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## 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
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)
}
}``````

## Community comments

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### What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

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