find a value & store in new variable (again)

Ubu
2012 3 19
3 个回答
5 次查看（30 天）

0 个投票

Dear Matlab friends,
We recently posted a question about finding values and storing them in new variables:
http://www.mathworks.com/matlabcentral/answers/31471-find-a-value-store-in-new-variable
Our question remains in the sense that we would like to know how to procede when we have a matrix rathen than a cell array.
Indeed, we have a 384 x 14 matrix A, and we would like to find all occurrences of number [2] in column 1, and save corresponded value in column 2 in a new variable "new_variable":
A =
3 5 6 ...
2 3 5
2 3 4
1 4 5
5 7 9
...
so the result would be:
new_variable =
2
2
The suggestion for cell arrays was
new_variable = A{2}(A{1} == 2);
We thank you very much for any suggestion!
Best,
Udiubu

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回答（3 个）

Geoff
Geoff 2012-3-19

1 个投票

Use logical indexing:
new_variable = A(A(:,1)==2, 2);
This indexes all those rows of A where column 1 is equal to 2, selects column 2 and assigns the result to new_variable.

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Aldin
Aldin 2012-3-19
Look at my answer it is same as yours
Ubu
Ubu 2012-3-19
Hey,
So let us understand where we are wrong here: the error we get is
??? undefined function or method 'eq' for input arguments of type 'cell'.
Our script imports a txt file with strings and numbers, converts all columns to cells so to be able to work on them and substitute numbers with strings, then creates a matrix A = [column1 column2 column3 ...]. Our last step is to be able to perform the action above: find a value & store in new variable.
Thanks thanks again for your precious help!
Best
Aldin
Aldin 2012-3-19
Which is the correct answer. Please press 'Accept answer' that we know that's the question closed successfully
Geoff
Geoff 2012-3-19
@Aldin: You used several lines and a loop, I used a one-line logical index. The result might be the same, but the solutions are not.
@Ubu: If you construct the matrix A from columns that are cells as you described, it will actually be a cell. If you want A to be a matrix, you need to then call A = cell2mat(A).
Aldin
Aldin 2012-3-19
@Geoff: Yes, but Your code is not exactly clear for beginners in MATLAB.
Ubu
Ubu 2012-3-19
The only problem with cell2mat is that I have different types of data - i.e. both strings and numbers. One radical solution would also be to remove those columns with strings. How would you remove them?
Ubu
Ubu 2012-3-19
column removal
A(row,column) = []
Geoff
Geoff 2012-3-19
@Ubu: Then you asked the wrong question. Your matrix is actually a cell. I'll answer separately so the code is formatted correctly. =)
@Aldin: Your profile states that you are "here to learn MatLab", yet if you indeed just learned something you don't seem to be very pleased.
Ubu
Ubu 2012-3-19
Geoff that's so kind of you.. We'll be waiting!
Aldin
Aldin 2012-3-19
@Geoff: How do you mean?

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Geoff
Geoff 2012-3-19

1 个投票

Okay, since your matrix actually contains cells the equality operator doesn't work. I'll split the code up for extra clarity, since another reader insists ;-)
I'm still not sure if you want to replace column 2 with the value 2 or just copy the filtered column 2 into new_variable... The question was confusing. Anyway,
I would do this:
rowidx = cellfun( @(x) x==2, A(:,1) );
new_variable = cell2mat(A(rowidx,2));
cellfun maps a function over all elements of a cell-array or matrix. The '@(x) x==2' part is an anonymous (on-the-fly/adhoc) function. Here we use it on the first column. The output is a vector of booleans, which we then use as a logical index.
Edit:
The other way is to just extract the numeric columns that are of interest and use my other solution that works on matrices:
B = cell2mat(A(:,1:2));
new_variable = B( B(:,1)==2, 2);

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Geoff
Geoff 2012-3-19
Note: Edited. Needed a call to cell2mat() in the first code section. Added an alternative.
Ubu
Ubu 2012-3-19
We wanted to copy the filtered column 2 into new_variable.
We'll try your code now..
Ubu
Ubu 2012-3-19
So Geoff, your cellfun code works - we indeed have a vector of 0 and 1 corresponding to the right matching - but in new_variable only the first matched value has been reported. We just need to tell the script to report all of them. Any 'for' loop suggestion?
As for your second suggestion: the numeric columns have 'NaN' strings as well, and this causes the same problem as before.
Geoff we would love your last effort.
Ubu
Ubu 2012-3-19
Possibly we found a solution:
rowidx = cellfun( @(x) x==2, A(:,1) );
new_var = A(rowidx(:,1) == 1, 14)
Now we'll gonna check it..
Geoff
Geoff 2012-3-19
That's odd. Did you use my edited version of the first code segment? Using A{rowidx,2} was incorrect.
I assume your matrix is generally numeric (ie the numbers themselves are not strings), but those NaNs get in the way. You could try converting them to numbers too:
A{cellfun(@(x) strcmpi(x,'nan'), A)} = NaN;
Ubu
Ubu 2012-3-19
A{cellfun(@(x) strcmpi(x,'nan'), A)} = NaN; right hand side has too few values to satisfy the left hand side.
However, we managed to delete those NaN, transform to mat and execute our mean and SD calculations.
It was a real pleasure to have you. You made a couple of guys very happy tonight.
We thank you very much Geoff!

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Aldin
Aldin 2012-3-19

0 个投票

Here, try this code:
A = [ 2 1 3 2 4; 4 5 3 6 2; 2 3 5 3 6; 1 2 5 3 6; 3 2 5 2 5]
for i = 1:5
if A(i,1) == 2
A(i,2) = 2;
end
end

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Ubu
Ubu 2012-3-19
Hi Aldin,
Thanks for your answer.
However we do not really get what you mean by using '= 2' in
A(i,2) = 2;
Best
Aldin
Aldin 2012-3-19
A(i,2) = 2; -----> "we would like to find all occurrences of number [2] in column 1, and save corresponded value in column 2 " (by Ubu).
It means to copy "2" in second column.
Aldin
Aldin 2012-3-19
oooo i know what you want :)
A = [ 2 1 3 2 4; 4 5 3 6 2; 2 3 5 3 6; 1 2 5 3 6; 3 2 5 2 5];
counter = 0;
for i = 1:5
if A(i,1) == 2
counter = counter + 1;
new_variable(count) = A(i,2);
end
end
Aldin
Aldin 2012-3-19
Have I solved your problem?

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Ubu
2012-3-19

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