Pascal's triangle in C weird output

Question

I'm trying to write a program to display Pascal's triangle up to a user-inputted number of levels. We aren't allowed to use the factorial method. My current code is this:

#include <stdio.h>

void trianglePrint(const int numLevels);

int main() {
  int numLevels;

  printf("Please enter how many levels of Pascal's Triangle you would 
  like to see: ");
  scanf("%d", &numLevels);

  trianglePrint(numLevels);


  return 0;
}

void trianglePrint(const int numLevels) {
  int pascalTriangle[28][28];
  int i, j;

  for (i = 0; i < numLevels; ++i) {
    for (j = 0; j <= i; ++j) {
      if (i == 0 || i == 1 || j == 0 || j == numLevels) {
        pascalTriangle[i][j] = 1;
        printf("%d ", pascalTriangle[i][j]);
      }
      else {
        pascalTriangle[i][j] = pascalTriangle[i - 1][j - 1] + 
        pascalTriangle[i - 1][j];
        printf("%d ", pascalTriangle[i][j]);
      }
    }
    printf("\n");
  }

}

We're only supposed to be able to go up to 28 levels, which is why I am using an array of size 28 in both dimensions.
This works fine for about 6 levels of the triangle, but for larger levels it gives really large integers. I assumed it was due to uninitialized arrays, but I'm not sure. Does anyone know where the error is?

It might help to show the expected output vs what it's actually outputting. — Carcigenicate, Nov 05 '17 at 16:50
`j == numLevels` is always false. `pascalTriangle[i - 1][j]` reads from an uninitialized variable. — melpomene, Nov 05 '17 at 16:54
@Yunnosch It means replacing `int pascalTriangle[28][28];` with `int pascalTriangle[28][28] = {{0}};`. — BLUEPIXY, Nov 05 '17 at 16:56
@BLUEPIXY I understood that, but it is unneeded (though generally wise), after fixing the logic. The correct algorithm does not need an initialised array. — Yunnosch, Nov 05 '17 at 16:58

score 0 · Answer 1 · answered Nov 05 '17 at 16:59

0

The problem is that you haven't set pascalTriangle[i - 1][j] before you use it to compute pascalTriangle[i][j] in the else clause.

answered Nov 05 '17 at 16:59

Yunnosch · Accepted Answer · 2017-11-05T17:15:02.970

0

If you change

if (i == 0 || i == 1 || j == 0 || j == numLevels)

to

if (i == 0 || i == 1 || j == 0 || j == i)

(thanks to Melpomene), then all accesses to your array end up on already intiailised members.

That solves the strange numbers.

Output:

Please enter how many levels of Pascal's Triangle you would like to see: 6
1
1 1
1 2 1
1 3 3 1
1 4 6 4 1
1 5 10 10 5 1

Process returned 0 (0x0)   execution time : 2.264 s
Press any key to continue.

Note:
Also initialising an array is a wise precaution. You could initialise with values which help finding an error, instead of hiding it, e.g. 42.

edited Nov 05 '17 at 17:15

answered Nov 05 '17 at 17:00

Yunnosch

26,130
9
42
54

@BLUEPIXY Thanks for spotting the error I fixed. Couldn't delete accepted answer for editing.... – Yunnosch Nov 05 '17 at 17:11
Upvoted, but I don't agree with your comment about initialising arrays. If you initialise an array, you prevent tools like valgrind from helping you,and in any case uninitialised values stand out a mile, so are informative. – Nov 05 '17 at 17:26

score 0 · Answer 3 · answered Nov 05 '17 at 17:01

0

Try the code

void Pascal(int n)
    {
      int arr[n][n]; 


      for (int i = 0; i < n; i++)
      {

        for (int j = 0; j <= i; j++)
        {
          // First and last values in every row are 1
          if (i == j || j == 0)
               arr[i][j] = 1;
          else // Other values are sum of values just above and left of above
               arr[i][j] = arr[i-1][j-1] + arr[i-1][j];
          printf("%d ", arr[i][j]);
        }
        printf("\n");
      }
    }

answered Nov 05 '17 at 17:01

shubhamj

778
1
7
13

You would deserve an upvote for a fast and good (not to say better than my first attempt) answer. But code-only answers are unlikely to be upvoted. Try explaining why your code does not have the problem, i.e. how you solved it. – Yunnosch Nov 05 '17 at 17:17

Pascal's triangle in C weird output

3 Answers3