Thread: problem with pointer notation in for-loop

Code:

for (ptr = (message -1) ; (*ptr = getchar()) != '\n' ; ptr++)  {

You're assigning a value to a pointer, for this in and of itself isn't illegal, though you will almost never get the results you intended, but you also haven't initialized the pointer (ie. told it to point to a valid area in memory). Try this
for ( ptr = &(message - 1); ( *ptr = getchar() ) != '\n'; ptr++ ) {

Though judging from your comments, this would actually be safer
ptr = &message;
for ( ; (*ptr = getchar() ) != '\n'; ) {

Code:

ptr = "\0";        /*  sets pointer to NULL   */

Two things. One: a pointer is null if the value is 0, not '\0' so it should be
ptr = NULL;
or
ptr = 0;
Two: "\0" isn't a null character, it's a string constant as dictated by the double quotes. A single character should be inside single quotes. If you wanted to assign a null character to the element of the array then you want to dereference the pointer first.
*ptr = '\0';

Code:

for ( ptr ; ptr >= message; ptr--){
      putchar(*ptr);
}

I'm guessing this would give you an undefined result. In the initialization part of the for loop, what are you assigning to ptr? An array always starts with 0, so the conditional expression
ptr >= message
is just asking for trouble.
Be more explicit with what you want the computer to do or it will give you problems when it tries to interpret your instructions. In this case

Code:

for ( ptr = &message[99]; ptr >= &message[0]; ptr-- ) {
/* 
**ptr is assigned to the address of the last element of the array
**so there's no doubt as to where the loop starts. You loop until 
**the pointer gets to the address of the 0th element of the array.
**Simple and easy to understand, for both the reader and the
**compiler.
*/

The problem was that while you declared your array, you didn't put anything in it and the chances of the random values put in it being '\0' are very slim, so when you printed out the entire array, you got the garbage that was there initially. To fix this problem, when you declare your array, also initialize it with one '\0' character and that will fill the array with nulls.

Here's the working code.

Code:

#include <stdio.h>

#define LENGTH 100    /* Maximum length of message array */

main()
{
   char message[LENGTH] = {'\0'};     /* Declare all variables */
   char *ptr;
 
   /*  
   **   This loop stores message in array with pointer pointing
   **   to it's elements.
   */

   printf("Enter your message: ");
   for ( ptr = message ; (*ptr = getchar()) != '\n' ; ptr++ ){
       ;
   }

   /*  
   **   This loop take message array and prints   
   **   in reverse using decrementation of pointer.
   */

    printf("Here's your message in reverse:");
    for ( ptr = &message[LENGTH]-1 ; ptr>=&message[0]; ptr-- ){
	  putchar(*ptr);
    }
    printf("\n");

    return 0;

}

Thanks.

I should say, that is the second type problem where I forget
about initializing an array.