java - Multiply elements with other elements in a list

Question

I'm new to programming and I'm writing a program in Java.

How do I multiply every element in a list with every other element i the list? Like this: [1, 3, 5, 7] should be multiplied like this: 1 * 3 + 1 * 5 + 1 * 7 + 3 * 5 + 3 * 7 + 5 * 7

How do I write an algorithm for this? I know it is something in this way but my mind can't figure out what I have to add / change.

    for (int index = 0; index < list.size(); index++) {
        sum += (list.get(index) * (list.get());
    }

Thank you!

question from:https://stackoverflow.com/questions/65831115/multiply-elements-with-other-elements-in-a-list

Answer 1

These approaches are provided based on your implied requirement that you must explicitly multiply each pair of elements and take their sum. There are other ways to achieve the sum that are more effiicent.

You need a nested loop.

• the outer loop starts at i = 0.
• the inner loop starts a k = i+1 = 1.
• this ensures the sum doesn't include the square of each entry.

List<Integer> nums = List.of(1,3,5,7);
int sum = 0;
for (int i = 0; i < nums.size(); i++) {
    for (int k = i+1; k < nums.size(); k++) {
        sum += nums.get(i)*nums.get(k);
    }
}
System.out.println(sum);

Prints

86

You could also do it like this using an enhanced for loop as the outer loop. But the first method is, imo, more straight forward.

int sum = 0;
int i = 1; 
for (int v : nums) {
    for (int k = i++; k < nums.size(); k++) {
        sum += v*nums.get(k);
    }
}