Changing adjacent elements in matrix with input from user

for i=1:n
   for j=1:n

        if  matrixM(i,j)==0
            matrixM(i+1,j)=-1;
            matrixM(i,j+1)=-1;
            matrixM(i-1,j)=-1;
            matrixM(i,j-1)=-1;
        end
   end 
end

Unrecognized function or variable 'n'.

I want to change elements to -1 which are adjacent (only lateral and vertical) to zero. How can I fix this code.

Answer 1

[Walter Roberson]

You start j at 1, but you try to store into matrixM(i,j-1) . When j==1 then that would try to store into matrixM(i,0)

Likewise you start i at 1 but try to store into matrixM(i-1,j) which is matrixM(0,j)

You allow j to go to n but try to store into matrixM(i,j+1) . If n+1 is larger than the array, then you have a problem.

You have several different situations:

• i is equal to 1. In that case, you must not access row i-1
• i is equal to the number of rows in the array. In that case you must not access row i+1
• j is equal to 1. In that case, you must not access column j-1
• j is equal to the number of columns. In that case you must not access column j+1
• otherwise neither i nor j are on the border and you can access the elements without restriction.

Now, you can do individual tests within your double-nested loop to be sure each location would be in range before doing the corresponding assignment. But there are other ways that might be more efficient.