For loops & lengths/size command

Cant seem to figure out where to start. Please help!

[julian gaviria]

Why does the iteration with length function never stops the iteration?

Context: Using copyfile function for copying and pasting indexed files. To note, the files are rightly copied and pasted. But the iteration never stops:

%%%

clc

clear

SourcePath ='\\nasac-m2.isis.unige.ch\m-GAubry\GAubry\YODA\RAW_DATA\Live\S008\V1\';

Participant = 'Session_2\scans\87627';

SourceFolder = fullfile([SourcePath,Participant],'RUN_1_MIST_1_0004');

DistPath ='\\nasac-m2.isis.unige.ch\m-GAubry\GAubry\YODA\jg\prp_glm\Sub_08\';

DistFolder = fullfile(DistPath,'FunRaw_S1');

if ~exist(DistFolder)

mkdir(DistFolder)

end

for i=1:length(dir(fullfile(SourceFolder,'f*')))

copyfile(fullfile(SourceFolder,'f*'),DistFolder);

end

[Geoff Hayes]

See comment at https://www.mathworks.com/matlabcentral/answers/1671129-issue-with-length-function?s_tid=srchtitle

Answer 1

[Geoff Hayes]

Hi Samantha,

I guess that you are asking how to go about iterating (via a for loop) over a vector or matrix or whatever using the length command or size command. Typically, I use the length command if I am iterating over the contents of a (row or column) vector:

x = [1 2 3 4 5];

if isvector(x)

      for i=1:length(x)
          % do something on x(i)
      end
  end

In the above example, length(x) would return 5. As per the documentation, if you call the length command on a matrix Y, then the result is the max(size(Y)) i.e. the greatest dimension. So if Y is a 4x4 matrix, then length(Y) is 4; if Y is a 4x8 matrix then length(Y) is 8; and if Y is a 12x3 matrix, then length(Y) is 12. (Of course if Y has more than two dimensions then the length will be the largest/max value across all dimensions.

I generally use the size command if I am iterating over the contents of a matrix:

Y = [1 2 3; 4 5 6; 7 8 9; 10 11 12];
if ismatrix(Y)
    [r,c] = size(Y);
    for i=1:r
        for j=1:c
            % do something against Y(i,j)
        end
     end
end

In the above example, size(Y) returns a pair of values for the two-dimensional matrix with the first value (the number of rows) equal to 4 and the second value (the number of columns) equal to 3. If the matrix were three-dimensional, then [r,c,d] = size(Y) would retrieve the size of each dimension. (Note that if you only want the size for the nth dimension you can simply do size(Y,n) where n is the dimension of Y that you are interested in. So size(Y,1) returns the number of rows, size(Y,2) the number of columns, etc.)

An alternative to either of the above is the numel command which returns the number of elements in the vector or matrix. This is equivalent to length for a vector or the product of all dimensions in a matrix. Using the above examples, numel(x) =5 and numel(Y) =12 (since there are four rows and three columns).

Hope that this helps!

Geoff

Answer 2

[Saanvi Raj]

a=[3 4 4 5;4 5 6 7];

y=length(a)