The challenge
Create a function multiplyAll/multiply_all which takes an array of integers as an argument. This function must return another function, which takes a single integer as an argument and returns a new array.
The returned array should consist of each of the elements from the first array multiplied by the integer.
Example:
multiplyAll([1, 2, 3])(2) = [2, 4, 6];
You must not mutate the original array.
The solution in Java code
Option 1:
import static java.util.stream.IntStream.of;
import java.util.function.Function;
public class Solution {
public static Function<Integer,int[]> multiplyAll(int[] array) {
return i -> of(array).map(a -> a*i).toArray();
}
}
Option 2:
import java.util.Arrays;
import java.util.function.Function;
public class Solution {
public static Function<Integer, int[]> multiplyAll(int[] array) {
return (n) -> Arrays.stream(array).map(x -> x*n).toArray();
}
}
Test cases to validate our solution
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.junit.jupiter.api.Assertions.assertArrayEquals;
import java.util.Arrays;
import java.util.concurrent.ThreadLocalRandom;
class SolutionTest {
@Test
void isArray() {
assertEquals(int[].class, Solution.multiplyAll(new int[]{1}).apply(1).getClass());
}
@Test
void hasCorrectLength() {
assertEquals(2, Solution.multiplyAll(new int[]{1, 2}).apply(1).length);
}
@Test
void basicTests() {
assertArrayEquals(new int[]{1, 2, 3}, Solution.multiplyAll(new int[]{1, 2, 3}).apply(1));
assertArrayEquals(new int[]{2, 4, 6}, Solution.multiplyAll(new int[]{1, 2, 3}).apply(2));
assertArrayEquals(new int[]{0, 0, 0}, Solution.multiplyAll(new int[]{1, 2, 3}).apply(0));
assertArrayEquals(new int[0], Solution.multiplyAll(new int[0]).apply(10), "should return an empty array");
}
@Test
void randomTests() {
ThreadLocalRandom rnd = ThreadLocalRandom.current();
for(int i = 0; i < 200; ++i) {
int l = rnd.nextInt(10);
int[] array = rnd.ints(0, 100).limit(l).toArray();
int n = rnd.nextInt(100);
int[] expected = Arrays.stream(array).map(v -> v * n).toArray();
assertArrayEquals(expected, Solution.multiplyAll(array).apply(n));
}
}
}
