What Data Is Available?
Yes, I know, data are, but the title just sounded terrible that way.
Ever wonder what data you have stored in some files, particularly MATLAB MAT-files? Sometimes I'd like to know what's there without making the commitment to load the data. Fortunately, there's a good way to accomplish this in MATLAB programmatically, and another good way when viewing the Current Folder. I will show you the programmatic way.
Contents
Files to Look at
Let's first identify some files to look at. I happen to know that we have some data for demos stored on the MATLAB path. These include the files durer.mat, seamount.mat, and clown.mat.
Peek Inside
Let's take a peek inside the seamount data. I can do this using a variant of the whos function.
whos -file seamount.mat
Name Size Bytes Class Attributes caption 1x229 458 char x 294x1 2352 double y 294x1 2352 double z 294x1 2352 double
And what's lurking inside topo.mat?
whos -file topo
Name Size Bytes Class Attributes topo 180x360 518400 double topolegend 1x3 24 double topomap1 64x3 1536 double topomap2 128x3 3072 double
Notice in this case, there are 4 variables, including 2 starting with the name "topomap". Suppose I want to find all the variables with names beginning with "topom". I can use the wildcard character "*".
whos topom* -file topo
Name Size Bytes Class Attributes topomap1 64x3 1536 double topomap2 128x3 3072 double
Loading Some Data
Assuming I have enough memory, I can easily load the data from a MAT-file. But sometimes I don't want to load everything, perhaps because some of the variables are large. And if I don't need them, why use the memory and clutter my workspace? I can choose to load selected variables only.
NOTE: Before anyone asks, I can't currently (R2010a) load part of an array in MATLAB; and yes, that's on our enhancement list.
So let me load in just the topomap* variables. I have two ways to manage this. First
load topo topom* whos
Name Size Bytes Class Attributes topomap1 64x3 1536 double topomap2 128x3 3072 double
and you can see that the variables topomap1 and topomap2 have been loaded into my workspace. There have been many discussions on the MATLAB newsgroup that "poofing" variables into the workspace, especially inside a function, is a poor idea. Instead, I can load them into a known variable. To do so, I must use the functional form of load instead of the command form.
mydata = load('topo','topom*')
mydata = topomap1: [64x3 double] topomap2: [128x3 double]
Now I have a known variable (mydata) which is a struct. mydata has fields with the names of the selected variables from the topo.mat MAT-file.
Here's more information to help you as you manage loading your data into MATLAB.
Do You Load Data Selectively?
Do any of you use some of these techniques for managing and loading selected portions of your data? I'd love to hear more here.
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