I was going through a dp problem, there I have to initialize the matrix with default(-1) values for memoized solution. And felt the need.
We can initialize the 1D array in java like this => Arrays.fill(arr, -1) what for 2D array...?
Otherwise I have to run a nested for loop to initialize the matrix with default values, that includes more lines of code, and doesn't look appropriate when you are solving a dp problem.
Try the approach below:
public void fill2D(int[][] matrix, int value) {
int m = matrix.length;
int n = matrix[0].length;
for (int i = 0; i < m * n; i++) {
int row = i / n;
int col = i % n;
matrix[row][col] = value;
}
}
Usage:
@Test
public void test() {
int[][] matrix = new int[5][6];
fill2D(matrix, -1);
for (int[] row : matrix) {
System.out.println(Arrays.toString(row));
}
}
Benchmark test
private final int M = 500;
private final int N = 500;
@Benchmark
public void method1() {
int[][] matrix = new int[M][N];
for (int i = 0; i < M * N; i++) {
int row = i / N;
int col = i % N;
matrix[row][col] = -1;
}
}
@Benchmark
public void method2() {
int[][] matrix = new int[M][N];
for (int i = 0; i < M; i++) {
for (int j = 0; j < N; j++) {
matrix[i][j] = -1;
}
}
}
@Benchmark
public void method3() {
int[][] matrix = new int[M][N];
for (int[] row : matrix) {
Arrays.fill(row, -1);
}
}
Result
Benchmark Mode Cnt Score Error Units
BenchMarkTest.method1 avgt 20 0,600 ± 0,002 ms/op
BenchMarkTest.method2 avgt 20 0,191 ± 0,003 ms/op
BenchMarkTest.method3 avgt 20 0,216 ± 0,021 ms/op
Try This:
int[][] dp = new int[rows][cols];
for (int[] row : dp) {
Arrays.fill(row, -1);
}
Full Code:
public static void main(String[] args) {
int rows = 5;
int cols = 5;
int[][] dp = new int[rows][cols];
for (int[] row : dp) {
Arrays.fill(row, -1);
}
for (int i = 0; i < rows; i++) {
for (int j = 0; j < cols; j++) {
System.out.print(dp[i][j] + " ");
}
System.out.println();
}
}
Output :
-1 -1 -1 -1 -1
-1 -1 -1 -1 -1
-1 -1 -1 -1 -1
-1 -1 -1 -1 -1
-1 -1 -1 -1 -1
You could combine Arrays.fill with Arrays.setAll using a function. Whether it is more readable or saves many lines compared to a simple loop is up to the viewer:
public static void main(String[] args) {
int row = 4;
int col = 4;
int[][] matrix = new int[row][];
IntFunction<int[]> func = i -> {
int [] arr = new int[col];
Arrays.fill(arr, -1);
return arr;
};
Arrays.setAll(matrix, func::apply);
System.out.println(Arrays.deepToString(matrix));
}
Or you can make use of streams and create your 2d array:
public static void main(String[] args) {
int row = 4;
int col = 4;
int[][] matrixStream = IntStream.range(0, row).mapToObj(i -> IntStream.range(0, col).map(j -> -1).toArray())
.toArray(int[][]::new);
System.out.println(Arrays.deepToString(matrixStream));
}
But under the hood, all approaches use a loop
fillin classArrays? It contains aforloop, so in essence you are executing a loop even when you are initializing a one-dimensional array with that method.new int[row][col]rather than doubly initialise the arrays.Arrays.xxx()do not return the array as you could then have usedmatrix = setAll(new int[row][], i -> fill(new int[col], -1));