CODEWITHSUNDEEP
c#algorithm
Gloop
Gloop

Reputation: 23

Pascal's Triangle function

I am trying to write a function to print out Pascal's triangle up to the step that the user inputs. The program works until step 5, and prints out 0 1 4 0 4 1 0 instead of 0 1 4 6 1 0 which is expected. It takes two values from another list, which are both 3 at the time, and adds them so i am not sure how it is changed to 0. Code:

static void PascalsTri(int input)
  {
    //Declare lists starting at 0 1 0
    int counter = 0;
    int[] start = { 0, 1, 0 };
    List<int> TriList = new List<int>(start);
    List<int> TempTriList = new List<int>();
    //Check if input is possible
    if(input < 1)
    {
      return;
    }
    //Write starting list to console
    Console.WriteLine(string.Join(" ", TriList));
    //Run the function as many times as the user input
    for(int i = 1; i < input; i++)
    {
      //Start off with the first two digits
      TempTriList.Add(0);
      TempTriList.Add(1);
      //Loop through writing the rule for as many numbers as there are
      while(counter < i)
      {
        //Takes the previous number and adds it to the correlating number
        TempTriList.Insert(counter+1, TriList[counter] + TriList[counter+1]);
        counter++;
      }
      TempTriList.Add(0);
      TriList.Clear();
      //Records the output in the permanent list, and prints it to the console
      foreach(int j in TempTriList)
      {
        TriList.Add(TempTriList[j]);
      }
      TempTriList.Clear();
      counter = 0;
      Console.WriteLine(string.Join(" ", TriList));
    }
  }

Upvotes: 2

Views: 307

Answers (2)

Dmitrii Bychenko
Dmitrii Bychenko

Reputation: 186668

If you want to print Pascal Triangle line after line you can just loop while computing next line in the process (fiddle)

Code:

    static void PascalTri(int rowsToPrint) {
      // 1st current line
      int[] current = new int[] { 0, 1, 0 };
      
      Console.WriteLine(string.Join(" ", current));

      // we compute all the other lines in a simple loop
      for (int r = 1; r < rowsToPrint; ++r) {
        // next line is 1 number longer than current
        int[] next = new int[current.Length + 1];

        // compute next line items
        for (int c = 0; c < current.Length - 1; ++c)
          next[c + 1] = current[c] + current[c + 1];

        // after computation, next line becomes current one
        current = next;

        // finally, we print the line
        Console.WriteLine(string.Join(" ", current));
      }
    }

Demo:

PascalTri(10);

Outcome:

0 1 0
0 1 1 0
0 1 2 1 0
0 1 3 3 1 0
0 1 4 6 4 1 0
0 1 5 10 10 5 1 0
0 1 6 15 20 15 6 1 0
0 1 7 21 35 35 21 7 1 0
0 1 8 28 56 70 56 28 8 1 0
0 1 9 36 84 126 126 84 36 9 1 0

Upvotes: 1

computercarguy
computercarguy

Reputation: 2454

That had me stumped for a while, too. Then I realized that a single line of code in your foreach is wrong.

TriList.Add(TempTriList[j]);

It should be:

TriList.Add(j);

The variable "j" isn't an index, but the array value itself. Once you make that change, it all works.

This code might be better arranged if it used a recursive method, but since what you have works (now), I'd leave it. Making that change to use recursion can be difficult and far beyond the scope of this question.

Upvotes: 1

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