如何在2D数组上用foreach循环表达for循环？

Question

public static boolean doit(int[][] a, int b){
    for(int i=0; i<a.length; i++){
        for (int j = 0; j < a[i].length; j++)
        {
            if(a[i][j] ==b) {
                return true;
            }
        }
    }
    return false;
}

So basically I want to use the ForEach loop, for checking if the array contains the int b , but I'm not sure how to use it.

public static boolean doitTwo(int[][] a, int b){
    for(c :a){
        for ( d:a){
              if(a[c][d] ==b) {
                return true;
            }
        }
    }

}

Answer 1

Nearly there, but you're iterating over a in both loops when the inner loops should be using c . Also, when you are iterating over the values instead of the indexes like this, you don't need to index into the arrays (like you were doing with a[c][d] ), as you already have the values in hand.

public static boolean doitTwo(int[][] a, int b){
    for(int[] c : a){
        for (int d : c){
              if(d ==b) {
                return true;
            }

    }

}

I also added types to your for loops, not strictly necessary as they can be inferred, but I prefer being explicit in Java.

The first for loop c : a takes a and iterates over its values. As it's a 2D array, each of it's elements are a 1D array! You then iterate over that 1D array and each values of the 1D array is int.

Example pseudocode:

# Original 2D array which is an array, containing arrays of ints. i.e. int[][]
x = [[1, 2, 3], [4, 5, 6];

for (a : x) {
  # at this point on the first iteration, a = [1,2,3]. i.e. int[]
  for (b : a) {
    # at this point on the first iteration, b = 1. i.e. int
  }
}