Multiple arrays in a nested for loop?

Question

Multiple arrays in a nested for loop?

mechanate (Mechanical)

(OP)

15 Oct 10 01:03

Hey guys, I've tried searching for this but haven't been able to find it.

It's been a little since I've done anything relevant in MATLAB and I've run into a little problem with my code. My problem is that I want a program to grab an input for a number of vectors and then I want that program to fill out those vectors based on a number given for a vector size. Let me try and clarify that. I want to calculate a result using 4 column vectors (4 is an arbitrary number given just as an example). I then want to be able to input values for each one of the elements in those vectors. So, I've got a for loop that runs from 1 to the upper bound of the number of vectors and within that loop I've got another for loop that runs from 1 to the upper bound of the vector size. So, in the end I'll end up with A1, A2, A3, A4, all the same size, but with values I enter for each element. A1 could look like [1;7;5;9], A2 like [5;7;5;1] and so on. Has anyone done anything like this before? I'll post my crummy code so you may have a better idea. I know the code doesn't wrong and I know it's at least a problem with my syntax. Thanks in advance!

function [numvectors] = MGS(numvectors)
check = isa(numvectors, 'numeric');
while check == 0;
   numvectors = input('Please enter a number\n')
   check = isa(numvectors, 'numeric');
end
vectorsize = input('Enter the size of the column vector (A x 1, where A is the size')
for i = 1:numvectors;
    str = num2str(i);
    for j = 1:vectorsize;
    num = input ('Enter number j value for number i matrix')
    A(str)(1, j) = num;
end
end

mechanate (Mechanical) · Answer 1 · 2010-10-24 19:27:48

Okay, so I've got this far with the code:

num_arrays = input('Specify the number of arrays');
length_array = input('Specify the number of rows in the array');

for i = 1:num_arrays
    for j = 1:length_array
        temp = input(['Enter data for array ', num2str(i), ', element ', num2str(j), ':'], 's');
        data(j,i) = str2num(temp);
    end
end

for i = 1:num_arrays
    eval([ sprintf('x%d = data(:,%d);', i,i)])
end

But, I've heard that using eval is a bad idea. Any reason as to why this is? If it is a bad idea to use eval, is there an alternate way to get the same result this code produces? Thanks in advance!

GregLocock (Automotive) · Answer 2 · 2010-10-24 19:35:40

eval is bad because it allows you to do almost anything. As such lazy programmers tend to use it when they should be using safer alternatives. That being said I use it a fair bit.

Try assignin.

Cheers

Greg Locock

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mechanate (Mechanical) · Answer 3 · 2010-10-24 19:39:47

Haha, yeah, I got caught up in the ease of using it. I figured it out though using cell arrays. I didn't know you could do something like:

for i = 1:num_arrays
x{i}=data(:,i)
end

This makes actually doing a transpose easier than trying to figure out how to get " ' " to read correctly within an eval statement. I also read that some times the MATLAB C compiler bugs out with the eval statement. Don't know if this is correct or not. Either way, thanks for the response!

questionguy (Electrical) · Answer 4 · 2010-10-27 17:14:04

Just a suggestion, but if all the arrays are the same length, why not use a 2d matrix with each column being one of the vectors. Cells might be fine for this case, but I've found cells to be cumbersome at times.

mechanate (Mechanical) · Answer 5 · 2010-10-28 16:50:43

Well, I had considered that, but this is part of a code to work a modified Grahm-Schmidt (not sure if I spelled that right) process. I didn't see any way to perform the calculations after grabbing all the values without having individual arrays.

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Multiple arrays in a nested for loop?