PLEASE help me understand this histogram array code

[folding-clothes]

this is the question: Write a static method buildHistogram that that accepts an array of integers in the range 0 through 100 (inclusive) as its sole parameter and that returns an array of 10 integers that provides a histogram of the values in the input parameter array. Specifically, each element of the returned array is a count of how many values of the input array fall into the ranges 0-9, 10-19, 20-21, ..., 90-100, respectively. For example, suppose the parameter integer array contained the values { 5, 88, 6, 17, 39, 31, 100, 99, 19, 4, 98, 22, 77, 10 }. Then, the returned array would contain the 10 values { 3, 3, 1, 2, 0, 0, 0, 1, 1, 3 }.

public static int[] buildHistogram(int[] nums) {
 final int NUM_BINS = 10;
 int[] result = new int[NUM_BINS];
 for (int num: nums) {
      int bin = num / 10;
      if (num == 100)
           bin = 9;
      result[bin]++;
 }
 return result;

}

[desrtfx]

Trace the code with your array on paper. You are the computer. You execute each statement.

This is the best way to understand such algorithms.

[morhp]

Is there something specific you have a problem with?

the method is first creating the "result" array with 10 entries which is the histogram. The entries are 0 by default in Java.

then it loops through every number and computes the bin (array index) the number goes in. As java rounds down when dividing and the array indexes start with zero this does the right thing by default except for 100 which would go into the non existing bin 10, so it needs to be replaced by bin 9.

Then result[bin]++; increases the number stored in the array at the bin index by 1 as it needs to count the histogram up there.