first time user syntax help

goaddr

Hey, I have a 2048x2048 matrix with values of either 0 or 1 which is called "white". I need to find the total number of 3x3 blocks in the matrix where each data point has a value of 1, ie,

1 1 1
1 1 1
1 1 1

Im a first time user and I think that my problem is syntax. My code is as follows,

x=0

for i=2:2047 j=2:2047

if white(i-1,j-1)==1 & white(i-1,j)==1 & white(i-1,j+1)==1 & white(i,j-1)==1 & white(i,j)==1 & white(i,j+1)==1 & white(i+1,j-1)==1 & white(i+1,j)==1 & white(i+1,j+1)==1

x=x+1

end

y=sum(x)

end

Ive been at it for a few hours and any help would be really appreciated. Ill be spending the rest of the afternoon trying to solve this problem....

Thanks for your help!

drasnor

To loop through two variables you need a nested loop. The structure for that looks like:[php]for i=1:10
for j=1:10
things = do(stuff);
end
end[/php]As an aside, 'x' is only a single element so y = sum(x) will always be equal to x.

kryyst

Just setting aside that your initial syntax is wrong. From a logical side it just wouldn't work either because as you have it, it would increment i & j equally. Meaning you'd only go through your grid on a diagonal so you'd be checking:

2:2, 3:3, 4:4 etc.... You'd miss situations like 2:3, 2:4 etc....

That's why you need to do it in 2 loops just from a basic logical reason.

Also if you have a grid thats is 2048x2048 you don't need to do a loop that goes from 1-2048 you only need to loop from 1-2046 because if you are checking a 3x3 grid then you don't need to do a check when you are on the 2nd from the last outer edges.

drasnor

C'mon kryyst, you know as well as I do that MATLAB array indices are from 1 to n, not 0 to n-1. He's got it right.

kryyst

You know I read that three times last night and never noticed that he was going from 2-2047 in his loop. I'm guess that's why I probably stop myself from trying to code any of my own things past 3 beers and 10pm.