如何找到数组列表中属于某个范围的元素数？

Question

I'm making a program which allows the user to enter in mark values, and it outputs the number of students in a certain mark range.

Ex. Level 2 ( (mark >= 60) && (mark < 70) )

Level 3 ( (mark >= 70) && (mark < 80) )

Level 4 (mark >= 80).

I thought of using a for loop and if statements to see which range each mark falls into, but I can't figure out how to count how many of them fall into which category.

ArrayList<Integer> marks = new ArrayList<>();

 private void btnSortActionPerformed(java.awt.event.ActionEvent evt) {                                        

    Collections.sort(marks);
    String output = "";
    for (int i=0; i<marks.size(); i++) {
        output += marks.get(i) + "\n";
    }
    txtOutputSort.setText(output);
}       

Answer 1

You can use an array of ints for each count. For example (if you have 2 levels):

ArrayList<Integer> marks = new ArrayList<>();
int[] marksCount = new int[2];
//initialize each int in marksCount
for (int i=0; i<marksCount.length; i++) {
marksCount[i] = 0;
}
...
for (int i=0; i<marks.size(); i++) {
if(marks.get(i)<60)
marksCount[0]++;
else if(marks.get(i)>=60 && marks.get(i)<70)
marksCount[1]++;

Now, you have the marks of each level counted and stored within the marksCount array.

Answer 2

The idea of using a for loop with if-else cases is a viable option. For the ranges that you have provided, the following code should work.

int marks_60_70 = 0;
int marks_70_80 = 0;
int marks_80 = 0;  

for(int mark: marks) {
    if((mark >= 60) && (mark < 70)) {
        marks_60_70++;
    } else if ((mark >= 70) && (mark < 80)) {
        marks_70_80++;
    } else if(mark >= 80) {
        marks_80++;
    }
}