编写一个名为 arraySum 的 function，它返回 int 值的二维数组中所有值的总和（作为 int）（Java）

Question

I am trying to sum all the int values of a 2D array. I named it array. my function is then called arraySum. If arraySum is null, I return 0. Otherwise, it goes through two for-loops, summing the values of these arrays.

int i = 0;
int j = 0;
static int sum = 0;
int[][] array = new int[i][j];
static int arraySum(int[][] array) { 
    if (array == null) {  // test if the incoming param is null
        return 0;
    } else { 
        for (int i = 0; i < array.length; i++) {  // length of the outer array
            for (int j = 0; j < array[i].length; j++) {  // length of the inner array
                sum += array[i][j];
            }
        }
        return sum;  // moved out of the loop
    }
} 

my error message:

java.lang.AssertionError: Incorrect result:  expected [-217085] but found [-308126] 

Answer 1

1. Fixing the method signature is the first step.
2. Then you'll need to fix the null check.
3. Then your loops need to check the size of the inner and outer arrays.
4. Move the return statement.

Here's the fixed code:

static int arraySum(int[][] array) {  // fix the signature
    if (array == null) {  // test if the incoming param is null
        return 0;
    } else { 
        int sum = 0;  // you need this in the scope of the method - it will be returned at the end
        for (int i = 0; i < array.length; i++) {  // length of the outer array
            for (int j = 0; j < array[i].length; j++) {  // length of the inner array
                sum += array[i][j];
            }
        }
        return sum;  // moved out of the loop
    }
}

Edit: I've concentrated on just the method now - how you call it is up to you. Please note that the method will not affect any externally defined sum variable. It will return the sum and it's up to the caller to store that value somewhere.

Answer 2

Avoid using primitive for statement. See following:

    static int arraySum(int[][] array) {
        int sum = 0;
        if (array == null) return 0;

        for(int[] row: array){
            for(int col : row) {
                sum += col;
            }
        }
        return sum;
    }

Answer 3

Streams can do this too:

//Create 2D array
int[][] array = {{1,2,3},{4,5},{6}}; 

//Sum all elements of array
int sum = Stream.of(array).flatMapToInt(IntStream::of).sum(); 

System.out.println(sum);

Picking apart the summing code:

---

Stream.of(array)

Turns the 2D array of type int[][] into a Stream<int[]> . From here, we need to go one level deeper to process each child array in the 2D array as the stream only streams the first dimension.

So at this point we'll have a stream that contains:

• an int[] array containing {1,2,3}
• an int[] array containing {4,5}
• an int[] array containing {6}

---

.flatMapToInt(IntStream::of)

So far we have a Stream of int[] , still two-dimensional. Flat map flattens the structure into a single stream of ints.

flatMapToInt() expands each int[] element of the stream into one or more int values. IntStream.of() takes an array int[] and turns it into an int stream. Combining these two gives a single stream of int values.

After the flattening, we'll have an int stream of:

{1,2,3,4,5,6}

---

.sum()

Now we have an IntStream with all elements expanded out from the original 2D array, this simply gets the sum of all the values.

---

Now the disclaimer - if you are new to Java and learning about arrays, this might be a little too advanced. I'd recommend learning about nesting for-loops and iterating over arrays. But it's also useful to know when an API can do a lot of the work for you.