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How do I ask for a workspace variable input in a running program?

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I'd like to be able to set a variable in a code to a workspace variable that the user can choose.
Assume I have some variable B in the workspace A=input('Please pick the variable you would like to use... ');
How do I set A equal to B while the program is running?

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John D'Errico
John D'Errico 16 Jun 2016
A simple answer is uigetvar , posted on the file exchange.
It pops up a dialog box, listing the names of all variables in your base workspace. You select the desired variable, and it returns the contents of that variable to your function of script.

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Star Strider
Star Strider 17 Jun 2016
+1 John!
Leave it to you to have already solved this problem — 4 years ago!

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추가 답변(3개)

Stephen Cobeldick
Stephen Cobeldick 16 Jun 2016
편집: Stephen Cobeldick 16 Jun 2016
Put all of your variables into a structure. Then you can simply do this:
>> S.a = 1;
>> S.b = 2;
>> S.c = 99;
>> A = input('input variable: ','s');
input variable: c
>> S.(A)
ans =
Solutions to problems can be so simple, when you are prepared to think a little outside the box.

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Stephen Cobeldick
Stephen Cobeldick 16 Jun 2016
  1. there are ways of manipulating arbitrary variables, but they are slow, and will cause more bugs than you can imagine... bugs that are very difficult to debug. And beginners love this option!!
  2. The idea of manipulating arbitrary variables in the base workspace... it sounds like a bad idea. What about wrapping your operations in functions, to keep them independent?
  3. Don't try to create an entire parser using MATLAB. Although beginners often want that "the user can input any variable, and pick any function...", this is inefficient and slow. Think about your task: what are you actually trying to achieve ?
Give us a bit more info about your task and we can help find a neat and efficient way to code it.
Aaron Greenbaum
Aaron Greenbaum 16 Jun 2016
I made a function with no arguments to browse for an hdf file on the computer and extract an image in the form of an 1024x1024 matrix the function finds the peak locations in the image. I have another script that takes the matrix and warps it and exports a new matrix. I would like to be able to use one of these new matrices in the peakfinding function.
I am pretty sure that I could make this work by adding an argument to my function and having a default if no argument is input, but I was wondering if there were otherways to do this. (Without using eval)
[centroids] = PeakFind(imagevarname)
if nargin==0
Stephen Cobeldick
Stephen Cobeldick 17 Jun 2016
The core question is: how do you get lots of matrices into your workspace?
This might happen is several ways, but lets consider two common ways:
  1. the functions returns a matrix, which are allocated to variables.
  2. the matrices are magically "poofed" into existence in the base workspace using assignin.
In the the first case the user allocates this matrix to a variable, so equally they can supply this variable as an input argument when calling the second function. In the second case, a poor program design makes the rest of the code more difficult... Unfortunately we don't have enough information on how you are doing these steps.
Some general advice though: remember that the KISS principal applies to writing code! If the task is basically to pass data from one function to another, then pass them simply by using the input/output arguments (which is fast and reliable). Concentrate your activities on actually solving your tasks, not on trying to replicate a functionality that already essentially exists (passing variables from one function to another).
For keeping track of lots of variables, you can put them all into one matrix, one cell array, one structure, etc. This is the simplest way to store them because you can trivially loop over that data. There are also many MATLAB operations that operate on entire matrices/cell arrays/structures all at once, which makes working on the data much faster and neater than if you try to access separate variables.
A numeric array is simplest, a structure lets you use fieldnames...
A little thinking, experimenting, and planning of how to achieve your task goes a long way:

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Star Strider
Star Strider 16 Jun 2016
If you have a list of variables already in your workspace, and you want the user to choose one, I would use the listdlg function.
It is much easier than using input and the associated logic involved in selecting a variable.

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Star Strider
Star Strider 16 Jun 2016
... and occasionally necessary.
Stephen Cobeldick
Stephen Cobeldick 16 Jun 2016
... to fix a beginner's poor program design.

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Aaron Greenbaum
Aaron Greenbaum 22 Jun 2016
편집: Aaron Greenbaum 22 Jun 2016
You can use John's Uigetvar and putvar to do this, but it does use the dreaded Eval function (evalin). Overall it is better to improve the way you structure your code such as putting your variables into a structure which can then be called for the required variable. In the end I ended up using nargin to have a more flexible function that I could use with or without an input image. Thanks everyone for your help!

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