How can I change backspace to '\b'? - c++

Question

I see a question in The C Programming Language. It's like this:
Write a program to copy its input to its output, replacing each tab by \t , each backspace by \b , and each backslash by \ . This makes tabs and backspaces visible in an unambiguous way.
Then I write a program like this:
#include<stdio.h>
int main(void)
{
int c;
while((c=getchar())!=EOF)
{
if(c=='\t')
{
putchar('\\');
putchar('t');
}
if(c=='\b')
{
putchar('\\');
putchar('b');
}
if(c=='\\')
{
putchar('\\');
putchar('\\');
}
else{
putchar(c);
}
}
return 0;
}
But when I input backspace, I can't get '\b', so how can I get the output '\b'? I.e., I mean, how can I output backspace?

Answer 1

Keyboard input is preprocessed by the operating system. Most characters are fed directly as input to your program, but some are handled specially.
On UNIX-like systems, standard input is usually line-buffered. The system reads a whole line of text and doesn't pass it on to your program until you press Enter. While reading that line, the system processes Backspace itself; rather than adding a backspace character to the buffer, it erases the most recent character. Your program never sees the '\b' character.
To enter a literal backspace character that can be seen by your program, on a UNIX-like system you can precede it with Ctrl-V. And depending on your tty and terminal emulator settings, typing the Backspace key might transmit an ASCII DEL character rather than backspace. To make sure your program sees a backspace character, type Ctrl-V Ctrl-H.
On non-UNIX-like systems (most likely Windows), there's probably a way to do something similar, but I don't know what it is.
You can also run your program with input from a file. Getting a literal backspace character into the input file is left as an exercise (it depends on the workings of your text editor).

Answer 2

The getchar function buffers the user input and sends it to your program only when the user presses Enter. As suggested, you must use getch() to detect each keypress.
Your code must be:
#include<stdio.h>
#include<conio.h>
int main(void)
{
int c;
while ((c = getch()) != EOF)
{
if (c == '\t')
{
putchar('\\');
putchar('t');
}
else if (c == '\b')
{
putchar('\\');
putchar('b');
}
else if (c == '\\')
{
putchar('\\');
putchar('\\');
}
else if (c == '\r')
{
putchar('\n');
}
else{
putchar(c);
}
}
return 0;
}
Enter is detected as \r by getch, so you have to manually replace it by \n if you want to go to the next line.

Answer 3

The backspace is usually eaten up by the shell. Use Ctrl + H to test for backspace.
See 5. Delete and Backspace for more detail.

Answer 4

In your code, you have three ifs with only one else at the end... so your else is only tied to the last if. For tab and backspace, since they're not a backslash, the last if test is false, and the else block executes printing the original character. This means after printing '\t' you print a tab (and likely see some empty space in your output), and after printing '\b' you print a backspace which eats the 'b' or causes it to get overwritten by the next character. To fix it, either use:
if(c == '\t')
{
putchar('\\');
putchar('t');
}
else if(c=='\b') /* "else if" rather than "if" */
{
putchar('\\');
putchar('b');
}
else if(c=='\\')
{
putchar('\\');
putchar('\\');
}
else{
putchar(c);
}
...or simply rewrite it with a switch:
switch(c)
{
case '\t':
puts("\\t");
break;
case '\b':
puts("\\b");
break;
case '\\':
puts("\\\\");
break;
default:
putchar(c);
}