如何检查字符串是否包含 C 中的某个值

Question

void main
{
printf("\n Key in value: ");
    fgetchar();
    fgets(str , CHAR_SIZE, stdin);
    
    //Remove white space 
    remove_white_spaces(str);
    printf("%s",str);

while (str[i])
    {
        if(str[i] == '0' && str[i] == '1')
        {
            i++;
            continue;
        }
        else
        {
            printf("Wrong number input")
            break;
        }

I want to check whether the user has type in the correct input. For example, I have key in 0 01111110 10100000000000000000000. After removing the white space, the str input became 00111111010100000000000000000000. From this str, I want to check that the user has only key in 0 and 1. However, the str value does not go in the if condition but the else condition instead. What is wrong with the code?

Output: Key in value: 0 01111110 10100000000000000000000 00111111010100000000000000000000

Answer 1

First of all, you are checking that str[i] should be equal to 0 and equal to 1 – and that doesn't make any sense, because an element in the array can be only one value, 0 or 1; so, you should test if (str[i] == '0' || str[i] == '1') . And, before that, you should initialize i: int i = 0 .

Answer 2

initialize i: putting the equivalent of C's

int i = 0; 

in your prog lang before entering the while loop should do the job.

Answer 3

You could use strtok to extract your characters. Also there's a flaw in your logic. it should be if (str[i] == '0' || str[i] == '1' to check if the value is '0' OR '1'. Here's a sample implementation you could refer to:-

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define CHAR_SIZE 100

int main()
{
    char str[CHAR_SIZE];
    printf("\n Key in value: ");
    getchar();
    fgets(str, CHAR_SIZE, stdin);
    char *tok;
    tok = strtok(str, "\n");
    int i = 0;
    tok++; //skip the first character which is a space
    while (*tok != 0x00)
    {

        if (*tok <= 0x31 && *tok >= 0x30)
            tok++;

        else
        {
            printf("Wrong number input ==> %c \n", *tok);
            break;
        }
    }
}