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rootulp's solution

to Accumulate in the Java Track

Published at Jul 13 2018 · 2 comments
Instructions
Test suite
Solution

Implement the accumulate operation, which, given a collection and an operation to perform on each element of the collection, returns a new collection containing the result of applying that operation to each element of the input collection.

Given the collection of numbers:

  • 1, 2, 3, 4, 5

And the operation:

  • square a number (x => x * x)

Your code should be able to produce the collection of squares:

  • 1, 4, 9, 16, 25

Check out the test suite to see the expected function signature.

Restrictions

Keep your hands off that collect/map/fmap/whatchamacallit functionality provided by your standard library! Solve this one yourself using other basic tools instead.

Running the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.

Source

Conversation with James Edward Gray II https://twitter.com/jeg2

Submitting Incomplete Solutions

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

AccumulateTest.java

import org.junit.Test;
import org.junit.Ignore;

import java.util.Arrays;
import java.util.LinkedList;
import java.util.List;

import static org.junit.Assert.assertEquals;

public class AccumulateTest {


    @Test
    public void emptyAccumulateProducesEmptyAccumulation() {
        List<Integer> input = new LinkedList<>();
        List<Integer> expectedOutput = new LinkedList<>();
        assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x * x));
    }

    @Ignore("Remove to run test")
    @Test
    public void accumulateSquares() {
        List<Integer> input = Arrays.asList(1, 2, 3);
        List<Integer> expectedOutput = Arrays.asList(1, 4, 9);
        assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x * x));
    }

    @Ignore("Remove to run test")
    @Test
    public void accumulateUpperCases() {
        List<String> input = Arrays.asList("hello", "world");
        List<String> expectedOutput = Arrays.asList("HELLO", "WORLD");
        assertEquals(expectedOutput, Accumulate.accumulate(input, x -> x.toUpperCase()));
    }

    @Ignore("Remove to run test")
    @Test
    public void accumulateReversedStrings() {
        List<String> input = Arrays.asList("the quick brown fox etc".split(" "));
        List<String> expectedOutput = Arrays.asList("eht kciuq nworb xof cte".split(" "));
        assertEquals(expectedOutput, Accumulate.accumulate(input, this::reverse));
    }

    private String reverse(String input) {
        return new StringBuilder(input).reverse().toString();
    }

    @Ignore("Remove to run test")
    @Test
    public void accumulateWithinAccumulate() {
        List<String> input1 = Arrays.asList("a", "b", "c");
        List<String> input2 = Arrays.asList("1", "2", "3");
        List<String> expectedOutput = Arrays.asList("a1 a2 a3", "b1 b2 b3", "c1 c2 c3");
        assertEquals(expectedOutput, Accumulate.accumulate(
                input1, c ->
                        String.join(" ", Accumulate.accumulate(input2, d -> c + d))
        ));
    }
}
import java.util.List;
import java.util.ArrayList;
import java.util.function.Function;

public class Accumulate {
  public static <T> List<T> accumulate(List<T> list, Function<T, T> f) {
    List<T> results = new ArrayList<>();

    for (T item : list) {
      results.add(f.apply(item));
    }

    return results;
  }
}

Community comments

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Avatar of Zapata

It is always best to initialize collections size, otherwise you could have performance issues.

Avatar of Zapata

It is always best to initialize collections size, otherwise you could have performance issues.

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