MATLAB: How to form a loop to perform calculations on series of variables in workspace

for loop

Matlab novice here; I have a set of data sheets with numbers loaded into workspace named, data1, data2,…,data50.

I am hoping to extract certain columns of data for analysis from data1 to data50 in a loop e.g.

for j=1:50
A = data{j}(:,3);
B = myAnalysis(A)

The idea is to extract the 3rd column from dataj and then use it for calculations. Any help would be much appreciated!

Best Answer

Firstly, have a look at this:

http://matlab.wikia.com/wiki/FAQ#How_can_I_create_variables_A1.2C_A2.2C....2CA10_in_a_loop.3F

Naming the variables "data1, data2,...,data50" creates trouble and extra work.

"data sheets". Have you read the data from Excel?

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MATLAB: Error: Unable to resolve the name workbooks.Open.

After excel = actxserver(), you have not defined "workbooks". Need this?

workbooks= excel.Workbooks;

MATLAB: Need help with loops

Rewriting the above to compute for any number of bets would seem simplest--

function [stat,mxIdx] = bestbets(odds)
% return betting odds prediction statistic and max row given odds array
    C = nanmean(odds,2);              %computes averages of rows 
    [D,mxIdx] = max(odds,[],2);       %finds max value of rows
    E = 1/sum(1./D);                  %ACE
    F = 1/sum(1./C);                  % Mean ACE
    G = C./F;                         % True odds
    stat=zeros(size(D));              % preallocate the statistic
    indx=1:length(D);                 % index array 1:N observations
    for i=1:length(D)
      stat(i,:) = 1/sum([1/D(i); 1./G(~(ix==i))])-1;  % compute for each
    end

Above reproduces your check array values for the given dataset.

NB: Above presumes the input odds array has been cleaned up before being passed, the problem about generating a suitable file format is a different discussion than than the algorithm implementation.

The loop could be eliminated with accumarray but left so could more easily get the implementation.

The "trick" is in building the logical addressing vector ~(indx==i) to select the elements from the G array with the exception of the one element being computed.

PS: While the computer doesn't care, as a general best practice I'd suggest using descriptive names for variables rather than just C, D, etc. It'll be bound to help in the long run as you develop into using Matlab and other programming languages more. I'll give high marks for brevity; that is also in my (nsh :) ) opinion an objective as exceedingly long variable names tend to just add clutter more than clarify, but at least some sort of a mnemonic to relate to the purpose is important for legibility.

ERRATUM Corrected order of function line to get function keyword first...

Best Answer

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MATLAB: Error: Unable to resolve the name workbooks.Open.

MATLAB: Need help with loops

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