Need help understanding this C Code (Array) - c

Question

I need help to understand this code clearly, please help. I can't figure out how this program keep track of how many number has given in responses array.
I don't understand what's going on the for loop and specially this line ++frequency[responses[answer]];
#include<stdio.h>
#define RESPONSE_SIZE 40
#define FREQUENCY_SIZE 11
int main(void)
{
int answer; /* counter to loop through 40 responses */
int rating; /* counter to loop through frequencies 1-10 */
/* initialize frequency counters to 0 */
int frequency[FREQUENCY_SIZE] = {0};
/* place the survey responses in the responses array */
int responses[RESPONSE_SIZE] = {1,2,6,4,8,5,9,7,8,10,1,6,3,8,6,10,3,8,2,7,6,5,7,6,8,6,7,5,6,6,5,6,7,5,6,4,8,6,8,10};
/* for each answer, select value of an element of array responses
and use that value as subscript in array frequency to determine element to increment */
for(answer = 0 ; answer < RESPONSE_SIZE; answer++){
++frequency[responses[answer]];
}
printf("%s%17s\n", "Rating", "Frequency");
/* output the frequencies in a tabular format */
for(rating = 1; rating < FREQUENCY_SIZE; rating++){
printf("%6d%17d\n", rating, frequency[rating]);
}
return 0;
}

Answer 1

++frequency[responses[answer]] is a dense way of writing
int r = response[answer];
frequency[r] = frequency[r] + 1;
with the caveat that frequency[r] is only evaluated once.
So, if answer equals 0, then responses[answer] equals 1, so we add 1 to frequency[1].
Edit
The following table shows what happens to frequency through the loop (old value => new value):
answer response[answer] frequency[response[answer]]
------ ---------------- ---------------------------
0 1 frequency[1]: 0 => 1
1 2 frequency[2]: 0 => 1
2 6 frequency[6]: 0 => 1
3 4 frequency[4]: 0 => 1
... ... ...
10 1 frequency[1]: 1 => 2
etc.

Answer 2

for(answer = 0 ; answer < RESPONSE_SIZE; answer++){
++frequency[responses[answer]]; // <---
}
This above loop just counts the number of times a number appear in array responses and that is stored at that number's index in array frequency. This line does that in first loop -
++frequency[responses[answer]];
So , it increments value at index responses[answer] of array frequency.
Lets say responses[answer] has value 1 , then value at index 1 of array frequency is incremented.
Second for loop is just for output as mentioned.