"Index exceeds matrix dimensions." But it doesn't

Masks=[0 1 0;...
    1 -4 1;...
    0 1 0;];

a=imread('C:\Users\***\Desktop\Matlab\figure\Fig3.40(a).jpg');
x=size(a);
m=x(1,1);
n=x(1,2);
b(m,n)=0;
Newres=uint8(b);
for i=1:m
    for j=1:n
        if i==1 && j==1
        Newres(i,j)=Masks(2,2)*a(i,j)+Masks(2,3)*a(i,j+1)+Masks(3,2)*a(i+1,j)+Masks(3,3)*a(i+1,j+1);
        elseif i==1 && 1<j<=n-1
            Newres(i,j)=Masks(2,1)*a(i,j-1)...
                +Masks(2,2)*a(i,j)...
                +Masks(2,3)*a(i,j+1)... %%<-----Line 18
                +Masks(3,1)*a(i+1,j-1)...
                +Masks(3,2)*a(i+1,j)+Masks(3,3)*a(i+1,j+1); %%<--------Line 20
        elseif i==1 && j==n
            Newres(i,j)=Masks(2,1)*a(i,j-1)...
                +Masks(2,2)*a(i,j)...
                +Masks(3,1)*a(i+1,j-1)...
                +Masks(3,2)*a(i+1,j);
        elseif 1<i<=m-1 && j==1
            Newres(i,j)=Masks(1,2)*a(i-1,j)...
                +Masks(1,3)*a(i-1,j+1)+Masks(2,2)*a(i,j)...
                +Masks(2,3)*a(i,j+1)+Masks(3,2)*a(i+1,j)+Masks(3,3)*a(i+1,j+1); %%<--- Line 29
        elseif 1<i<=m-1 && j==n
            Newres(i,j)=Masks(1,1)*a(i-1,j-1)...
                +Masks(1,2)*a(i-1,j)+Masks(2,1)*a(i,j-1)...
                +Masks(2,2)*a(i,j)+Masks(3,1)*a(i+1,j-1)+Masks(3,2)*a(i+1,j); %%<---- Line 33
        elseif i==m && j==1
            Newres(i,j)=Masks(1,2)*a(i-1,j)...
                +Masks(1,3)*a(i-1,j+1)...
                +Masks(2,2)*a(i,j)+Masks(2,3)*a(i,j+1);
        elseif i==m && 1<j<=n-1
            Newres(i,j)=Masks(1,1)*a(i-1,j-1)+Masks(1,2)*a(i-1,j)...
                +Masks(1,3)*a(i-1,j+1)+Masks(2,1)*a(i,j-1)...
                +Masks(2,2)*a(i,j)+Masks(2,3)*a(i,j+1);
        elseif i==m && j==n
            Newres(i,j)=Masks(1,1)*a(i-1,j-1)...
                +Masks(1,2)*a(i-1,j)...
                +Masks(2,1)*a(i,j-1)+Masks(2,2)*a(i,j);
        else
        Newres(i,j)=Masks(1,1)*a(i-1,j-1)+Masks(1,2)*a(i-1,j)...
            +Masks(1,3)*a(i-1,j+1)+Masks(2,1)*a(i,j-1)+...
            Masks(2,2)*a(i,j)+Masks(2,3)*a(i,j+1)+...
            Masks(3,1)*a(i+1,j-1)+Masks(3,2)*a(i+1,j)+...
            Masks(3,3)*a(i+1,j+1);
            
        end
        if Newres(i,j)>255
            Newres(i,j)=0;
        end
        
    end
end

This is a code for image processing. When I run it, it says Index exceeds matrix dimensions. for line 18. When I make a(i,j), this time line 20 gives the same error. On line 20 I turn (j+1) to j and then matlab says line 29. There I turn i+1 to i. And line 33 gives the exact same error. The promblem is matrix dimensions are not exceeded. If you use

for k=1:n-1 a(1,k+1)
end

on your command window you will get results.

So why is this happening?

To better understand me use any image on your computer.

Answer 1

[Walter Roberson]

You set m and n to size() of the image. You have for j=1:n . Your loop tries to index the image a(i,j+1) . But when j reaches n in the loop, reaches the number of columns in the image, then a(i,j+1) tries to go one past the number of columns.

