Question

I am working on some code that filters text before it is sent further into a program (this code removes everything but all alphanumeric characters and and underscores), the code itself works perfectly except for the fact that I cannot find a way to store the output of of it for use in other parts of the program, If i had to guess, this probably involves saving the stdout from putchar into a variable, but i cannot find much info for doing so online, if someone could point me in the right direction for this I would really appreciate it, thanks!

#include <stdio.h>
#include <string.h>
#include <ctype.h>
int main(void) {
    int i;
    char *p;
    char stg[] = "hello";
        for (p = &stg[0]; *p != '\0'; p++) {
           if (isalnum(*p) || *p == '_') {
           putchar (*p);
           }
        }
        putchar ('\n');
    return 0;
}

Answer 1

Perhaps I don’t understand your “need” to use putchar() while doing the filtering, but you can filter the input into an output array of chars to use however needed after the filtering as shown below.

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

int main(void) {
    int i;
    char *p;
    char stg[] = "hel123*^_lo";
    char output[200] = {0x00};
    int index = 0;


    p = stg;
    while( *p )
    {
        if (isalnum(*p) || *p == '_')
        {
            output[index++] = (char)putchar(*p);
        }       
        p++;
    }
    putchar('\n');
    printf("[%s]\n", output);
    return 0;
}

Output:

hel123_lo
[hel123_lo]

EDIT:

And if you want to just filter the string into an array without displaying the string using putchar() you’d do something like this:

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

int main(void) {
    int i;
    char *p;
    char stg[] = "hel123*^_lo";
    char output[200] = {0x00};
    int index = 0;


    p = stg;
    while( *p )
    {
        if (isalnum(*p) || *p == '_')
        {
            output[index++] = *p;
        }       
        p++;
    }

    printf("[%s]\n", output);
    return 0;
}

And what exactly are you trying to do with the output of the filtered text?