CODEWITHSUNDEEP
matlabfor-loop
user334911
user334911

Reputation:

The for loop doesn't iterate

In MATLAB the following for loop:

for i = [1:100]' 
    %'// Do something, such as disp(i)
end

isn't apparently really implemented by iteration, rather i becomes the matrix [1 2 3 ... 100] and the "loop" is only executed once on this matrix i. You can verify this by printing the value of i or other tracking information. Only a single pass is made through the loop.

Is it possible to force MATLAB to do genuine looping? The reason I ask is that the above approach is fine for many cases but much more painful when you have nested loops that need to run.

Example:
The following code won't do what you would expect if you thought you were getting actual iteration over a loop:

for i = outlier
    data(i) = median(data(i-100:i+100))
end

One would expect at each outlier index this would replace data(i) with the median of the data from i-100 to i+100, but it doesn't. In fact, the median returns a single value computed on a conglomerate of all the ranges you cared about, and every data(i) point is replaced with that single value.

Upvotes: 7

Views: 3218

Answers (2)

Jonas
Jonas

Reputation: 74930

If you write

for i = (1:100)' %'# square brackets would work as well
   doSomething
end

the loop is executed only once, since a for-loop iterates over all columns of whatever is to the right of the equal sign (it would iterate 200 times with a 100-by-200 array to the right of the equal sign).

However, in your example, you have i=[1:100], which evaluates to a row vector. Thus, the loop should execute 100x.

If you iterate over an array that might be nx1 instead of 1xn, you can, for safety reasons, write:

for i = myArray(:)'  %'# guarantee nx1, then transpose to 1xn
end

Upvotes: 12

eykanal
eykanal

Reputation: 27017

This is not correct. The code:

for i=1:100
    disp(i)
end

will print all the values 1 through 100 consecutively. While Matlab does encourage vectorization, you can definitely use traditional loops with the style of coding you used above.

Upvotes: 7

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