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pogiaron's solution

to Matching Brackets in the Elm Track

Published at Jun 11 2020 · 0 comments
Instructions
Test suite
Solution

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 code you have to write is located inside the src/ directory of the exercise. Elm automatically installs packages dependencies the first time you run the tests so we can start by running the tests from the exercise directory with:

$ elm-test

To automatically run tests again when you save changes:

$ elm-test --watch

As you work your way through the tests suite in the file tests/Tests.elm, 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 Expect
import MatchingBrackets exposing (isPaired)
import String
import Test exposing (..)


tests : Test
tests =
    describe "MatchingBrackets"
        [ 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 MatchingBrackets exposing (isPaired)


isPaired : String -> Bool
isPaired input =
    isPairedHelp input []

isPairedHelp : String -> List Char -> Bool
isPairedHelp input stack =
    case String.uncons input of
        Nothing -> 
            (List.length stack) == 0
        Just (first,rest) ->
            if String.contains (String.fromChar first) "[{(" then
                isPairedHelp rest (first :: stack)
            else if String.contains (String.fromChar first) "]})" then
                if mostRecentlyAdded (openingBracket first) stack then
                    isPairedHelp rest (List.drop 1 stack)
                else
                    False
            else
                isPairedHelp rest stack

mostRecentlyAdded : Char -> List Char -> Bool
mostRecentlyAdded bracket stack =
    case stack of
        [] -> 
            False
        first :: _ -> 
            first == bracket

openingBracket: Char -> Char
openingBracket closingBracket =
    case closingBracket of
        ']' -> '['
        '}' -> '{'
        ')' -> '('
        x -> x

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