[Arda Nova]

I don't understand the problem

There is a elseif part for i==1 and j==n

So, it shouldn't even go to line 18 when j==n.

Besides, this works one command window too.

for k=2:n-1
Hey(1,k)=Masks(2,1)*a(1,k-1)...
                +Masks(2,2)*a(1,k)...
                +Masks(2,3)*a(1,k+1)...
                +Masks(3,1)*a(1+1,k-1)...
                +Masks(3,2)*a(1+1,k)+Masks(3,3)*a(1+1,k+1);
end

But not in the code's itself. Why? It's infuriating

[Walter Roberson]

In this for k code, you use an upper bound of n-1, so when you have the k+1 then that gives a maximum of n, which is fine.

There is no elseif part for i==1 and j==n in your code. There is elseif i==1 && 1<j<n-1 . As far as MATLAB is concerned, that means the same as

elseif (i==1) && ((1<j)<n-1)
    

1<j is either false (0) for j = 1, or is true (1) for j > 1 . Either value, 0 or 1, are <n-1 unless n is 0 or 1 itself.

MATLAB has no documented chain inequalities. (But in practice chain inequalities can be used with symbolic expressions.)

[Arda Nova]

But there is

 elseif i==1 && j==n
            Newres(i,j)=Masks(2,1)*a(i,j-1)...
                +Masks(2,2)*a(i,j)...
                +Masks(3,1)*a(i+1,j-1)...
                +Masks(3,2)*a(i+1,j);

I mean before matlab, I used c++. I understand how codes works more or less. Let's say we have 400x300 matrix. So m=400 and n=300. So when j=299

code is like

                    elseif i==1 && 1<j<=300-1
            Newres(1,300)=Masks(2,1)*a(1,299-1)...
                +Masks(2,2)*a(1,299)...
                +Masks(2,3)*a(1,299+1)... %%<-----Line 18
                +Masks(3,1)*a(1+1,299-1)...
                +Masks(3,2)*a(1+1,299)+Masks(3,3)*a(1+1,299+1);
            

as soon as j=300 this code is not used but elseif i==1 && j==n is used.

At least this is what should happend. Where do I falter?

[Walter Roberson]

But the elseif i==1 && 1<j<n-1 is before the elseif i==1 && j==n test, and the 1<j<n-1 is true when you do not want it to be because 1<j<n-1 means the same as

temp = 1<j;   %true or false
double(temp) < n-1   %double(false) is 0, double(true) is 1

C++ does not handle chain inequalities specially either. Comparisons return type bool, but bool false in C++ is defined as having value 0 and bool true in C++ is defined as having value 1, so the C++ meaning of 1<j<n-1 would be the same as the MATLAB understanding: neither one would treat it as a range test.

[Arda Nova]

I changed the order of elseif's and now they are working. It was very helpful. I still don't really understand the reason why, sorry for that, but I will figure it out myself. Thank you very much.

[Walter Roberson]

You need to recode the 1<j<n-1 to be 1<j & j<n-1 and make all such similar changes.

[Arda Nova]

I see. Even If I don't figure it out why, at least I will follow this proposal. Thank you again.

[Walter Roberson]

As I explained, MATLAB does not treat A < B < C as meaning to test whether B is strictly inside the range A to C (at least not except sometimes if you are working with symbolic expressions.)

In numeric MATLAB, there is only one trinary operator (meaning that it has three inputs), namely the colon operator start:increment:stop . All other operators are unary (meaning it has one input) such as the - sign in sin(-x) or binary (meaning it has two inputs) such as 5 + x . All of the numeric relation operators are binary. There is a standard precedence ordering so for example MATLAB knows that 3 + 5 * x is to be treated as (3 + (5*x)) and not as ((3+5)*x) . In MATLAB in all cases where the operators are equal precedence, evaluation starts at the left and moves to the right, such as 3 + x + 5 being intepreted as ((3+x)+5) . A < B < C is treated the same way, as a series of binary operators to be handled left to right, so it is ((A<B)<C) . The A<B part returns a logical value, false (0) or true (1), and that 0 or 1 result is then compared to C.

Computer languages that permit range comparisons such as A < B < C are not common; I have had difficulty finding any languages with that feature other than a few symbolic languages.