Why is "\b" not clearing one character at left? - c

Question

I'm trying to print 01-99 in words and I am somewhat successful.
Here is the source code:
#include <stdio.h>
#include <string.h>
int main(int argc, char *argv[]) {
char a, b;
char *digit1;
char *digit2;
// get digit character by character
scanf("%c%c", &a, &b);
switch(a) {
case '1':
switch(b) {
case '0':
digit1 = "ten";
break;
case '1':
digit1 = "eleven";
break;
case '2':
digit1 = "twelve";
break;
case '3':
digit1 = "thirteen";
break;
case '4':
digit1 = "fourteen";
break;
case '5':
digit1 = "fifteen";
break;
case '6':
digit1 = "sixteen";
break;
case '7':
digit1 = "seventeen";
break;
case '8':
digit1 = "eighteen";
break;
case '9':
digit1 = "nineteen";
break;
default:
digit1 = "";
break;
}
break;
case '2':
digit1 = "twenty-";
break;
case '3':
digit1 = "thirty-";
break;
case '4':
digit1 = "forty-";
break;
case '5':
digit1 = "fifty-";
break;
case '6':
digit1 = "sixty-";
break;
case '7':
digit1 = "seventy-";
break;
case '8':
digit1 = "eighty-";
break;
case '9':
digit1 = "ninty-";
break;
default:
digit1 = "";
}
switch(b) {
case '1':
digit2 = "one";
break;
case '2':
digit2 = "two";
break;
case '3':
digit2 = "three";
break;
case '4':
digit2 = "four";
break;
case '5':
digit2 = "five";
break;
case '6':
digit2 = "six";
break;
case '7':
digit2 = "seven";
break;
case '8':
digit2 = "eight";
break;
case '9':
digit2 = "nine";
break;
case '0':
digit2 = "\b";
break;
default:
digit2 = strcpy(digit1, "\b");
}
if (a != 1) {
printf("%s%s\n", digit1, digit2);
}
else {
printf("%s\n", digit1);
}
return 0;
}
I'm successful in printing from 20-99 until now. But there is an error. If I enter any of 20, 30, 40... - is not removed as it should be cause I used \b to remove that.

Answer 1

You could replace
digit2 = "\b";
by
if (digit1[0] != '\0')
digit1[strlen(digit1) - 1] = '\0';
which effectively removes the trailing character from digit1.
Since this works completely on the level of your strings, you avoid relying on device-specific behaviour as \b would do.

Answer 2

This is a little outside of the C standard as it concerns the behavior of the input/output environment, but usually the backspace character just repositions the cursor. To back up and erase, try the sequence BACKSPACE SPACE BACKSPACE.
This is the behavior of the DEC VT100 terminal, which is pretty ubiquitous (also called ANSI terminal, same is used by XTERM-derived terminals). As #jxh points out, adding terminal-specific sequences is only appropriate if the output device is a terminal. If you're writing to a file, this will not erase, but simply add these additional bytes to the output stream. However, these delayed sequences would still be interpreted correctly if written to the terminal at a later time, perhaps using cat. You could use the POSIX function isatty(3) to get a good guess at whether these sequences will be interpreted.
It may be possible to unwrite a character in a buffered write by adjusting the file position.
FILE *myfile = /*initialization*/;
/* ... */
fsetpos (myfile, -1, SEEK_CUR);
Please also consider MvG's valuable comments for more ways these methods may fail. (It really is shaky territory, being outside of the standard. brrrrr. :)

Answer 3

The escape code "\b" emits a terminal-control character; that is, it writes an additional control character to the output stream, it does not remove previous characters from the output stream: On most systems "hellO\bo" produces a string literal of 8 bytes equivalent to { 'h', 'e', 'l', 'l', 'O', '\b', 'o', '\0' };
It is important, though, to remember that this is a terminal-control character. What it does is terminal dependent: some terminals move the cursor left stopping at column 0, some move the cursor left with wrap, some erase the character the cursor arrives at, others don't.
But further more, if you are viewing the data other than through a terminal, it is just a byte. E.g. the code
#include <stdio.h>
int main(int argc, char* argv[]) {
printf("Hello!\b\n");
}
will generate a sequence of ascii values that, if viewed with linux "cat" might display "Hello", with Windows/DOS "more" command, "Hello!" (DOS is a non-destructive backspace) and if loaded in notepad produce "Hello!" followed by what looks like a wing-ding.
Don't confuse terminal control with io-stream control.