Vector within vector. What does it do in this context?

Question

Forty students were asked the level of quality of food in the student canteen, on a scale from 0 to 10. ( 1 meaning horrific and 10 meaning excellent). Put the 40 responses into an entire array and summarize the search results.

    public static void main(String[] args) {

    Scanner sc = new Scanner(System.in);
    final int tamanhoResposta = 40, frequencySize = 11;
    int response[] = {5, 10, 6, 7, 3, 7,5 ,6 ,3, 10, 4,5,4,9,8,7,5,6,5,6,4,5,6,7,8,4,3,8,9,10,9,6,5,3,9,5,9,10};
    int frequency[] ={0};


    for(int i = 0; i < tamanhoResposta; i++){
        ++frequency[response[i]];
    }

    System.out.println(" Avaliação ===================== Frequência " );
    for(int ii = 0; ii < frequencySize; ii++){
        System.out.println(ii + " ");
        System.out.println(frequency[ii]);
    }

I don’t understand what this part is for, but what it does?

++frequency[response[i]];

He makes that exception:

Exception in thread "main" java.lang.Arrayindexoutofboundsexception: 5

Answer 1

The operator ++ is an incrementer. If you have a variable called x can do:

x++;

that the same as:

x += 1;

which is the same as:

x = x + 1;

which is the same as:

++x;

The difference is that the latter increments one before using the variable value. This is useful in an expression, but not in a statement, as is the case used. That is, in that case could have written frequency[response[i]]++; that gave in the same. Or could have written frequency[response[i]] = frequency[response[i]] + 1;. A lot longer, right?

Obviously what he’s incrementing is an element of array frequency. Knows arrays, right? What is this element? Is it the first element? That is, it is the 0? It is the 1? It is determined by a variable. In this case it is the response[i];, which in turn is also an element of a array. So he’s taking the element of response determined by i, and this element is being used as an index of frequency.

N]ao confuses the array with its elements. All are variables. The array is a variable that contains a portion of variables, but each element is an individual variable.

A different way of writing the same thing:

int x = response[i];
int y = frequency[x];
y = y + 1;
frequency[x] = y;

It’s become clearer now?