Checking whether input is number in C

Question

I need to check whether the input is number. My code looks something like this:

int input;

while ( scanf(" %s %d", string, &input) != EOF) {

if ( isNotANumber(input) ) {
    printf("Not a number"); }

doSomethingElse(input, string);
}

EDIT: I need to be accepting input and calling the function doSomethingElse(input) until the user enters EOF. isNotANumber is a mock function, I don't have that function, I'm asking how could I write it.

EDIT 2: Variable string needs to be a string, variable input needs to be an integer.

EDIT 3: I tried separating my code into this:

while (scanf(" %s", string) != EOF) {
    if (scanf("%d",&input) != 1) {
    printf("not a number");
}
doSomething();
}

But it stil doesn't work for input like "4a".

I think `isNotANumber(input)` cannot inspect `input` in the loop. — BLUEPIXY, Dec 03 '16 at 12:46
As `input` is defined `int` its value will always and ever be an integer, at least as long `scanf()` did not fail. — alk, Dec 03 '16 at 13:04
If you don't know that the input is a number, you cannot use scanf. (Or, rather, you can use scanf, but if scanf doesn't read the input because it isn't a number, then you're going to have to read it some other way, so doing the scanf was pointless.) You'll need to input the data some other way. Try `fread` and then check if it is an integer using `strtol`. BTW, do you mean "number", or "integer"? Be precise. — William Pursell, Dec 03 '16 at 13:11
Apologies for not clarifying clearly. I've edited my answer. — Kate, Dec 03 '16 at 13:23

score 1 · Accepted Answer · edited May 23 '17 at 12:13

1

For example, you can change it as follows.

#include <stdio.h>

#define doSomethingElse(input) do{ printf("your input is %d\n", input); }while(0)

int main(void){
    int input;
    int status;

    while ((status = scanf("%d", &input)) != EOF) {
        if ( status == 0 ) {
            printf("Not a number\n");
            while(getchar() != '\n'); //clear input
        }
        else {
            doSomethingElse(input);
        }
    }
}

However, this can not check input like 123.456. (accept 123)
So, It is recommended to input with fgets and check with strtol.

As already pointed out, like scanf(" %s %d", string, &input) can not check the input after the number.

So, For convenience, check backward input.

char string[32], ch;
int input;
int status;


while ((status = scanf("%31s %d%c", string, &input, &ch )) != EOF) {
    if ( status == 3 && ch == '\n') {
        doSomethingElse(input);
    }
    else {
        printf("Not a number\n");
        while(getchar() != '\n'); //clear input
    }
}

Example using fgets and strtol
(mystrtoi has reorganized the answer of chux. thanks)

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

#define doSomethingElse(input) do{ printf("your input is %d\n", input); }while(0)

int mystrtoi(const char *str, int *err) {
    char *endptr;
    *err = errno = 0;
    long l = strtol(str, &endptr, 0);
    if (errno == ERANGE || *endptr != '\0' || str == endptr) {
        *err = 1;
    }
    // Only needed if sizeof(int) < sizeof(long)
    if (l < INT_MIN || l > INT_MAX) {
        *err = 1;
    }
    return (int) l;
}


int main(void){
    char line[128];
    char string1[32], string2[128];
    int num, err;

    while (fgets(line, sizeof line, stdin)){
//      if(2 != sscanf(line, "%31s %31s", string1, string2)){// or use strtok to split
        if(2 != sscanf(line, "%31s %127[^\n]", string1, string2)){
            printf("invalid input\n");
            continue;
        }
        num = mystrtoi(string2, &err);
        if(err) {
            printf("Not a number\n");
        }
        else {
            doSomethingElse(num);
        }
    }
}

edited May 23 '17 at 12:13

Community

1
1

answered Dec 03 '16 at 13:10

BLUEPIXY

39,699
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Thank you. I've had to edit the code, how can I check my input now? – Kate Dec 03 '16 at 13:20
@Kate The situation does not change much. input fails if `status == 1`. – BLUEPIXY Dec 03 '16 at 13:25
It still accepts input as "string 4a" and that should also be seen as "Not a number.". How can I fix that? – Kate Dec 03 '16 at 13:29
I already recommended to input with `fgets` and check with `strtol`. – BLUEPIXY Dec 03 '16 at 13:31
I looked here https://www.tutorialspoint.com/c_standard_library/c_function_fgets.htm . But I still don't see how I can use it in my case. Help is greatly appreciated. – Kate Dec 03 '16 at 13:36
@Kate validate using `strtol` [string convert to int sample](http://stackoverflow.com/a/22866001/971127) – BLUEPIXY Dec 03 '16 at 13:46
The problem is that my scanf functions looks like this: scanf(" %s %d", string, &input). And now I don't know how to adjust it. – Kate Dec 03 '16 at 13:52
@Kate Do you necessarily have to use `scanf` ? – BLUEPIXY Dec 03 '16 at 14:04
Not necessarily. But I don't know how to use fgets. I've separated my code into this: `while (scanf("%s", string) != EOF) { if (scanf("%d", input) != 1) { printf("Not an integer");} doSomething();}` But it still doesn't work properly for input like 4a. – Kate Dec 03 '16 at 14:56
@Kate Sample was added. – BLUEPIXY Dec 03 '16 at 15:24

artm · Answer 2 · 2016-12-03T12:49:35.940

0

while ( scanf("%d", input) != EOF)

Your scanf code has two problems:

scanf returns the number of successfully read item, not EOF. Here you want to check if scanf has successfully read one integer input
scanf expects the address of the variable

You should re-write that line as:

while ( scanf("%d", &input) == 1 )

edited Dec 03 '16 at 12:49

answered Dec 03 '16 at 12:45

artm

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if your `scanf` is successful, then `input` is already a number – artm Dec 03 '16 at 12:56

Checking whether input is number in C

2 Answers2