# How to avoid using 'for' loop

3 views (last 30 days)
moh mor on 22 Oct 2023
Edited: Walter Roberson on 25 Oct 2023
'u' and 'v' are 401*401 matrices and 'R' is a 401*1 matrix. I have a function which gets 'u' and 'v' vectors and one value of 'R' per iteration and calculates the value of the function. Then after that, I have to sum all these values to get the final result. I wish to achieve my goal without using 'for' loop. Is it possible?
Example:
% Initialize 'u', 'v', and 'R' matrices
u = ...; % 401x401 matrix
v = ...; % 401x401 matrix
R = ...; % 401x1 matrix
functionValues =zeros(401,401);
% Calculate the values of the function for each element
for i=1:numel(R)،
functionValues = functionValues + calculateValue(u, v, R(i)); % 401x401 matrix
end
In fact I wish to avoid using 'for' loop to increase running speed.
moh mor on 22 Oct 2023
Note: 'functionValues' has the same dimension as 'u' and 'v'. In fact I wish to sum up values of 'functionValues' for different 'R' at each position of 'functionValues'.

Image Analyst on 22 Oct 2023
Just modify youir calculateValue() so that it takes multiple values of R (can handle R being a vector) and returns a vector instead of a single scalar number. Then just call the function -- no for loop needed.
moh mor on 22 Oct 2023
Sorry, I think sth is misunderstood. 'functionValues' has the same dimension as 'u' and 'v'. In fact I wish to sum up values of 'functionValues' for different 'R' at each position.
Image Analyst on 22 Oct 2023
Then why not
theSum = 0;
for k = 1 : numel(R)،
theSum = theSum + calculateValue(u, v, R(k)); % 401x401 matrix
end
And I agree with @Dyuman Joshi with only a few iterations (401*401) you'd most likely not notice any difference at all. It might be a different story if you had hundreds of millions of elements.

Dyuman Joshi on 22 Oct 2023
Vectorize the function calculateValue() and use sum for calculateValue(u, v, R).
However, this assumes that calculateValue() can be vectorized.
And if it can be, I doubt if you will see any noticeable speed increase by using vectorization instead of using a for loop.
moh mor on 22 Oct 2023
Edited: Walter Roberson on 25 Oct 2023
As much as I could, I've Vectorized my problem.
Dyuman Joshi on 22 Oct 2023
Edited: Dyuman Joshi on 25 Oct 2023
@moh mor, is there still a problem?
Does it work properly? And did you check the speed of the two methods for comparison?

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