这个java post增量运算符在做什么？

Question

So there's a piece of an radix sort implemented Java code that reads as below:

aux[count[a[i]]++] = a[i];

Why use a post increment operator? Why not aux[count[a[i]]+1] ? Is the post increment simply to increment the value in count[a[i]] by 1 and store it there?

radixSort.java

int N = a.length;
int[] count = new int[R+1];
for (int i = 0; i < N; i++)
  count[a[i]+1]++;
for (int r = 0; r < R; r++)
  count[r+1] += count[r];

for (int i = 0; i < N; i++)
  aux[count[a[i]]++] = a[i];
for (int i = 0; i < N; i++)
  a[i] = aux[i];

Answer 1

Is the post increment simply to increment the value in count[a[i]] by 1 and store it there?

Yes, exactly. There are two side-effects of the statement: one is a modification to an element of aux , and the other is a modification to an element in count .

Personally I'd avoid writing it like that - I'd probably write:

// We don't use i other than to index into a, so use an
// enhanced for loop instead
for (int value : a)
{
    aux[count[value]] = value;
    count[value]++;
}

Note that even if the change to count weren't required, aux[count[a[i]]+1] wouldn't do the same thing - because aux[count[a[i]]++] refers to the element in aux with index count[a[i]] , before the increment, because ++ is being used as a post-increment here.

Answer 2

This aux[count[a[i]]++] = a[i]; means:

first, take a[i] value, and use it as index for count. Then that value use as index in aux array and place the value of a[i] to its place. Then increment the value on position count[a[i]] for 1 .

Its like this:

aux[count[a[i]]] = a[i];
count[a[i]] = count[a[i]] + 1;

but shorter.

You have also this: aux[++count[a[i]]] = a[i]; . This will take value from count[a[i]] and first increment for 1 and then use it as index in aux array, so now we search it as:

count[a[i]] = count[a[i]] + 1;
aux[count[a[i]]] = a[i];

As you can see, aux[count[a[i]]+1] is not like aux[count[a[i]]++] , since it will not store increased value of a[i] for 1 in a[i] , and it will take index a[i] + 1 from aux , but aux[count[a[i]]++] takes index of a[i] .

aux[count[a[i]]+1] is similar to aux[++count[a[i]]] but the difference is you didn't increment the value in count[a[i]] .