The C Programming Language, Ch.1 Exercise 1.10 (Getchar and Putchar)

Question

I've been working on this for 2 hours and I am stuck... I found the answer online, but that isn't going to help me learn the concept that I'm obviously missing.

Prompt: 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.

Here's what I came up with, it doesn't replace a tab or \ with the indicated putchar, it just adds it in front of it.(I didn't do backspace because I can't really input a backspace...):

This is how I read the code. What am I missing?:

"There is some integer c. c is equal to the input. When the input is not equal to end of file proceed. If input is tab then the output \t. If input is \ then output \\. Output the input to console."

int c;

while((c=getchar())!=EOF)
{
    if(c=='\t')
        {
            putchar('\\');
            putchar('t');
        }

    if(c=='\\')
        {
            putchar('\\');
            putchar('\\');
        }
    putchar(c);
}

"If input is tab then the output \t. If input is \ then output \\. Output the input to console" -- and what if more than one of those happens? — Jim Balter, Dec 30 '21 at 12:05

paxdiablo · Accepted Answer · 2014-04-07T04:20:01.510

Your main problem is that you are outputting the character regardless of the fact that you may have already output its translation. Those if statements will do what you expect but, in their present form, they simply drop through to the next statement.

Hence you'd be looking for something more like this:

while ((c = getchar()) != EOF) {
    // Detect/translate special characters.

    if (c == '\t') {
        putchar ('\\');
        putchar ('t');
        continue;              // Go get next character.
    }

    if (c == '\b') {
        putchar ('\\');
        putchar ('b');
        continue;              // Go get next character.
    }

    if (c == '\\') {
        putchar ('\\');
        putchar ('\\');
        continue;              // Go get next character.
    }

    // Non-special, just echo it.

    putchar (c);
}

Another possibility, shorter and more succinct would be:

while ((c = getchar()) != EOF) {
    // Detect/translate special characters, otherwise output as is.

    switch (c) {
        case '\t': putchar ('\\'); putchar ('t');  break;
        case '\b': putchar ('\\'); putchar ('b');  break;
        case '\\': putchar ('\\'); putchar ('\\'); break;
        default:   putchar (c);
    }
}

or instead of so many `continue` you can also use `else if` instead of `if` — theharshest, Apr 07 '14 at 04:12
@theharshest, I prefer a consistent group of conditions at the start than the possibility of indent-hell :-) In any case, I'd probably implement it with the (new) `switch` variant myself). — paxdiablo, Apr 07 '14 at 04:21

score 3 · Answer 2 · answered Nov 14 '17 at 06:56

I know im late to the party, but this question pops up in chapter one before else, case, continue, and functions are introduced.

Here is a working solution to exercise 1-10 that involves only concepts introduced up to the point of the exercise. You need to keep track of whether an escaped character was found and then display the copied character only if one was not found.

#include <stdio.h>

int main() {

  int input;

  while((input = getchar()) != EOF){

    int escaped = 0;

    if(input == '\t'){
        putchar('\\');
        putchar('t');
        escaped = 1;
    }

    if(input == '\b'){
        putchar('\\');
        putchar('b');
        escaped = 1;
    }

    if(input == '\\'){
        putchar('\\');
        putchar('\\');
        escaped = 1;
    }

    if(escaped == 0){
      putchar(input);
    }
  }
}

score 1 · Answer 3 · answered Apr 07 '14 at 06:06

There are many ways to implement this and paxdiablo gave a couple of good ones. Here's one that illustrates the DRY principle via functional decomposition:

void putesc(char c)
{
    putchar('\\');
    putchar(c);
}

void ioloop(void)
{
      for (int c;;)
          switch (c = getchar())
          {
               case EOF:  return;
               case '\t': putesc('t'); break;
               case '\b': putesc('b'); break;
               case '\\': putesc(c); break;
               default:   putchar(c); break;
          }
 }

score 1 · Answer 4 · edited Jul 20 '21 at 20:40

Adding another solution! It's handy for us newbies to enrich us, seeing variety of solutions.

#include <stdio.h>

/* a program to copy its input to its output, replacing tab by \t,
    backspace by \b, backslash by \\  */
    
/* need double backslash to output a single backslash */

int main(){
    
    int c;          /*  to store next character from getchar() */
        
    while((c = getchar()) != EOF){
        if( c != '\t' && c != '\b' && c != '\\')        /* print all characters except special one's */
            putchar(c);                             
            
        else{
            if(c == '\t'){          /* replacing tab by \t */
                putchar('\\');
                putchar('t');
                }
            
            if(c == '\b'){          /* replace backspace by \b */ 
                putchar('\\');
                putchar('b');
                }
                
            if(c == '\\'){          /* replace backslash by \\ */
                putchar('\\');
                putchar('\\');
                }       
        }
        
    }

}

score 1 · Answer 5 · edited Mar 01 '22 at 06:56

I'm a rookie. But considering only what they taught until to that point, I came up with this answer.

#include <stdio.h>

main() 
{
    int c;

    c = getchar();

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

score 0 · Answer 6 · answered Oct 17 '19 at 06:01

You are very close. Simply change the second "putchar()" in each block into an assignment statement and you have the correct output.

int c;

while((c=getchar())!=EOF)
{
    if(c=='\t')
        {
            putchar('\\');
            c = 't';
        }

    if(c=='\\')
        {
            putchar('\\');
            c = '\\';
        }
    putchar(c);
}

Amit Thaware · Answer 7 · 2022-10-15T14:25:05.003

    #include <stdio.h>

    int main(int argc, const char * argv[]) {
    
        int c;
        int tab = 't';
        int backspace = 'b';
        int backslash = '\\';

        while((c = getchar()) != EOF){

            if (c == '\t'){
                putchar('\\');
                putchar(tab);
            }
            else if(c == '\b'){
                putchar('\\');
                putchar(backspace);
            }
            else if(c == '\\'){
                putchar('\\');
                putchar(backslash);
            }
            else{  // All other characters EXCLUDING "tab, backspace and backslash"
                  // will always be printed.
                putchar(c);
            }
        }

       return 0;
   }

Your answer could be improved with additional supporting information. Please [edit] to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers [in the help center](/help/how-to-answer). — Community, Oct 19 '22 at 01:24

Marcelo Coronel · Answer 8 · 2018-03-21T05:20:06.810

-2

Just before the exercise, the book mentions ASCII code and not more advanced statements. In consequence, I think the solution was oriented to use ASCII.

    int c;
    while ( (c = getchar()) != EOF ){
        //92 is the ASCII code for the backslash \
        if ( c == '\t'){
            putchar(92);
            putchar('t');
        }else if ( c == '\\' ) {
            putchar(92);
            putchar(92);
        }else if ( c == '\b' ) {
            putchar(92);
            putchar('b');
        }else{
            putchar(c);
        }
    }

edited Mar 21 '18 at 05:20

answered Mar 21 '18 at 05:02

Marcelo Coronel

17
3

1

The book also said that using 'A' instead of 65 is better and more useful for programmer reading the code. – knoftrix Nov 28 '20 at 16:54

The C Programming Language, Ch.1 Exercise 1.10 (Getchar and Putchar)

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