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katrinleinweber's solution

to Pascal's Triangle in the R Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

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

Installation

See this guide for instructions on how to setup your local R environment.

How to implement your solution

In each problem folder, there is a file named <exercise_name>.R containing a function that returns a NULL value. Place your implementation inside the body of the function.

How to run tests

Inside of RStudio, simply execute the test_<exercise_name>.R script. This can be conveniently done with testthat's auto_test function. Because exercism code and tests are in the same folder, use this same path for both code_path and test_path parameters. On the command-line, you can also run Rscript test_<exercise_name>.R.

Source

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.

test_pascals-triangle.R

source("./pascals-triangle.R")
library(testthat)

context("pascals triangle")

test_that("zero rows", {
  expect_equal(pascals_triangle(0), list())
})

test_that("single row", {
  expect_equal(pascals_triangle(1), list(1))
})

test_that("two rows", {
  expect_equal(pascals_triangle(2), list(1, c(1, 1)))
})

test_that("three rows", {
  expect_equal(pascals_triangle(3), list(1, c(1, 1), c(1, 2, 1)))
})

test_that("four rows", {
  expect_equal(pascals_triangle(4), list(1, c(1, 1), c(1, 2, 1), c(1, 3, 3, 1)))
})

test_that("negative rows", {
  expect_error(pascals_triangle(-1))
})

test_that("null/no rows", {
  expect_error(pascals_triangle(NULL))
})

message("All tests passed for exercise: pascals-triangle")
pascals_triangle <- function(n) {
  
  # catch invalid input & edge cases 
  
  if (n < 0 | is.null(n))
    stop("Invalid! Please provide n > 0!")
  else if (n == 0)
    list()
  else if (n == 1)
    list(1)
  else if (n == 2)
    list(1, c(1, 1))
  else if (n >= 3) {
    
  # construct triangle
    t <- list(1, c(1, 1))
    
    for (i in seq(3, n)) {
      t <- append(t, list(c(i)))
      t[[i]][1] <- 1
      t[[i]][i] <- 1
      
  # construct row
      for (j in seq(2, i-1))
        t[[i]][j] <- t[[i-1]][j-1] + t[[i-1]][j]
    }
    return(t)
  }
}

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