Manipulating an Array into a New Array

Question

I am trying to write a method that takes an array, and adds adjacent cells together to produce a new array. It's easier to explain with examples:

{11, 4, 3, 18, 23, 9} --> {15, 21, 32}

{5, 5, 21, 13, 1} --> {10, 34} (the sixth cell is just ignored)

public static int[] collapse(int[] a) {
    int[] b = new int[a.length / 2];
    for (int j = 0 ; j < (a.length - 1) ; j+2) {    //goes thru original array
        for (int i = 0 ; i < b.length ; i++) {      //populates new array
            b[i] = a[j] + a[j+1];
        }
    }
    return b;
}

I figured this required using nested for-loops, the first one to go thru the original array, and the second one to populate the new array. I know that the j+2 in the first for-loop is syntactically incorrect, but I cannot figure out another way to accomplish the same idea.

ajb · Accepted Answer · 2014-04-03 23:22:35Z

2

Problem #1: The third expression in a for usually needs to be something that modifies the index. j+2 by itself doesn't do that; j = j + 2 would. (Or j += 2.)

Problem #2: You do not want a nested for loop. What this does is: first it sets j to 0 and lets i go through the whole b array. Then it increments j (after you fix the first problem) and lets i go through the whole b array again. This isn't what you want; you want them to be increased in parallel.

One way is just to declare i as a variable outside the loop:

int i = 0;
for (int j = 0; j < a.length - 1; j += 2) {
     b[i++] = a[j] + a[j + 1];
}

making sure the body of the loop increments i. Or, you can get fancy:

for (int i = 0, j = 0; j < a.length - 1; i++, j += 2) {
    b[i] = a[j] + a[j + 1];
}

and let the for increment both indexes.

answered Apr 3, 2014 at 23:22

ajb

31.8k4 gold badges63 silver badges86 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Jason · Accepted Answer · 2014-04-03 23:18:44Z

1

Don't use two loops. Just keep a counter (outputIndex) that represents your position in the output array, and increment it each time after you use it:

public static int[] collapse(int[] a) {
    int[] b = new int[a.length / 2];
    int outputIndex = 0;
    for (int j = 1 ; j < a.length ; j+=2) {    //goes thru original array
        b[outputIndex++] = a[j - 1] + a[j];
    }
    return b;
}

Also, I started the loop at j = 1, so that it handles the case where you are missing the last item to be added. Just remember that the two items you are adding are at [j-1] and [j], not [j] and [j+1].

answered Apr 3, 2014 at 23:18

Jason

11.8k3 gold badges45 silver badges48 bronze badges

Comments

geoff · Accepted Answer · 2014-04-03 23:20:12Z

0

Maybe you meant j=j+2, not just j+2?

In fact, this can be done with one for loop.

for(int i = 0; i < oldArray.length - oldArray.length % 2; i+=2) {
    newArray[i/2] = oldArray[i] + oldArray[i+1]
}

The i < oldArray.length - oldArray.length % 2 ensures that for an odd array, the last element is ignored.

answered Apr 3, 2014 at 23:20

geoff

2,2741 gold badge19 silver badges36 bronze badges

Collectives™ on Stack Overflow

Manipulating an Array into a New Array

3 Answers 3

Comments

Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related