Avatar of milogert

milogert's solution

to Matching Brackets in the Elm Track

Published at May 20 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!

Source

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)


isPaired : String -> Bool
isPaired input =
    let
        inputChars =
            input
                |> String.filter (\b -> isOpen b || isClose b)
                |> String.toList
    in
    case inputChars of
        [] ->
            True

        next :: rest ->
            doCheck [] next rest


doCheck : List Char -> Char -> List Char -> Bool
doCheck brackets currentChar nextBrackets =
    case nextBrackets of
        [] ->
            case brackets of
                [] ->
                    False

                prev :: prevRest ->
                    matches prev currentChar

        next :: rest ->
            if isOpen currentChar then
                doCheck
                    (currentChar :: brackets)
                    next
                    rest

            else if isClose currentChar then
                case brackets of
                    [] ->
                        False

                    prev :: prevRest ->
                        if matches prev currentChar then
                            doCheck
                                prevRest
                                next
                                rest

                        else
                            False

            else
                False


isOpen : Char -> Bool
isOpen b =
    List.member b [ '(', '[', '{' ]


isClose : Char -> Bool
isClose b =
    List.member b [ ')', ']', '}' ]


matches : Char -> Char -> Bool
matches open close =
    case ( open, close ) of
        ( '(', ')' ) ->
            True

        ( '[', ']' ) ->
            True

        ( '{', '}' ) ->
            True

        _ ->
            False

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.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?