Rot13 implementation: error in translate_string function - memory

Question

I wrote a rot13.c program but I can tell something in my loop inside rot13_translate_string is causing the program to just print out blank lines.
Any thoughts?
Thank you!
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
char rot13_translate_character(char c)
{
if( 'A' <= c && c <= 'M' )
{
return c + 13;
}
else if( 'N' <= c && c <= 'Z' )
{
return c - 13;
}
else if( 'a' <= c && c <= 'm' )
{
return c + 13;
}
else if( 'n' <= c && c <= 'z' )
{
return c - 13;
}
else
{
return c;
}
}
char *rot13_translate_string(const char *str)
{
int len = strlen(str);
char *translation = calloc(len, sizeof(char));
int i;
do //****HERE IN THIS SECTION
{
/* Translate each character, starting from the end of the string. */
translation[len] = rot13_translate_character(str[len]);
len--;
} while( len < 0 ); //<
return translation;
}
And here is the main (part of the same file) - is the condition for my for i = 1 ok?
int main(int argc, char **argv)
{
if( argc < 2)
{
fprintf(stderr, "Usage: %s word [word ...]\n", argv[0]);
return 1;
}
/* Translate each of the arguments */
int i;
for( i = 1; i < argc; i++) //*****IS this right?
{
char *translation = rot13_translate_string( argv[i] );
fprintf(stdout, "%s\n", translation);
}
return 0;
}

Answer 1

As just it was pointed out by Janis is the control on the loop do ... while. It should be
while( len >= 0 );
A "while" loop runs while the control expression is true (and terminates once the expression becomes false). You define the variable len just before the loop and it cannot be <0.
So you never really enter in the loop.
You obtain a line for each input word because of fprintf(stdout, "%s\n", translation); line, where you print for each (empty) word a line (\n).
In other languages, for example in Pascal, there is "repeat until" loop construction, which continues to run until the control expression is true, and only after that it changes it terminates.
In that case you could use a condition with <0.
In C to follow the same logic you can use while loop and negate the condition. In your case
} while (! (len < 0) );