# How to Assign a Variable Name to a Point in a Matrix

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Sophie Culhane on 16 Dec 2020
Commented: Ameer Hamza on 16 Dec 2020
I am trying to assign variable name 'centersquare' to the center point in an NRows x NCols matrix. In my program, I had the center calculated every time I went through a for-loop and it worked. However, my professor instructed me to only calculate it once outside of the for-loop to be more efficient. I have tried various things in attempt to only calculate my centersquare once, however it does not work with my program.
Here is what I had before:
for pebbles = 1:NP
A(ceil(NRows/2),ceil(NCols/2)) = A(ceil(NRows/2),ceil(NCols/2)) + 1;
...
end
Here is what I am trying to do:
centersquare = A(ceil(NRows/2),ceil(NCols/2));
for pebbles = 1:NP
centersquare = centersquare + 1;
...
end
Can someone tell me why this isn't working? Thank you

Ameer Hamza on 16 Dec 2020
Edited: Ameer Hamza on 16 Dec 2020
I guess your professor asked to pre-calculate the indexes. Something like this
row = ceil(NRows/2);
col = ceil(NCols/2);
for pebbles = 1:NP
A(row,col) = A(row,col) + 1;
...
end
btw, I will be surprised if there is a significant difference. MATLAB JIT compiler is probably intelligent enough to see that the same thing is being used in each iteration. However, this is definitely more readible.
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Ameer Hamza on 16 Dec 2020
I am glad to be of help!