List values of variables with wildcard

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Greg 2016-2-9

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https://ww2.mathworks.cn/matlabcentral/answers/267329-list-values-of-variables-with-wildcard

编辑： Stephen23 2017-5-4

采纳的回答： Guillaume

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Is there a command or other simple way to list the values of variables specified by a wildcard? For example, if I type

>> command a*

MATLAB would give me the names and values of all variables that start with "a". (who and whos do not give the values.) Thanks. Greg Reese

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Stephen23 2016-2-9

编辑：Stephen23 2016-2-9

A much better solution would be to stop writing buggy, slow and obfuscated programming that accesses variable names dynamically (see Guillaume's answer), and to switch to using a structure. Then you can easily access all of its fieldnames, filter them, sort them, or do whatever is required. Much easier:

http://www.mathworks.com/help/matlab/structures.html

Here is why accessing variable names dynamically is a bad idea ("iffy", as Guillaume said), although beginners seem to love doing this regardless of the fact it creates slow, buggy code and always ignore our advice to use better programming methods:

http://www.mathworks.com/matlabcentral/answers/196952-how-to-create-new-variables-in-batches-with-strcat

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Answer 1

Guillaume 2016-2-9

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编辑：Guillaume 2016-2-9

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What you're trying to do sounds iffy. You should not be manipulating data by their variable names. Saying that, it's very easy but it involves eval which should raise some alarm bells:

varnames = who('a*');
values = cellfun(@eval, varnames, 'UniformOutput', false);
cell2table([varnames, values], 'VariableNames', {'Variable', 'Value'})

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Greg 2016-2-9

"What you're trying to do sounds iffy."

It may sound like I'm being lazy (because I am being lazy!) but it's not iffy. Let me give an example:

Suppose I have four variables - power_alpha, power_beta, power_delta, and power_theta. Right now, to see the values of all four variables, I have to type the four lines

>> power_alpha

>> power_beta

>> power_delta

>> power_theta

What I'm looking for is some command, let's call it ShowValue, with which I can get the output to the four lines above by just typing in the one line

>> ShowValue power_*

In other words, it would operate just like the MATLAB command "who" but show the variable names and their values.

"You should not be manipulating data by their variable names."

I'm not sure what you mean by that.

"Saying that, it's very easy but it involves eval which should raise some alarm bells:"

I agree, using eval should raise some alarms! I could easily write a function to carry out such a command, perhaps with code like you posted, but this seems like such a simple functionality that I would like to verify that it doesn't already exist.

Perhaps we need a flag for the "who" command, something like

who power_* -ShowValue

Thanks for your help.

Greg Reese

Guillaume 2016-2-9

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I'm afraid there is no built-in way of doing what you want and if you want the behaviour of who to be modified, you'll have to ask Mathworks.

All programmers are lazy. That's why they write code so the computer does the work for them. But, I don't think you're being lazy enough, or you're not lazy about the right things.

Your four power_* variables are obviously related, therefore there should all be in the same variable (just power?). If these powers are all scalar, then store them in a vector. If they're vectors of the same length, store them in a 2d matrix, etc. If they're different size, store them in a cell array. This is even lazier, since to see the value of all four of them at once you'd just type:

power

As a rule, if you start numbering variables, or giving them sequential names (such as your *_{alpha, beta, delta, theta}), you're doing it wrong. At some point you're going to want to apply the same operation to all of them and be forced to eval them or copy paste the code. Not lazy enough! Put them all in array and write a loop that does the same thing to all.

You basically have an inherent relationship between all four variables that is only expressed through their names. It is better to express that relationship explicitly by putting them all into a common container. That's what I meant by my bady worded "You should not be manipulating data by their variable names".

Aditya Asopa 2017-5-3

Thanks Guillaume, Sometimes the variable names are in not our hands. I need to process a file from which I need to pick columns as variables. All column headers are names as A1,A2, A3 and so on.

Could you suggest a possible way out?

Thanks a ton - Adi

Stephen23 2017-5-4

编辑：Stephen23 2017-5-4

@Aditya Asopa: read these:

https://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

https://www.mathworks.com/matlabcentral/answers/57445-faq-how-can-i-create-variables-a1-a2-a10-in-a-loop

"Could you suggest a possible way out?"

It would be very easy to put them into a structure.

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List values of variables with wildcard

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