Android/Kotlin 中的操作顺序

Question

Could anyone explain to me why this code works:

 if (questionList[currentIndex].answer == inputAnswer) {
        correctAnswers += 1*100/questionList.size
        percentage.text = "Правильно: $correctAnswers%"

and this one doesn't:

 if (questionList[currentIndex].answer == inputAnswer) {
            correctAnswers += 1/questionList.size*100
            percentage.text = "Правильно: $correctAnswers%"

When I click a button, set up with the first bit of code, everything works fine and my textView gets updated, but when I change the order of operations, nothing happens (textView.text doesn't change it's value).

Am I missing something?

Thank you in advance!

Answer 1

When you do math with integers, fraction components are not preserved. Suppose the size of the question list is 10.

In your first block of code, you have

1 * 100 / 10

Operations are done from left to right, so after the first multiplication, you have

100 / 10

and that resolves to 10.

Now with your second block of code you have

1 / 10 * 100

The first division with floating point numbers would be 0.1, but with integers, since the fraction is not preserved, it evaluates to 0.

0 * 100

which resolves to 0. So it will always result in zero if the dividend is smaller than the divisor.

If you really want to present fractional numbers, you should use floating point numbers, and if you want the result to be an Int, use roundToInt() on the result. If you just use toInt() , it will just drop the fraction rather than rounding to the nearest integer.