Plot loop n variables

Question

Jorge Rodriguez 2017-8-17

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编辑： Jan 2017-8-18

Hello Community,

I have been trying to create a plot of several arrays in loops, but I have problems assigning the change of name of the variable to the plot. Also if you know how to assign the legend name according to the plot it would be great. So, I have A1,A2,A3,... and B1,B2,B3,... respectively each pair makes a plot. So far I have written: Arrays A, B are a size (5x10000), and I'm trying to plot the second column of A (date) with the second column of B with:

%nrows is the number of pairs of A and B

for i=1:nrows
    x=datetime(A{i}(:,2)', 'ConvertFrom', 'datenum');
    y=B{i}(:,2)';
    figure
    plot(x,y);
    xlabel('A');ylabel('B'); grid on; grid minor; legend ('B',num2str(i));
    hold on
end

The error that I get is "Undefine variable A or Class A"

Any help is very appreciated. Thanks

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Image Analyst 2017-8-18

I'm wondering if per is right and he just has two cell arrays A and B, and when he wrote A1, A2, etc. he really meant A(1}, A{2}, etc. In fact his code tries to reference A like that, but it appears the problem is that A was never defined/created/passed-in in the first place.

Jorge Rodriguez 2017-8-18

Thanks for the suggestions. As you can figure out from my code. I'm new in MATLAB or coding in general, that the reason I ask.

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Answer 1

Sebastian Castro 2017-8-18

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编辑：Sebastian Castro 2017-8-18

This is happening because the syntax A{1} doesn't mean you're accessing a variable named A1. It's accessing the first element of a variable named A.

To put John D'Errico's suggestions in a ... more positive way, the 2 syntaxes above show the difference in stringing together things in a single array vs. having to manage a bunch of hard-coded variable names like A1, A2, etc. The former approach ends up being better to work with because it's less manual effort.

Based on your previous question, if you used accumarray to split your original data you would get a cell array. This is a way to store data of non-uniform size and/or types, which was the way to handle splitting your particular dataset. If you used that approach, you can then access each individual array as A{1}, A{2}, etc.

- Sebastian

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Jorge Rodriguez 2017-8-18

Thanks Sebastian, I will try to implement this format.

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Answer 2

John BG 2017-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/353142-plot-loop-n-variables#answer_278345

在 MATLAB Online 中打开

Hi Mr Rodriguez

this is John BG <mailto:jgb2012@sky.com jgb2012@sky.com>

while it's a good general guideline to try to work with variables that use matrix indices instead of putting the index in the name of the variable, it's a common practice, it happens, and since you already have the data in such format, let's work with what we've go available:

1.

Let's say you have the following

A1 =
    1.4897    1.4172
    1.4090    0.6715
>> A2
A2 =
   -1.2075    1.6302
    0.7172    0.4889
>> A3
A3 =
    1.0347   -0.3034
    0.7269    0.2939
>> B1
B1 =
   -0.8095    1.4384
   -2.9443    0.3252
>> B2
B2 =
   -0.7549   -1.7115
    1.3703   -0.1022
>> B3
B3 =
   -0.2414    0.3129
    0.3192   -0.8649

2.

now capture the names of the variables of interest

S=whos
S = 
  10×1 struct array with fields:
    name
    size
    bytes
    class
    global
    sparse
    complex
    nesting
    persistent
S.name
ans =
    'A'
ans =
    'A1'
ans =
    'A2'
ans =
    'A3'
ans =
    'AB'
ans =
    'B1'
ans =
    'B2'
ans =
    'B3'
ans =
    'S'
ans =
    'ans'

3.

Now have the choice to either

3.1.- work with the names of the variables contained in, for instance

D={S.name}
D =
  1×10 cell array
    'A1'    'A2'    'A3'    'AB'    'B1'    'B2'    'B3'    'D'    'S'    'ans'

there may not be need for a cell, but if you have more than 9 pairs A B then the length of the variable names varies with the increasing numerals

3.2.- or use the code you have already included in your question

A={}
for k=1:1:length(D)
if findstr(D{k},'A')
    A=[A;D{k}]
end
end
B={}
for k=1:1:length(D)
if findstr(D{k},'B')
    B=[B;D{k}]
end
end

now you can start working on the plots mentioned in the question, or any other processing

if you find this answer useful would you please be so kind to consider marking my answer as Accepted Answer?

To any other reader, if you find this answer useful please consider clicking on the thumbs-up vote link

thanks in advance

John BG

<mailto:jgb2012@sky.com jgb2012@sky.com>

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John BG 2017-8-18

编辑：John BG 2017-8-18

it will be 'long' as in 'langsam' if the name of the variable is complicated and there are many complicated variable names.

But how complicated and slow can it be to retrieve A1 A2 .. and B1 B2 ..

People come on, when Coberdick and Simon say 'slow' without bringing any time measurement, what's the point?

Jan 2017-8-18

编辑：Jan 2017-8-18

"Slow" means, that using arrays instead of a set of variables with numbered names will be "faster" - measured in runtime and taking into account the the time for programming, debugging and maintaining the code.

You find enough arguments to get the point e.g. in https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval.

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Plot loop n variables

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