Chapter 3 - Control Flow 2

Answer to Exercise 3-2, page 60
Solution by Paul Griffiths
Write a function escape(s,t) that converts characters like newline and tab into visible escape sequences like \n and \t as it copies the string t to s . Use a switch . Write a function for the other direction as well, converting escape sequences into the real characters.

 

/*
 
  EX3_2.C
  =======
 
  Suggested solution to Exercise 3-2

*/

#include <stdio.h>

void escape(char * s, char * t);
void unescape(char * s, char * t);

int main(void) {
    char text1[50] = "\aHello,\n\tWorld! Mistakee\b was \"Extra 'e'\"!\n";
    char text2[51];
   
    printf("Original string:\n%s\n", text1);
   
    escape(text2, text1);
    printf("Escaped string:\n%s\n", text2);
   
    unescape(text1, text2);
    printf("Unescaped string:\n%s\n", text1);
   
    return 0;
}

/*  Copies string t to string s, converting special
    characters into their appropriate escape sequences.
    The "complete set of escape sequences" found in
    K&R Chapter 2 is used, with the exception of:
   
    \? \' \ooo \xhh
   
    as these can be typed directly into the source code,
    (i.e. without using the escape sequences themselves)
    and translating them is therefore ambiguous.          */

void escape(char * s, char * t) {
    int i, j;
    i = j = 0;
   
    while ( t[i] ) {
       
        /*  Translate the special character, if we have one  */
       
        switch( t[i] ) {
        case '\n':
            s[j++] = '\\';
            s[j] = 'n';
            break;
           
        case '\t':
            s[j++] = '\\';
            s[j] = 't';
            break;
           
        case '\a':
            s[j++] = '\\';
            s[j] = 'a';
            break;
           
        case '\b':
            s[j++] = '\\';
            s[j] = 'b';
            break;
           
        case '\f':
            s[j++] = '\\';
            s[j] = 'f';
            break;
           
        case '\r':
            s[j++] = '\\';
            s[j] = 'r';
            break;
           
        case '\v':
            s[j++] = '\\';
            s[j] = 'v';
            break;
           
        case '\\':
            s[j++] = '\\';
            s[j] = '\\';
            break;
           
        case '\"':
            s[j++] = '\\';
            s[j] = '\"';
            break;
           
        default:
           
            /*  This is not a special character, so just copy it  */
           
            s[j] = t[i];
            break;
        }
        ++i;
        ++j;
    }
    s[j] = t[i];    /*  Don't forget the null character  */
}

/*  Copies string t to string s, converting escape sequences
    into their appropriate special characters. See the comment
    for escape() for remarks regarding which escape sequences
    are translated.                                             */

void unescape(char * s, char * t) {
    int i, j;
    i = j = 0;
   
    while ( t[i] ) {
        switch ( t[i] ) {
        case '\\':
           
            /*  We've found an escape sequence, so translate it  */
           
            switch( t[++i] ) {
            case 'n':
                s[j] = '\n';
                break;
               
            case 't':
                s[j] = '\t';
                break;
               
            case 'a':
                s[j] = '\a';
                break;
               
            case 'b':
                s[j] = '\b';
                break;
               
            case 'f':
                s[j] = '\f';
                break;
               
            case 'r':
                s[j] = '\r';
                break;
               
            case 'v':
                s[j] = '\v';
                break;
               
            case '\\':
                s[j] = '\\';
                break;
               
            case '\"':
                s[j] = '\"';
                break;
               
            default:
               
                /*  We don't translate this escape
                    sequence, so just copy it verbatim  */
               
                s[j++] = '\\';
                s[j] = t[i];
            }
            break;
           
        default:
           
            /*  Not an escape sequence, so just copy the character  */
           
            s[j] = t[i];
        }
        ++i;
        ++j;
    }
    s[j] = t[i];    /*  Don't forget the null character  */
}