 # stephl001's solution

## to Matching Brackets in the Elm Track

Published at Sep 25 2019 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

# Bracket Push

Given a string containing brackets [], braces {}, parentheses (), or any combination thereof, verify that any and all pairs are matched and nested correctly.

## Elm Installation

Refer to the Installing Elm page for information about installing elm.

## Writing the Code

The first time you start an exercise, you'll need to ensure you have the appropriate dependencies installed. Thankfully, Elm makes that easy for you and will install dependencies when you try to run tests or build the code.

Execute the tests with:

$elm-test  Automatically run tests again when you save changes: $ elm-test --watch


As you work your way through the test suite, be sure to remove the skip <| calls from each test until you get them all passing!

Ginna Baker

## Submitting Incomplete Solutions

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

### Tests.elm

module Tests exposing (tests)

import BracketPush exposing (isPaired)
import Expect
import String
import Test exposing (..)

tests : Test
tests =
describe "BracketPush"
[ test "paired square brackets" <|
\_ ->
Expect.equal True (isPaired "[]")
, skip <|
test "empty string" <|
\_ ->
Expect.equal True (isPaired "")
, skip <|
test "unpaired brackets" <|
\_ ->
Expect.equal False (isPaired "[[")
, skip <|
test "wrong ordered brackets" <|
\_ ->
Expect.equal False (isPaired "}{")
, skip <|
test "wrong closing bracket" <|
\_ ->
Expect.equal False (isPaired "{]")
, skip <|
test "paired with whitespace" <|
\_ ->
Expect.equal True (isPaired "{ }")
, skip <|
test "partially paired brackets" <|
\_ ->
Expect.equal False (isPaired "{[])")
, skip <|
test "simple nested brackets" <|
\_ ->
Expect.equal True (isPaired "{[]}")
, skip <|
test "seceral paired brackets" <|
\_ ->
Expect.equal True (isPaired "{}[]")
, skip <|
test "paired and nested brackets" <|
\_ ->
Expect.equal True (isPaired "([{}({}[])])")
, skip <|
test "unopened closing brackets" <|
\_ ->
Expect.equal False (isPaired "{[)][]}")
, skip <|
test "unpaired and nested brackets" <|
\_ ->
Expect.equal False (isPaired "([{])")
, skip <|
test "paired and wrong nested brackets" <|
\_ ->
Expect.equal False (isPaired "[({]})")
, skip <|
test "math expression" <|
\_ ->
Expect.equal True (isPaired "(((185 + 223.85) * 15) - 543)/2")
, skip <|
test "complex latex expression" <|
\_ ->
Expect.equal True (isPaired "\\left(\\begin{array}{cc} \\frac{1}{3} & x\\\\ \\mathrm{e}^{x} &... x^2 \\end{array}\\right)")
]
module BracketPush exposing (isPaired)

type Symbol
= Bracket
| Brace
| Parenthese

type PairSymbol
= Open Symbol
| Close Symbol
| Unknown

parseSymbol : Char -> PairSymbol
parseSymbol c =
case c of
'[' ->
Open Bracket

']' ->
Close Bracket

'(' ->
Open Parenthese

')' ->
Close Parenthese

'{' ->
Open Brace

'}' ->
Close Brace

_ ->
Unknown

parse : String -> List PairSymbol
parse =
String.toList >> List.map parseSymbol

buildStack : List PairSymbol -> List PairSymbol -> List PairSymbol
buildStack stack list =
case ( stack, list ) of
( _, [] ) ->
stack

( _, Unknown :: ys ) ->
buildStack stack ys

( [], y :: ys ) ->
buildStack [ y ] ys

( (Open os) :: xs, ((Close cs) as currentSymbol) :: ys ) ->
if os == cs then
buildStack xs ys

else
buildStack (currentSymbol :: xs) ys

( _, y :: ys ) ->
buildStack (y :: stack) ys

isPaired : String -> Bool
isPaired =
parse >> buildStack [] >> List.isEmpty