java：检查array [i] [j]是否在范围内

Question

i have a 2-dimensional array of ints which i'm looping over. i want to check if an array's element i'm pointing at is within the bounds of that array. say, if i had an array of size 3x2, i would like to be able to do this:

for(int i=some_variable;i<arrayWidth;i++) {
    for (int j=other_variable;j<arrayHeight;j++) {
        if (!array[i][j].ISOUTOFBOUNDS) {
            // do something
        }
    }
}

is there an eloquent way of achieving that or should i just check the indices manually?

ps. i and j can turn out to be negative, too, so i would like to check that as well.

Answer 1

You can avoid performing this comparison n² time by pre-computing the valid ranges:

int maxValidI = Math.min(array.length, arrayWidth);
int maxValidJ = Math.min(array[0].length, arrayHeight);
int minValidI = Math.max(0, some_variable);
int minValidJ = Math.max(0, other_variable);

And then iterate i and j from minValidI and minValidJ to maxValidI and maxValidJ .

I hope it will help. Thanks to Andy for notifying me about OP's edit.

Answer 2

Why not just get the length property of the arrays and use them as you limits in the control structure of if

int arrayWidth = array.length;
for(int i = 0; i < arrayWidth; i++) {
    int arraySize = array[i].length;
    for (int j = 0; j < arraySize; j++) {

    }
}

// or if arrayHeight is from somewhere else

for(int i = 0; i < arrayWidth; i++) {
    int arraySize = array[i].length;
    for (int j = 0; j < arrayHeight; j++) {
        if (arrayHeight <= arraySize) {
            // do something
        }
    }
}

Also note array.length is the size of the array, but does not guarantee the index contains a non null value. I would use ArrayList instead since it contains size() which is the size of the non null values.

https://docs.oracle.com/javase/7/docs/api/java/util/ArrayList.html#size()

https://docs.oracle.com/javase/7/docs/api/java/util/Arrays.html#asList(T...)

Answer 3

You can't do it in the form you wrote it, but you can do something close:

if (isInBounds(array,i,j)) { ... }

Then you just need to implement isInbounds .

You could make that method completely generic if you wanted to. Eg the following works for any array or array of arrays or ..., even nonuniform ones (eg an array of different sized arrays). Only catch is it won't work on arrays of primitive types cuz of the cast to Object[] . Probably need more reflection stuff to handle that.

public static boolean isInBounds(Object arrayObj, int...indexes) {
  for (int i = 0; i < indexes.length; i++) {
    if (arrayObj == null || !arrayObj.getClass().isArray()) {
      return false; // we had too many indexes... ended up indexing a leaf element
    }
    Object[] array = (Object[]) arrayObj;
    if (indexes[i] < 0 || indexes[i] >= array.length) {
      return false;
    }
    arrayObj = array[indexes[i]];
  }
  return true;
}

Answer 4

i figured out a different way to do this.

for(int i=some_variable;i<arrayWidth;i++) {
    for (int j=other_variable;j<arrayHeight;j++) {
        try {
            int tmp=array[i][j];
            // do something
        } catch (ArrayIndexOutOfBoundsException e) {
            continue;
        }
    }
}

what do you think of this approach? it saves us from having to pre-compute the ranges and makes the code shorter and easier.