Make escapes visible

K&R Exercise 1-10: write a program that copies input to output, but makes three characters obvious: a tab prints as \t, a backspace as \b, and a backslash as \\. Those characters become visible; a real backslash cannot be mistaken for an escape.

You already copied input in Collapse runs of blanks. Same getchar loop; this time three characters print as two-character escapes instead of themselves.

Why these three

A tab is '\t'. On the screen it just makes a gap. A backspace is '\b'. You rarely see it; it moves the cursor back. A backslash is '\\'. If you copy it unchanged, then a following t looks exactly like a tab you had made visible.

So the program prints \t for a tab, \b for a backspace, and \\ for a backslash. Everything else is copied as-is.

Write the program

Create a file called make-escapes-visible.c:

#include <stdio.h>

int main(void)
{
    int c;

    while ((c = getchar()) != EOF) {
        if (c == '\t') {
            printf("\\t");
        } else if (c == '\b') {
            printf("\\b");
        } else if (c == '\\') {
            printf("\\\\");
        } else {
            putchar(c);
        }
    }
    return 0;
}

printf("\\t") prints a backslash and a t — two characters, not a tab. Same idea for \b and \\.

Compile and run

In Terminal, from the folder that holds make-escapes-visible.c:

gcc make-escapes-visible.c -o make-escapes-visible
./make-escapes-visible

Type hello, press Tab, type world, type a backslash, type path, press Return, then Ctrl+D on an empty line. You should see:

hello\tworld\\path

The tab became \t. The single backslash became \\.

If gcc is missing, finish Getting your macOS ready for C first.

Testing backspace

The Backspace key on macOS usually deletes the last character. It does not send '\b' into your program. Feed a real backspace byte with printf instead:

printf 'ab\bc\n' | ./make-escapes-visible

You should see:

ab\bc

That is a, b, a backspace, c, then a newline. The program prints \b for the backspace. In some terminals Ctrl+H also sends ASCII 8 — the same byte.

What you should take away

A copy loop can rewrite characters, not only pass them through. \t, \b, and \\ in the output are ordinary letters with a backslash in front — that is what makes tabs, backspaces, and backslashes unambiguous.

Next

→ Test the word count program — Exercise 1-11