Changing for to while inside a nested for in Java

Question

When I try to change the nested for loop the program doesn't work as supposed

This is the original code:

public class AandB
{
    public static void main (String [] args) {
        int a = 0, b = 2;
        int j = 1;
        for (int i = 2; i>= 1; i--) {
            for(int j = 1; j <=3; j++) {
                if(j%2 == 0) {
                    b = b* 2;
                } else {
                    a= a + b;
                }

                System.out.println("a=" + a + " b=" + b);

            } 

        }

    }
}

It has this output:

a=2 b=2 a=2 b=4 a=6 b=4 a=10 b=4 a=10 b=8 a=18 b=8

I changed this code above so that it could work with a while loop inside the for loop:

public class AandB
{
    public static void main (String [] args) {
        int a = 0, b = 2;
        int j = 1;
        for (int i = 2; i>= 1; i--) {
            while(j <4) {
                if(j%2 == 0) {
                    b = b* 2;
                } else {
                    a= a + b;
                }

                System.out.println("a=" + a + " b=" + b);
                j++;
            } 

        }

    }
}

It has this output:

a=2 b=2 a=2 b=4 a=6 b=4 a=6 b=8

It should output the same results, but I can't get it right. The problem is that after the while loop ends, the for iterates but ignores the while because the condition no longer applies. I suppose I need to clear the variable j? Or create something like a counter variable?

Debug, debug, debug! Simply printing out the values of i and j at different places would have showed you where things where going wrong. — user1531971
– user1531971, Commented Apr 13, 2016 at 2:15

Elliott Frisch · Accepted Answer · 2016-04-13 02:15:18Z

1

Move int j = 1; inside the for loop. That's the difference between your existing for loop and while loop implementations. Something like,

for (int i = 2; i >= 1; i--) {
    int j = 1;
    while (j < 4) {

When your for loop had for(int j = 1; j <=3; j++) { that shadowed the outer j and re-initialized to 1 on every iteration of the outer loop.

edited Apr 13, 2016 at 2:15

answered Apr 13, 2016 at 2:10

Elliott Frisch

202k20 gold badges166 silver badges265 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Adrian Shum · Accepted Answer · 2016-04-13 02:21:30Z

1

As in C/C++/Java 101, for is (almost) equivalent to while by the following conversion:

for (A; B; C) {
  D;
}

Can be written as while in form of:

A;
while(B) {
    D;
    C;
}

So your

for (int i = 2; i>= 1; i--) {
    for(int j = 1; j <=3; j++) {
        // do something
    }
}

Should be written as (if just changing the inner loop)

for (int i = 2; i>= 1; i--) {
    int j = 1;
    while(j <=3) {
        // do something
        j++;
    }
}

answered Apr 13, 2016 at 2:21

Adrian Shum

40.3k11 gold badges89 silver badges134 bronze badges

Comments

fr3d · Accepted Answer · 2016-04-13 02:11:25Z

0

In the original loop, your conditional is "j <=3" in the while loop it changes to "j <=4".

answered Apr 13, 2016 at 2:11

fr3d

35 bronze badges

1 Comment

Rafa_Asp1 Over a year ago

Sorry, I meant j < 4.

Birdie · Accepted Answer · 2016-04-13 02:12:06Z

0

I believe that it's your initialization on the 2nd one. You're saying while(j <=4), meaning that it loops 5 times, rather than on the first one, which loops 2 times.

answered Apr 13, 2016 at 2:12

Birdie

577 bronze badges

Collectives™ on Stack Overflow

Changing for to while inside a nested for in Java

4 Answers 4

Comments

Comments

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

Comments

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related