Selectivly remove values from a matrix with a loop

I wrote a program that takes data from different databases and plots them. However, sometimes one or more of the databases inputting data yields no values from a query.

When this happens, I get an error message,

'index exceeds matrix dimensions'

Right now, my guess on how to solve this is to run a 'for' loop. (I want to remove the null set of data from a data array as well as a 'names' array (for the plot legend))

I have n data sets and a names array with n names.

I want to do:

for i=1:n %Check each database
     if dataarray{i}=0  
%If the data is null (This might not be the right way to do this...but basically if there IS data...it is stored as a 350x2 double array...and if there is NOT data...its stored as 0 (a double array I believe)

            delete ith value from dataarray 
             %I dont know how to do this
            delete ith value from namesarray
             %Also dont know how to do this
             n=n-1 
             %For plotting in the future..we have one less dataset..now n-1 datasets

end

end

So thats what I am trying to accomplish. I have been trying to read about error handling from matlab, and have also browsed this forum, but cannot find exactly what I need.

Any help would be great!

Answer 1

[Matt Fig]

It looks like you are using a cell array.

dataarray = {9, [8 8;9 9], 7, 0, [5 9], 0, magic(2), 0, 'string'}
dataarray = dataarray(~cellfun(@(x) isequal(x,0),dataarray))

The reason why you should not use a FOR loop for this is that you set the loop to iterate over the original length of the variable. Then if you delete a value in the variable, the length of the variable decreases, but the loop will keep right on going. Here is a simple example to follow:

A = [1 2 3 4];
for ii = 1:length(A)  % ii is going to 4, unless an error occurs...
   ii  % Display current value of ii
   A(ii) = []  % What will happen when ii is 3??
end

[daniel.x16]

Thanks!

Answer 2

[Fangjun Jiang]

Follow this example:

a=1:9
a(5)=[]

Please note that using this approach, you should not do it in a loop. Also follow this example:

b=randint(5)
b(b==0)=[]

[daniel.x16]

does a(5)=[] delete the 5th row? Is that how that works?

[Matt Fig]

In this example, the 5th column is deleted. a is a row vector. To delete the 5th row of a matrix:

a(5,:) = []

[daniel.x16]

Ahh...well that makes sense for the 'how to delete' part. You said to not use it in a loop? Is there any way for the program to delete a column that has nothing in it? I was thinking an 'if' loop, but if there is a better way?

[Fangjun Jiang]

The reason not to do it in the loop is because a(5)=[] as an example above reduces the length of a from 9 to 8. Thus if it is in a for-loop (for i=1:9), it will mess up with the index. If you don't want to remove that element, you can assign a special value like inf or nan. Anyway, maybe you should provide an example with some data so we'll better understand the question and provide answers.

[Walter Roberson]

daniel.x16: there is no such thing as an if loop.