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Loading arrays from text file...again :P

Posted on 2003-10-29
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Last Modified: 2010-04-02
I asked a very similar qustion earlier here and got the answer I needed,
but now my program has changed and I'm running into some small problems.
I can load the values from the file into the 2d array perfectly, but when I try to
run an if to test the values i get screwed. This code should printf("%s", textures[0]); but
prints nothing.  the first value of map.txt is 9, i.e.
cat map.txt
9 0 0 1 1 3 3 4 5 6 7

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
using namespace std;

int main () {
  FILE * pFile;
  long lSize;
  char * buffer;

  pFile = fopen ("map.txt" , "rb" );
  if (pFile==NULL) exit (1);
  fseek (pFile , 0 , SEEK_END);
  lSize = ftell (pFile);
  rewind (pFile);
.
  buffer = (char*) malloc (lSize);
  if (buffer == NULL) exit (2);
  fread (buffer,1, lSize,pFile);

  char * pch;
  pch = strtok (buffer," ");

  int x = 0;
  int y = 0;

  char* matrix[15][20];

  while (pch != NULL)
  {
    if(y <= 19)
    {
      matrix[x][y] = pch;
      pch = strtok (NULL, " ,.");
      y++;
    }
    else
    {
     y = 0;
     x++;
    }
  }
/* ======================================================================*/
  char *textures[] = {".\\images\\stone.bmp", ".\\images\\woodtile.bmp"};
 
  for(int row = 0; row<15; row++){
    for(int col = 0; col<20; col++){
      if(matrix[0][0] == "9"){        //test matrix[0][0] to see if to equals "9"
        printf("%s", textures[0]);    //if it does then printf
      }
    }
  }
/* ======================================================================*/
  fclose (pFile);
  free (buffer);
  return 0;
}
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Question by:w00t
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Author Comment

by:w00t
ID: 9647190
that period around line 19 shouldn't be there :P
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Expert Comment

by:Exceter
ID: 9647260
>> char *textures[] = {".\\images\\stone.bmp", ".\\images\\woodtile.bmp"};
>>
>> ...
>> 
>> printf("%s", textures[0]);    //if it does then printf

textures is a char array. Its type is char*. textures[0] is a char, not a char*. You are attempting to display a string using the first char in the array as the pointer to the array. You could do this by saying,

printf("%s", &textures[0]);

but it is easiuer to simply say,

printf("%s", textures);

Exceter
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Expert Comment

by:Exceter
ID: 9647348
Oops, strike that, I didn't notice that textures had multiple dimensions...
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Expert Comment

by:Exceter
ID: 9647363
Here's the actual culprit, unless the sleepy seed demon is still haunting me,

>> char* matrix[15][20];

...

>>      if(matrix[0][0] == "9"){        //test matrix[0][0] to see if to equals "9"
>>        printf("%s", textures[0]);    //if it does then printf

matrix[0][0] is a char. "9" is a null terminated string containing the character 9 and a terminating null byte. Therefore its type is char*, not char. Replacing the double quotes with single quotes tells the compiler that this is a single character and not a string. Therefore, this should read,

if(matrix[0][0] == '9')
    printf("%s", textures[0]);

Cheers!
Exceter
0
 
LVL 8

Accepted Solution

by:
Exceter earned 100 total points
ID: 9647391
Grrr....

>> char* matrix[15][20];

Okay, I'm getting annoyed with myself now... This is an array of char* pointers and not chars...

>>      if(matrix[0][0] == "9"){        //test matrix[0][0] to see if to equals "9"
>>        printf("%s", textures[0]);    //if it does then printf

Understand that this comparison is still wrong, I simply stated the wrong reason for this. Both matrix[0][0] and "9" are char* pointers to a string. However, they can point to different arrays that contain the same data. Therefore, the pointer value of "9" is not equal to the pointer value for matrix[0][0] and the comparison fails. The actual data in the arrays is never compared. You need to use the strcmp function for this task, for example,

if( strcmp( matrix[0][0], "9" ) == 0 ) // If the data in the strings are equal, print textures[0]
   printf("%s", textures[0]);

I should have read your question more carefully from the beginning. My apologies.

Cheers!
Exceter
0
 
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Expert Comment

by:tinchos
ID: 10546306
No comment has been added lately, so it's time to clean up this question.
I will leave the following recommendation for this question in the Cleanup topic area:

Accept: Exceter {http:#9647391}

Please leave any comments here within the next four days.
PLEASE DO NOT ACCEPT THIS COMMENT AS AN ANSWER!

Tinchos
EE Cleanup Volunteer
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