Question

I found This program is K&R The C programming book I’m new to C programming
I’m not getting why the variable d is used and the value of d is change to d=0
then d=1 between program.

Can anyone explain me what this program logic does I know this program is to replace
tabs with \t likewise.

#include <stdio.h>

int main()
{
    int c, d;

    while ( (c=getchar()) != EOF) {
        d = 0;
        if (c == '\\') {
            putchar('\\');
            putchar('\\');
            d = 1;
        }
        if (c == '\t') {
            putchar('\\');
            putchar('t');
            d = 1;
        }
        if (c == '\b') {
            putchar('\\');
            putchar('b');
            d = 1;
        }
        if (d == 0)
            putchar(c);        
    }
    return 0;
}

Answer 1

d is used as a flag variable (i.e. a variable that communicates a single binary condition, yes or no). The program sets d to one when it has interpreted the character c as a special character, and printed its slash-prefixed value. The program checks that d flag later on to determine if it has printed c or not. If the answer is no, then the program prints c; otherwise, it continues to the next iteration of the loop.