Pascal's Triangle Row Sequence

Question

I'm currently working on finding the row sequences of Pascal's triangle. I wanted to input the row number and output the sequence of numbers in a list up until that row. For example, (Pascal 4) would give the result (1 1 1 1 2 1 1 3 3 1).

I am trying to use an algorithm that I found. Here is the algorithm itself:

V_c = V_c-1 * ((r - c)/c)

r and c are supposed to be row and column, and V₀=1. The algorithm can be specifically found on the wikipedia page in the section titled "Calculating and Individual Row or Diagonal."

Here is the code that I have so far:

(define pascal n)
  (cond((zero? n) '())
       ((positive? n) (* pascal (- n 1) (/ (- n c)c))))

I know that's hardly anything but I've been struggling a lot on trying to find scoping the function with a let or a lambda to incorporate column values. Additionally, I've also been struggling on the recursion. I don't really know how to establish the base case and how to get to the next step. Basically, I've been getting pretty lost everywhere. I know this isn't showing much, but any step in the right direction would be greatly appreciated.

Answer 1

I discussed Pascal's Triangle at my blog.

In your question, the expression for Vc is just for one row. That translates to code like this:

(define (row r)
  (let loop ((c 1) (row (list 1)))
    (if (= r c)
        row
        (loop (+ c 1) (cons (* (car row) (- r c) (/ c)) row)))))

Then you just put together a bunch of rows to make the triangle:

(define (rows r)
  (let loop ((r r) (rows (list)))
    (if (zero? r)
        rows
        (loop (- r 1) (append (row r) rows)))))

And here's the output:

> (rows 4)
(1 1 1 1 2 1 1 3 3 1)

The base case is (= r c) in the first function and (zero? r) in the second.

If you want to write subscripts clearly, you can adopt the notation used by TeX: subscripts are introduced by an underscore and superscripts by a caret, with braces around anything bigger than one character. Thus Vc in your notation would be V_c, and Vc-1 in your notation would be V_{c-1}.

Answer 2

Using as a guide the entry in Wikipedia, this is a straightforward implementation of the algorithm for calculating a value in the Pascal Triangle given its row and column, as described in the link:

#lang racket

(define (pascal row column)
  (define (aux r c)
    (if (zero? c)
        1
        (* (/ (- r c) c)
           (aux r (sub1 c)))))
  (aux (add1 row) column))

For example, the following will return the first four rows of values, noticing that both rows and columns start with zero:

(pascal 0 0)

(pascal 1 0)
(pascal 1 1)

(pascal 2 0)
(pascal 2 1)
(pascal 2 2)

(pascal 3 0)
(pascal 3 1)
(pascal 3 2)
(pascal 3 3)

Now we need a procedure to stick together all the values up until the desired row; this works for Racket:

(define (pascal-up-to-row n)
  (for*/list ((i (in-range n))
              (j (in-range (add1 i))))
    (pascal i j)))

The result is as expected:

(pascal-up-to-row 4)
> '(1 1 1 1 2 1 1 3 3 1)