Matlab: How to build array with strings at specific index

Question

I have a cell array of strings (length = 4):
A = {'a', 'b', 'c', 'd'}
I have a double matrix of indices (length = 4):
B = [2, 4, 6, 8]

How can I create a new cell array C (of strings) of length = 8, that uses the indices in B to place the strings from A into the new array C. For any index not specified in B, I want to enter a ' ' space (empty string). Note: my real data does not go "every-other".

C = {' ', 'a', ' ', 'b', ' ', 'c', ' ', 'd'}

How can this be done in Matlab?

how did you come with 8? Is it 4+4, or is it the maximum index in B? — Autonomous
– Autonomous, Commented Oct 31, 2014 at 1:04
No, the lengths of A, B, and C were arbitrary, except that the lengths of A and B need to be equal (because the indices in B correspond to the strings in A). — Todd
– Todd, Commented Oct 31, 2014 at 1:12
Yes, the values of B could be [100, 200, 300, 400], but then my new cell array C would need to be at least length = 400 or greater. — Todd
– Todd, Commented Oct 31, 2014 at 1:14

Autonomous · Accepted Answer · 2014-10-31 01:25:59Z

2

This is another approach, very similar to the above, but no repmat.

C(B)=A;
C(cellfun('isempty',C))={' '};

I have replaced traditional @isempty since it may be faster. Thanks @LuisMendo for mentioning that in a comment.

edited Oct 31, 2014 at 1:25

answered Oct 31, 2014 at 1:11

Autonomous

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1 Comment

Luis Mendo Over a year ago

C(cellfun('isempty',C)) may be faster than C(cellfun(@isempty,C))

Luis Mendo · Accepted Answer · 2014-10-31 10:51:15Z

1

One possible approach:

C = repmat({' '}, 1, max(B)); %// predefine with ' ';
C(B) = A; %// replace actual values

Or:

C(B) = A; %// this automatically fills missing values with [] 
ind = cellfun('isempty', C); %// find occurrences of [] 
C(ind) = repmat({' '}, 1, sum(ind)); %// replace them with ' '

The last line could be simplified as follows (no repmat needed), as noted by @ParagS.Chandakkar:

C(ind) = {' '}; %// replace them with ' '

edited Oct 31, 2014 at 10:51

answered Oct 31, 2014 at 1:03

Luis Mendo

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1 Comment

Todd Over a year ago

Great, thanks! I would only add this step before your code, to create the empty cell array C = repmat({''},1,8);

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