Selectivly remove values from a matrix with a loop

2011 6 6
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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!

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 采纳的回答

Matt Fig
Matt Fig 2011-6-6

0 个投票

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

更多回答（1 个）

Fangjun Jiang
Fangjun Jiang 2011-6-6

0 个投票

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)=[]

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daniel.x16
daniel.x16 2011-6-6
does a(5)=[] delete the 5th row? Is that how that works?
Matt Fig
Matt Fig 2011-6-6
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
daniel.x16 2011-6-6
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
Fangjun Jiang 2011-6-6
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
Walter Roberson 2011-12-8
daniel.x16: there is no such thing as an if loop.

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