Pascal's Triangle function-CodePudding

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));
    }
  }

CodePudding user response：

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.

CodePudding user response：

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