K&R exercise 1-19: reversing each line of input

Question

This is my solution for exercise 1-19 of K&R:

_{Write a function reverse(s) that reverses the character string s. Use it to write a program that reverses its input a line at a time.}¹

It works, but I'm trying to write good programs. I think the fact that I can't use the length of the vector when already got the line in function getlinea makes me think I could do better. The function reverse(s) uses 3 for sentences, which might be a little ugly. I know it doesn't mean it's not good but having to "read" the line 4 times is kind of a little stupid to me. I still can't think of a different solution.

/*
 * main.c
 *
 *  Created on: 19/3/2015
 *      Author: utnso
 */

#include <stdio.h>

#define MAXLINE 1000

void reverse (char s [] );
int getlinea (char s [] , int lim);

int main ()
{
    int len;
    char palabra [MAXLINE];
    while ( ( len = getlinea ( palabra, MAXLINE ) )  >= 0 )
        if (len > 0)
            printf ("%s\n", palabra);
    return 0;

}

int getlinea ( char s [], int lim )
{
    int i, c;
    i = 0;
    while ( ( c = getchar() ) != EOF && c != '\n' && i < lim-1 )
    {
        s [i] = c ;
        ++ i;
    }
    s [i] = '\0';
    if ( i > 0 )
        reverse ( s ) ;
    else
        if ( c == EOF )
            return -1;
    return i ;
}

void reverse ( char turnaround [] )
{
    int i, j;
    int h = 0;
    char aux [MAXLINE];
    for ( i = 0; turnaround [i] != '\0' ; ++i )
        ;
    for ( j = i-1 ; j >= 0 ; --j)
    {
        aux [h] = turnaround [j];
        ++h;
    }
    for ( i = 0 ; turnaround [i] != '\0' ; ++i )
        turnaround [i] = aux [i];
}

This is the output for

$ ./1-19 < main.c

(it also deletes white lines)

*/
c.niam * 
* 
5102/3/91 :no detaerC  * 
osntu :rohtuA      * 
/* 
>h.oidts< edulcni#
0001 ENILXAM enifed#
;) ][ s rahc( esrever diov
;)mil tni , ][ s rahc( aenilteg tni
)( niam tni
{
;nel tni  
;]ENILXAM[ arbalap rahc   
) 0 =>  ) ) ENILXAM ,arbalap ( aenilteg = nel ( ( elihw   
)0 > nel( fi      
;)arbalap ,"n\s%"( ftnirp         
;0 nruter 
}
) mil tni ,][ s rahc ( aenilteg tni
{
;c ,i tni 
;0 = i    
) 1-mil < i && 'n\' =! c && FOE =! ) )(rahcteg = c ( ( elihw  
{ 
; c = ]i[ s       
;i ++     
} 
) 0 > i ( fi  
{ 
;'0\' = ]i[ s     
; ) s ( esrever       
} 
esle  
) FOE == c ( fi       
;1- nruter            
; i nruter    
}
) ][ dnuoranrut rahc ( esrever diov
{
;j ,i tni 
;0 = h tni    
;]ENILXAM[ xua rahc   
) i++ ; '0\' =! ]i[ dnuoranrut ;0 = i ( rof   
;     
)j-- ; 0 => j ; 1-i = j ( rof 
{ 
;]j[ dnuoranrut = ]h[ xua     
;h++      
} 
) i++ ; '0\' =! ]i[ dnuoranrut ; 0 = i ( rof  
;]i[ xua = ]i[ dnuoranrut     
}

---

¹_{http://cs.indstate.edu/~cbasavaraj/cs559/the_c_programming_language_2.pdf - Page 31}

Answer 1

• getlinea does too much: it reads the line and reverses it. A Single Responsibility Principle mandates restructuring the code:

  while((len = getlinea(...)) > 0) {
      reverse();
      print();
  }
  

• Avoid naked loops. Each loop represent an important action, and deserves a name. The code:

  reverse(char turnaround[])
  {
      char aux[...];
      int len = length_of_string(turnaround);
      copy_backwards(aux, turnaround, len);
      copy(turnaround, aux, len);
  }
  

  is much easier to reason about.

• The length of the input is known to the caller. Consider passing it as parameter:

  reverse(char turnaround[], int len)
  {
      char aux[...];
      copy_backwards(aux, turnaround, len);
      copy(turnaround, aux, len);
  }
  

• Finally, a string can be reversed in place. There is no need for aux. I hope you can figure a solution yourself.

Answer 2

While the answer is using concepts not covered in chapter 1 (and for that reason, is probably a better solution in real situations), here's my take on the exercise using only the covered material:

void reverse(char s[]) {
    int i;
    for (i = 0; s[i] != '\0' && s[i] != '\n'; ++i) { }
    for (i = i-1; i >= 0; --i) {
        putchar(s[i]);
    }
    putchar('\n');
}

The requirement of the exercise is actually to reverse its input one line at a time, so no character array is actually needed. This function gets the job done without allocating extra memory.

The function could actually perform better if another parameter len could be given, so you won't even need the first loop. I didn't add the second parameter only because the exercise seems to want a function taking only one argument s.