C Program Error?

linux_GNUbie · 11-30-2003, 06:43 AM

Hello all. Newbie to programming so please, be nice!

I'm teaching myself C and I'm doing problems & examples from various books I have.
I'm trying to solve a problem involving a 2-dimensional integer array that has the same
number of rows as columns (five for my example), where each cell contains a positive
integer value, and where the sum of each row/column/diagonal is equivalent. I'm using
the values 1 through 25 as input from the user to identify the starting position. The rules
for the problem are as follows:
1. Start at any cell and place a 1 there,
2. Move to the next cell by moving to the right one column and up two rows; if you move
past an edge wrap around to the other side,
3. If the new cell is NOT occupied, place the next number there; otherwise, go back to the
previous cell, drop down to the cell under that one and place the next number there,
4. Repeat steps 2 and 3 until the table is filled.

I've put a lot of time into this problem and I thought I came up with a pretty good solution.
However, the program does not fill the table completely and I can't seem to figure out why.
I was hoping someone here could offer a suggestion. I'm a bit frustrated now.

TIA!

Chris//CMS

Code:

#include <stdio.h>
#define SIZE 25

int main() {
   int x = 0;                /* counter used in loop initialized to zero */
   int y = 0;                /* counter used in loop initialized to zero */
   int start_row = 0;        /* position of starting point (row) initialized to zero */
   int start_column = 0;     /* position of starting point (column) initialized to zero */
   int old_row = 0;          /* position of starting point (row) initialized to zero */
   int old_column = 0;       /* position of starting point (column) initialized to zero */
   int table[5][5] = {0};    /* five-element, two-dimensional integer array initialized to zero */

   /* prompt and read in starting position (row) */
   printf("   ENTER STARTING POSITION -->\n");
   printf("   ENTER STARTING ROW NUMBER [ENTER 1 THROUGH 5]: ");
   scanf("%d", &start_row);
   fflush(stdin);

   while (start_row < 1 || start_row > 5) {
      printf("   INVALID ENTRY!\n");
      printf("   ENTER STARTING ROW NUMBER [ENTER 1 THROUGH 5]: ");
      scanf("%d", &start_row);
      fflush(stdin);
   }

   printf("   STARTING ROW IS %d\n", start_row);   /* ERROR CHECK */

   /* prompt and read in starting position (column) */
   printf("   ENTER STARTING COLUMN NUMBER [ENTER 1 THROUGH 5]: ");
   scanf("%d", &start_column);
   fflush(stdin);

   while (start_column < 1 || start_column > 5) {
      printf("   INVALID ENTRY!\n");
      printf("   ENTER STARTING COLUMN NUMBER [ENTER 1 THROUGH 5]: ");
      scanf("%d", &start_column);
      fflush(stdin);
   }

   printf("   STARTING COLUMN IS %d\n", start_column);   /* ERROR CHECK */

   /* this subtraction accounts for the difference between the user's input (the
    * numbers 1 through 5) and the locations in the table array (0 through 4)
    */
   start_row = start_row - 1;
   start_column = start_column - 1;

   /* this assigns the number one to the first place in the table array */
   table[start_row][start_column] = 1;

   for (x = 2; x <= SIZE; x++) {

      old_row = start_row;
      old_column = start_column;

      /* calculates new row position where rows are labeled zero through four */
      if (start_row >= 2)
         start_row = start_row - 2;
      else
         start_row = start_row + 3;

      /* calculates new column position where columns are labeled zero through four */
      if (start_column < 4)
         start_column++;
      else
         start_column = start_column - 4;

      if (table[start_row][start_column] != 0) {
         start_row = old_row;
         start_column = old_column;

         if (start_row == 4)
            start_row++;
         else
            start_row = start_row - 4;
      }

      table[start_row][start_column] = x;
   }

   printf("\n __________________________\n");

   /* loops used to iterate and print table */
   for (x = 0; x < 5; x++) {
      for (y = 0; y < 5; y++)
         printf(" | %2d", table[x][y]);
         printf(" |\n");
   }

   printf(" --------------------------\n\n");

   return 0;

}

MartinN · 11-30-2003, 08:14 AM

Hi!

You have a typo in your code

Code:

...

      if (table[start_row][start_column] != 0) {
         start_row = old_row;
         start_column = old_column;

         if (start_row == 4) /* HERE! Should read:   start_row < 4 */
            start_row++;
         else
            start_row = start_row - 4;
      }

...

Regards
Martin

linux_GNUbie · 11-30-2003, 08:39 AM

Cool. Thanks Martin. My mistakes so far have always dealy with simple logic and math.
I was stuck looking at the lines

Code:

      old_row = start_row;
      old_column = start_column;

just inside the for loop. I played around with these lines for a while think my mistake was
somehow associated with them. I moved those lines around inside--and outside--the for
loop. I got different results each time I moved them. I created the variables old_row and
old_column to hold the initial values that identified the starting position, in case the next
position was already occupied; I'd still have the initial values to revert back to. I can't seem
to grasp why I can't move those two lines outside the for loop--say, just before the for loop.
All I'm doing is copying the values from one location in memory to another. I'm not doing
anything to the initial values of start_row or start_column. Yet when I move those two lines
outside/before the for loop I get strange results. Odd!

Thanks! I'll read some more.

Chris//CMS

MartinN · 11-30-2003, 09:02 AM

Quote:

Originally posted by linux_GNUbie

I was stuck looking at the lines

Code:

old_row = start_row; old_column = start_column;

I can't seem to grasp why I can't move those two lines outside the for loop--say, just before the for loop.

I don't think it's strange. You save the old values of row and column there. But those values are different every time you go through the loop.

By the way. A little comment on your use of hard-coded variables. You make a

Code:

#define SIZE 25

in the beginning of your program. I would recommend that you change that to

Code:

#define MAX_COL 4
#define MAX_ROW 4

and then use those values every time when you loop and test (for example the test where you had the error). By doing that and removing all hard-coded values from the code, it's much easier to let your program calculate the same for, say a 6 * 6 matrix. Plus, it's easier to read and understand a line of code that says

Code:

      if (start_column < MAX_COL)
         start_column++;

than

Code:

      if (start_column < 4)
         start_column++;

Keep up the otherwise good work!

Regards
Martin

linux_GNUbie · 11-30-2003, 06:49 PM

Again, thank you Martin, for your time and incite. Does there come a time when
a person asks too many questions?

Chris//CMS

MartinN · 12-01-2003, 03:18 AM

Quote:

Originally posted by linux_GNUbie
Again, thank you Martin, for your time and incite. Does there come a time when
a person asks too many questions?

Chris//CMS

No problemo. Keep asking! If I don't have the answer someone else may be able to help you.

Programming is like learning a foreign language. You don't get really good at it by only reading the textbooks. You need to practice it for a while.

Regards
Martin