How to check some characters before and after another character in C?

Question

C++ / C#

From the keyboard, enter a character string, in which, in addition to words, there can be integers
and real numbers, in which the fractional part is separated by a comma.
Extract all numbers from this string. Printing an abbreviated string. Hint:
to remove numbers, you need to shift all subsequent words to the left.

Here is my code:

#include <stdio.h>

int main(void) {

    char in[100];   //массив символів для вхідної строки
    char out[100];  //массив символів для вихідної строки

    int indexIN = 0,  //індекс массиву символів вхідної строки
        indexOUT = 0; //індекс массиву символів результуючої строки

    printf("Vvedit stroku:\n");

    gets_s(in);

    while (in[indexIN] != '\0')
    {
        if (in[indexIN] >= '0' && in[indexIN] <= '9')
            indexIN++;
        else
        {
            out[indexOUT] = in[indexIN];
            indexIN++;
            indexOUT++;
        }
    }

    out[indexOUT] = '\0';  //додаємо символ кінца строки в результуючий массив

    puts(out);

    return 0;
}

I was able to implement removal of integers, but when I try to remove a fractional number (for example 4.5), only numbers are removed, and the comma remains

. Help me please.

PS If there are other code options for the entire program, I would also be very grateful

Answer 1

Just look two characters ahead, if they are a comma and a number then remove them.

#include <stdio.h>
#include <stdbool.h>

static inline bool isDigit(char c) {
    return c >= '0' && c <= '9';
}

int main() {
    char in[100], out[100];
    int indexIN = 0, indexOUT = 0; 

    puts("Введите строку");
    fgets(in, 100, stdin);

    while(indexIN < sizeof(in) && in[indexIN]) {
        if (isDigit(in[indexIN])) {
            // Если два следующих символа - запятая и цифра
            if(indexIN < sizeof(in)-2 && in[indexIN+1] == ',' && isDigit(in[indexIN+2]))
                indexIN += 2;
            indexIN++;
        } else {
            out[indexOUT++] = in[indexIN++];
        }
    }

    out[indexOUT] = 0;
    puts(out);
}

Answer 2

I'm thinking of doing a character check before and after the comma, but don't know how. Help me please.

Check for the number of characters in[indexIN - 1]and in[indexIN + 1], while taking into account the overflow when indexIN == 0.
To begin with, you can put the check for a number into a macro so as not to duplicate the code:

#define IS_DIGIT(I) (in[(I)] >= '0' && in[(I)] <= '9')

Well, the condition in the if will then look like this:

if (IS_DIGIT(indexIN) || (in[indexIN] == ',' && indexIN != 0 && IS_DIGIT(indexIN - 1) && IS_DIGIT(indexIN + 1)))
    indexIN++;