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.mat files not loading correctly

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Daniel Guilbert
Daniel Guilbert on 13 Apr 2017
Edited: dpb on 16 Apr 2017
Hi all,
I am very new to MATLAB, and the first program i wrote which loaded a file loaded a file named data_vector. Now, whenever I load any file it gets automatically stored in a variable of name data_vector. This means that the following code will not compile on my copy of MATLAB R2016b.
clear
load test_data;
plot(test_data(1,:)) %test_data is a 1000x500 2D array
Please help!
Edit: the specific error message is
Reference to a cleared variable test_data.
Error in phase_detector (line 4)
plot(test_data(1,:))

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dpb
dpb on 13 Apr 2017
The only problem appears to be your expectation...the variable is named data_vector, the file you wrote it to is test_data.mat.
Naming a file doesn't change the variable name inside it; the
whos -file test_data
line clearly reveals the content of the file is, as you asked it to be, the array data_vector. When you load the file, the variable(s) in the file are restored to the workplace with their original name.
This is all in
doc save
doc load
There simply is no variable test_data in sight here...if you want that as a variable name you'll have to assign something to it to make it such.
Daniel Guilbert
Daniel Guilbert on 14 Apr 2017
So what's the point of being able to save them under different names if you can only ever reference them using the initial name? Is there anyway to change the name of a variable besides cloning? Seems a bit weak.
dpb
dpb on 14 Apr 2017
This doesn't save them under different variable names, only as a different file; that's the point. SAVE/LOAD is primarily a shorthand for precisely that purpose; save/retrieve a given variable or set thereof for quickly being able to reproduce a snapshot of a previous session or the like without having to worry about extraneous code.
If you really want to save a set of data but read it back in a more general sense, use something other than SAVE (like fwrite/fread for unformatted) that doesn't include such additional information with it.
Or, as Steven notes, if you want to force rename a variable from a .mat file, there's always the structure route albeit it is a fair amount of overhead often compared to using simple arrays.

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Accepted Answer

Steven Lord
Steven Lord on 14 Apr 2017
  1. Calling clear to clear all variables is rarely necessary in a function.
  2. If you call load with an output argument, the output argument will be a struct array and the variables that were in the MAT-file will be read into fields in that output argument. This allows you to avoid conflicts with variables that already existed in the workspace. I recommend you use this approach; that way the name of the variable you have to access to use your data is predictable. You can use dynamic field names in conjunction with fieldnames if you don't know the names of the variables in your MAT-file but need to iterate over them.

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