CODEWITHSUNDEEP
javaloops
user193113
user193113

Reputation: 55

helping me understand what happens in this loop in java

PrintStream out = System.out;

    int sum = 0;
    for(int i = 0; i<5; i++)
        for(int j=i; j<5; j++)
            sum++;
    out.println(sum);

outputs: 15. i dont know how this gets 15. i tried solving it in my head, i keep getting 25.

If we ignore the second for loop, and put sum++ in the first for loop, you get 5. But how do we get 15 from the 2nd for loop?

PrintStream out = System.out;

    int sum = 0;
    for(int i = 0; i<5; i++)
        for(int j=i; j<5; j++);
            sum++;
    out.println(sum);

outputs: 1. What happens if you put ; after for statement. does it stop?

Upvotes: 1

Views: 3069

Answers (4)

assylias
assylias

Reputation: 328598

i = 0 => j goes from 0 to 4 => sum = 5  
i = 1 => j goes from 1 to 4 => sum = 5 + 4 = 9  
i = 2 => j goes from 2 to 4 => sum = 9 + 3 = 12  
i = 3 => j goes from 3 to 4 => sum = 12 + 2 = 14  
i = 4 => j goes from 4 to 4 => sum = 14 + 1 = 15

your second code snippet does not compile as it is.

EDIT

Now that your second snippet compiles:

for(int i = 0; i<5; i++)
    for(int j=i; j<5; j++);
        sum++;

is equivalent to (and this is why you should always use braces):

for(int i = 0; i<5; i++) {
    for(int j=i; j<5; j++) {
    }
}
sum++;

That explains why it outputs 1.

Upvotes: 6

pb2q
pb2q

Reputation: 59607

The loop is basically creating a triangular number, specifically the 5th triangular number, e.g. a number of the form n + (n - 1) + ... + 2 + 1.

The trick, and the reason that you don't see 25 as the result, is in the second loop: it only iterates from the current value of i up to 5.

On each iteration of the inner loop the sum is incremented once. Because of the loop trick, the inner loop iterates 5 times, then 4, and so on:

first iteration of the outer loop: j = 0 to 4 -> 5 increments
second iteration of the outer loop: j = 1 to 4 -> 4 increments  
... 3 increments
... 2 increments
... 1 increment

5 + 4 + 3 + 2 + 1 = 15

Upvotes: 0

KK.
KK.

Reputation: 783

To get 25, instead of 

    for(int j=i j<5; j++); // inner loop
        sum++;

use

//......here
for(int j=0 j<5; j++); // inner loop
        sum++;

What you are doing now is (1+1+1+1+1) + (1+1+1+1) + (1+1+1) + (1+1) + (1) whereas what you want is (1+1+1+1+1).. 5 times

p.s. why the loop in the second code is for(int j=il j<5; j++); ? il is not defined, and it 'll stop at the ; even if you have defined il somewhere earlier.

Upvotes: 0

jahroy
jahroy

Reputation: 22692

The output is 15 because j is initialized to the value of i in each iteration of the first loop.

The output would be 25 if j was initialized to zero in stead.

Upvotes: 0

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