Coding a subtraction loop that takes in a list

Question

I am making a calculator and need a subtraction loop for the code to work. Right now it is scanning the list but only doing the first and last numbers not the entire list.

if(a.equals("Subtract")) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        System.out.println("Enter integers one at a time in order");
        System.out.println("When completed type done");
        while (scan.hasNextInt()) {
            list.add(scan.nextInt());
        }
        Integer[] nums = list.toArray(new Integer[0]);
        for (int i = 0; i < nums.length-1; i++) {
            sum = nums[0];
            sum = sum - nums[i+1];
        }
        System.out.println("Your total is " + sum);
        System.out.println("-----------------------------");
        main(args);
        }

Only subtracts the first and last numbers of loop not entire loop in order

You're assigning sum each iteration in the loop. Also, I'm not sure what the point of sum -= sum - nums[i+1] is because you're subtracting sum from itself. — Compass
– Compass, Commented May 28, 2019 at 15:15
Run the loop line by line to see what it is doing, if the logic isn’t clear from the code itself. You’re assigning sum at every iteration, which probably isn’t what you want. And the line sum -= sum - nums[i+1] doesn’t seem rational because it basically means sum = sum + nums[i+1] — Sami Kuhmonen
– Sami Kuhmonen, Commented May 28, 2019 at 15:15

Conffusion · Accepted Answer · 2019-05-29 07:18:34Z

1

You're almost there. Move the assignment of the first value outside the loop and start the loop from the second element of the array (index=1):

sum = nums[0];
for (int i = 1; i < nums.length; i++) {
    sum = sum - nums[i];
}

EDIT: Thanks Joakim for your remark. Fixed the end condition in the for loop

edited May 29, 2019 at 7:18

answered May 28, 2019 at 15:48

Conffusion

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1 Comment

Joakim Danielson Over a year ago

This doesn't generate the correct result for obvious reasons. Please test your code.

Joakim Danielson · Accepted Answer · 2019-05-28 15:53:18Z

0

I suspect Java 8 streams is not what OP is after but for completeness here is a way to make use of the List instead of a for loop using streams

if (!list.isEmpty) {
    sum = list.get(0) - list.subList(1, list.size()).stream().reduce(0,Integer::sum);
}

If a loop is prefered then at least use the list already created

sum = list.get(0);
for (int i = 1; i < list.size(); i++) {
    sum -= list.get(i);
}

edited May 28, 2019 at 15:53

answered May 28, 2019 at 15:45

Joakim Danielson

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Comments

Dave Costa · Accepted Answer · 2019-05-28 17:15:18Z

0

You're re-initializing sum on each iteration of the loop, to the value of the first element:

sum = nums[0];

You should do this before the loop.

edited May 28, 2019 at 17:15

answered May 28, 2019 at 15:19

Dave Costa

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