) – Clifford Jan 20 '13 at 10:38
@Clifford Yeah, pretty much :P – Jan 20 '13 at 10:39
2

If you don't want to use `ctype.h` and `isdigit()`, just test for `if(*s<'0' || *s>'9')`. The C-standard guarantees that `c - '0'` has the characters numeric value for any character `c` that is a digit. That is true even for Unicode character sets. – con-f-use Apr 19 '16 at 12:46
1

This function returns `true` for empty string. – viator May 24 '20 at 13:53
You could change the algo to consider floats too. For example 2343.34 should also return true. `int digits_only(const char *s) { while (*s) { if(*s == '.') { *s++; continue; } else if ((isdigit(*s++) == 0)) return 0; } return 1; }` – Vendetta V Dec 22 '20 at 16:08
`int digits_only(const char *s) { if(*s == '0') return 0; while (*s) { if(*s>='0' && *s<='9'){ s++; } else return 0; } return 1; }` – Max Oct 12 '22 at 17:04

score -4 · Answer 2 · edited Jul 08 '15 at 17:27

-4

#include<stdio.h>
void main()
{
    char str[50];
    int i,len = 0,count = 0;
    clrscr();
    printf("enter any string:: ");
    scanf("%s",str);
    len = strlen(str);
    for(i=0;i<len;i++)
    {
            if(str[i] >= 48 && str[i] <= 57)    
            {
                  count++;
            }
    }
    printf("%d outoff %d numbers in a string",count,len);
    getch();
}

edited Jul 08 '15 at 17:27

shivtej

633
9
18

answered Jan 20 '13 at 09:09

Programmingcampus.com

7

3

`void main()` in 2013? Really? "`str[i]>=48 && str[i]<=57`" isn't exactly the right way to do that part either. – Flexo Jan 20 '13 at 09:59
1

Too much superfluous (and compiler specific) code for others to criticise and down-vote see H2CO3's answer - short and to the point, with no unnecessary extras. Also this counts the number of ASCII digit characters (in a somewhat obfuscated manner); that is not what the question is about. Sure you could test `count != len`, but as a go/no-go test it is sufficient to stop on first non-digit character. The "magic numbers" 48 and 57 could more intuitively be replaced with the caharacter constants `'0'` and `'9'`. – Clifford Jan 20 '13 at 10:32
I know my dear friend but i post simple answer for the problem. And i hope this is the best solution to those who beginner of c programming – Programmingcampus.com Jan 20 '13 at 12:11

How to check a given string contains only number or not in C

2 Answers2