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to Pascal's Triangle in the Common Lisp Track

Published at Mar 01 2020 · 0 comments
Test suite

Compute Pascal's triangle up to a given number of rows.

In Pascal's Triangle each number is computed by adding the numbers to the right and left of the current position in the previous row.

   1 1
  1 2 1
 1 3 3 1
1 4 6 4 1
# ... etc


Check out Installing Common Lisp for instructions to get started or take a look at the guides available in the track's side bar.


While Common Lisp doesn't care about indentation and layout of code, nor whether you use spaces or tabs, this is an important consideration for submissions to exercism.io. Excercism.io's code widget cannot handle mixing of tab and space characters well so using only spaces is recommended to make the code more readable to the human reviewers. Please review your editors settings on how to accomplish this. Below are instructions for popular editors for Common Lisp.


Use the following commands to ensure VIM uses only spaces for indentation:

:set tabstop=2
:set shiftwidth=2
:set expandtab

(or as a oneliner :set tabstop=2 shiftwidth=2 expandtab). This can be added to your ~/.vimrc file to use it all the time.


Emacs is very well suited for editing Common Lisp and has many powerful add-on packages available. The only thing that one needs to do with a stock emacs to make it work well with exercism.io is to evaluate the following code:

(setq-default indent-tabs-mode nil)

This can be placed in your ~/.emacs (or ~/.emacs.d/init.el) in order to have it set whenever Emacs is launched.

One suggested add-on for Emacs and Common Lisp is SLIME which offers tight integration with the REPL; making iterative coding and testing very easy.


Pascal's Triangle at Wolfram Math World http://mathworld.wolfram.com/PascalsTriangle.html

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


(ql:quickload "lisp-unit")
#-xlisp-test (load "pascals-triangle")

(defpackage #:pascal-test
  (:use #:common-lisp #:lisp-unit))
(in-package #:pascal-test)

(define-test zero-rows
    (assert-equal '()
                  (pascal:rows 0)))

(define-test single-row
    (assert-equal '((1))
                  (pascal:rows 1)))

(define-test two-rows
    (assert-equal '((1) (1 1))
                  (pascal:rows 2)))

(define-test three-rows
    (assert-equal '((1) (1 1) (1 2 1))
                  (pascal:rows 3)))

(define-test four-rows
    (assert-equal '((1) (1 1) (1 2 1) (1 3 3 1))
                  (pascal:rows 4)))

(define-test negative-rows
    (assert-equal '()
                  (pascal:rows -1)))
(let ((*print-errors* t)
      (*print-failures* t))
  (run-tests :all :pascal-test))
(in-package #:cl-user)
(defpackage #:pascal
  (:use #:cl)
  (:export #:rows))
(in-package #:pascal)

(defun rows (n)
  (flet ((next-row (row)
              :for val1 :in (append '(0) row '(0))
              :for val2 :in (append row '(0))
              :collect (+ val1 val2))))
    (if (< n 1)
        (do ((x 1 (1+ x))
             (ret '((1)) (cons (next-row (car ret)) ret)))
            ((>= x n) (reverse ret))))))

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