MATLAB: Creating a matrix from for loop values?

Question

I have the following code:

for i = 1450:9740:89910
    n = i+495;
    range = ['B',num2str(i),':','H',num2str(n)];
    iter  = xlsread('BrokenDisplacements.xlsx' , range);
    displ = iter;
    displ = [displ; iter];
end

Which takes values from an Excel file from a number of ranges I want and outputs them as matricies. However, this code just uses the final value of displ and creates the total matrix from there. I would like to total these outputs (displ) into one large matrix saving values along the way, how would I go about doing this?

enable displ as empty before the for loop and remove displ = iter; line. — Pavan Yalamanchili
– Pavan Yalamanchili, Commented Feb 26, 2013 at 23:41

Floris · Accepted Answer · 2013-02-27 17:56:46Z

1

Since you know the size of the block of data you are reading, you can make your code much more efficient as follows:

firstVals = 1450:9740:89910;
displ = zeros((firstVals(end) - firstVals(1) + 1 + 496), 7);
for ii = firstVals
    n = ii + 495;
    range = sprintf('B%d:H%d', ii, ii+495);
    displ((ii:ii+495)-firstVals(1)+1,:) = xlsread('BrokenDiplacements.xlsx', range);
end

Couple of points:

I prefer not to use i as a variable since it is built in as sqrt(-1) - if you later execute code that assumes that to be true, you're in trouble
I am not assuming that the last value of ii is 89910 - by first assigning the value to a vector, then finding the last value in the vector, I sidestep that question
I assign all space in iter at once - otherwise, as it grows, Matlab keeps having to move the array around which can slow things down a lot
I used sprintf to generate the string representing the range - I think it's more readable but it's a question of style
I assign the return value of xlsread directly to a block in displ that is the right size

I hope this helps.

edited Feb 27, 2013 at 17:56

answered Feb 27, 2013 at 0:12

Floris

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2 Comments

Mustafa Nasser Over a year ago

Thank you very much, but I couldn't get this to work, the first block of the matrix was empty, all zeroes

Floris Over a year ago

Yes there was a mistake in my code; I apologize - edited the answer to get rid of the empty block- see indexing of displ when I assigne after the xlsread.

Autonomous · Accepted Answer · 2013-02-26 23:43:28Z

1

How about this:

displ=[];

for i = 1450:9740:89910
    n = i+495;
    range = ['B',num2str(i),':','H',num2str(n)];
    iter  = xlsread('BrokenDisplacements.xlsx' , range);
    displ = [displ; iter];
end

answered Feb 26, 2013 at 23:43

Autonomous

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