I don't really understand the do { } while structure

Question

I'm trying to learn Java, I studied Pascal in high school and it has the repeat until..; instruction.

I want to solve an exercise where I'm supposed to keep entering numbers until the penultimate + antepenultimate numbers equal the last number I entered.(a[i-2]+a[i-1] = a[i]); I'm doing it without arrays but that doesn't really matter.

In Pascal it would be easy because repeat until is more easier to use For ex it would be

repeat  
...   
until ((a[i-2]+a[i-1] = a[i]) and (n=3)); 

n is the number of values I entered

I can't figure out how to introduce it in Java, so far I did this but it doesn't work if I enter 2 2 4. It should stop but it keeps asking for numbers

    int pen = 0, ant = 0, s = 0, n = 1;
    int ult = input.nextInt();
    s = s + ult;
    do {
        do {
            ant = pen;
            pen = ult;
            ult = input.nextInt();
            n++;
            s = s + ult;
        } while (ant + pen != ult);
        System.out.println(n);
    } while ((n == 3) || (n < 4));

ult is the last number I enter, s is the sum of the numbers entered.

Could anyone tell me how to set the conditions so it will stop if I enter the values 2 2 4?

Answer 1

A Do-While loop runs the code in the loop first. It evaluates the logic last, and then if it's true it repeats the code inside the loop, and so on until the logic is false.

The way to solve tricky problems like this is to get out a sheet of paper and record what each variable does. Step through each line like a debugger and record what's being stored in each variable as the program progresses.

It's the best way to do it. You'll find that you'll gain a deeper understanding of how your programs are working.