Avoid for loop which has an if-statement in it

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Shima Asaadi
Shima Asaadi 2016-3-30
回答： Jan 2016-3-31
I have a for-loop process, in which I have to check sth, so I have a if statement. How can I avoid for-loop to have a faster code:
for i=1:n
score = find sth in a cell array
if ~isempty(score)
M(i)=score;
do sth;
end
end
in the above code, I try to find sth in a cell array and if I find it then the score is not empty, and then fill an array M and do some other things.
Thank you
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Matthew Eicholtz
Matthew Eicholtz 2016-3-30
Also, how are i and j involved in the following two lines?
str = "independent of this code"
score = features_score(strcmp(str,features_score(:,1)),2);
Shima Asaadi
Shima Asaadi 2016-3-30
"str" is dependent of i and j. I mean that, for each element of the matrix, I have to set a value of an associated "str". But, I should find the value of "str" in features_score, and if it is not in the features_score, the element of the matrix is zero.

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回答（2 个）

Guillaume
Guillaume 2016-3-30
编辑：Guillaume 2016-3-30
Well, str can't be independent of the loop otherwise your code just repeat the exact same thing m x n times.
Assuming str is an m x n cell aray of strings, you can use ismember to replace the loops:
%demo data;
str = {'a', 'b', 'cc'; 'ff', 'notpresent', 'ddd'};
features_score = {'a', 100; 'b', 200; 'cc', 300; 'ddd', 400; 'e', 500; 'ff', 600};
[ispresent, row] = ismember(str, features_score(:, 1));
M = zeros(size(str));
M(ispresent) = [features_score{row(ispresent), 2}]
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Shima Asaadi
Shima Asaadi 2016-3-30
编辑：Shima Asaadi 2016-3-31
Thank you for reply.
sorry, I mean that it is not relevant to the code I provided, because I do some calculations, but yes, it is dependent of the variable i and j.

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Jan
Jan 2016-3-31
Avoiding FOR loops is often overestimated. Loops are not a general problem in Matlab. They can be accelerated as in all computer languages by avoiding repeated calculations inside the loops. If this matters in your case cannot be seen as long as you post some abstract pseudo-code only. This simply hides the interesting and important details.
As usual it is not worth to accelerate the loops, if they do not consume the main part of the computing time. So use the profile command to identify the bottleneck at first and care about this only. And acceleration of 100% of a piece of code, which needs only 2% of the total processing time, reduces the run time by only 1%.
I suggest to do both: Use the profile at first, and then post the relevant code in the forum - as real code with useful test data, not as pseudo code.

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