array indexing with repeated index, index size larger than the array

Hi

I want to do c = 1; for i = I B(c) = A(i); c = c + 1; end

where I is a vector.

For instance A = [a b c d]; I = [1 1 3 1 1 1 2 2 1]; then B = [a a c a a a b b a];

is there a fast way to do this without for loop?

Thanks

Answer 1

[Andrei Bobrov]

One way:

A = {'a' 'b' 'c' 'd'}; 
I = [1 1 3 1 1 1 2 2 1];
B = A(I);

[Sung-En Chiu]

tks, did a small test the speed seems to be slower than the for loop

len = 10000000;
A = [10 5 4 1 2];
I = randi(5,1,len);
B = zeros(1,len);
C = zeros(1,len);

tic();
tmpA = num2cell(A);
B = tmpA(I);
%B = cell2mat(B);
toc();

tic();
for i = 1:len
    C(i) = A(I(i));
end
toc();

with the conversion cell2mat it is

Elapsed time is 6.572784 seconds.
Elapsed time is 0.137823 seconds.

without conversion back still

Elapsed time is 0.093561 seconds.
Elapsed time is 0.093041 seconds.

The conversion cell2mat takes a lot of memory for some reason

this is test over Matlab 2018a

is there other way or this for loop actually not to bad?

[Stephen23]

@Sung-En Chiu: why do you use num2cell and cell2mat? Indexing is a very basic (and efficient) MATLAB operation, it works perfectly on numeric arrays, just as the introductory tutorials show:

https://www.mathworks.com/help/matlab/getting-started-with-matlab.html

[Sung-En Chiu]

Can u create C without forloop?

[Stephen23]

@Sung-En Chiu: sure, using basic MATLAB indexing. Just use the code given in Andrei Bobrov's answer:

C = A(I)

[Sung-En Chiu]

oh, I see. I read his code wrong. Didn't know that can create a larger matrix by this indexing before. tks