problem with loop- for: java

Question

I'm trying to change this code to a for loop, but i have some problems

panel[1].setBackground(Color.red);
            panel[2].setBackground(Color.white);
            panel[3].setBackground(Color.red);
            panel[4].setBackground(Color.white);
            panel[5].setBackground(Color.red);
            panel[6].setBackground(Color.white);
            panel[7].setBackground(Color.red);
            panel[8].setBackground(Color.white);
            panel[9].setBackground(Color.red);
            panel[10].setBackground(Color.white);

new code - for

for (int i = 0; i < panel.length; i++) {
                panel[(i*2)+1].setBackground(Color.red);//i think that is correct, or no?
                panel[(i*3)+1].setBackground(Color.white); //problem here
            }

thanks

Consider the values of (i*2)+1 and (i*3)+1 as your loop progresses. One the first iteration, i = 0, so they will be (0*2)+1 = 1 and (0*3)+1 = 1, so we're already off to a bad start. — Oliver Charlesworth
– Oliver Charlesworth, Commented Feb 23, 2011 at 21:52

Sean Patrick Floyd · Accepted Answer · 2011-02-23 21:52:23Z

8

Use a new-style for loop:

int ct = 0;
for(JPanel panel : panels){
   panel.setBackground((ct % 2 == 1) ? Color.Red : Color.White);
   ct++;
}

answered Feb 23, 2011 at 21:52

Sean Patrick Floyd

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13 Comments

OscarRyz Over a year ago

+1 I would rather have a reference for current color than modulo operator on a int var though. Looks a bit cleaner that way stackoverflow.com/questions/5097601/problem-with-loop-for-java/…

Oliver Charlesworth Over a year ago

Agree with @Oscar. There's little point using an iterator if you have to maintain an index/count independently anyway.

Sean Patrick Floyd Over a year ago

@Fel what does that mean? a) There is an item, but you don't want to change it's color b) you are leaving the zero index of your array empty?

user177800 Over a year ago

@Fel: you do have an [0] you are just not putting anything in it, it is still there and it would cause this answer to fail with a NullPointerException

bestsss Over a year ago

@Sean, you never do any library code, do you? you do have bounds checking and absolutely type safety... Also if you remove bit ops, java is dead on the servers, no protocols for you

|

score 3 · Accepted Answer · 2011-02-23 23:31:13Z

3

Solution

for (int i = 1; i < panel.length; i++)
{
    if ( i % 2 == 0 ) { panel[i].setBackground(Color.white); }
    else { panel[i].setBackground(Color.red); }   
}

Or a more concise expression using the ternary operator:

for (int i = 1; i < panel.length; i++)
{
     panel[i].setBackground( i % 2 == 0 ? Color.white : Color.red );  
}

Explaination

% is the modulo operator, i % 2 == 0 when i is even, != 0 when odd.

Caveats

Your array of panels referencing in your example starts at 1, arrays in Java start at ZERO, you might have a potential one off error here if you have anything in the (first) ZERO array element.

Using the type safe List classes is always better than working with arrays directly, you would not have to deal with the one off error problems you are creating by not using the first array slot.

edited Feb 23, 2011 at 23:31

answered Feb 23, 2011 at 21:53

user177800

4 Comments

x.509 Over a year ago

whats teh point of using if else conditions when you can increment an array with 2?

user177800 Over a year ago

that would not work with a list of odd items, read what the % operator is for.

bestsss Over a year ago

i % 2 == 0 ? panel[i].setBackground(Color.white) : panel[i].setBackground(Color.red); } this will not compile in java

user177800 Over a year ago

@bestsss: fixed it, copy and pasted the wrong thing originally

Owen · Accepted Answer · 2011-02-23 22:00:27Z

3

for(int i = 1; i<panel.length; i++)
{
    if(i%2 == 0)
    {
        panel[i].setBackground(Color.white);
    }
    else
    {
        panel[i].setBackground(Color.red);
    }
}

edited Feb 23, 2011 at 22:00

answered Feb 23, 2011 at 21:52

Owen

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Comments

OscarRyz · Accepted Answer · 2011-02-23 22:34:43Z

3

I would:

Color current = Color.white; 
for( Panel p : panels ) { 
   p.setBackground( current );
   current =  ( current == Color.white ? Color.red : Color.white );
}

edited Feb 23, 2011 at 22:34

answered Feb 23, 2011 at 21:57

OscarRyz

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2 Comments

bestsss Over a year ago

hi! in the 2nd example you have red/white, in the 1st white/red. i.e. need to start w/ red (before the loop)

OscarRyz Over a year ago

Removed 2nd example as only makes noise

x.509 · Accepted Answer · 2011-02-23 22:09:25Z

-1

for (int i = 1; i < length; i+=2)
{
    panel[i].setBackground(red);
    panel[i+1].setBackground(white);
}

answered Feb 23, 2011 at 22:09

x.509

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1 Comment

user177800 Over a year ago

this will cause an IndexOutOfBoundsException as well as not work odd numbers of items

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