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Why does str2num take much longer to convert 1001 long char array apposed to a 1000 long char array (MatLab R2010b 64bit, on Win 64bit)

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Serge
Serge on 1 Jun 2016
Commented: Walter Roberson on 2 Jun 2016
Example:
%make 1001-by-3 char array of numbers, can replace 3 with any size
t = '0123456789';
t = t(ceil(rand(3,1001)*10))';
%big difference in time needed to convert 1000 vs 1001 numbers
tic,n1000 = str2num(t(1:1000,:));toc
tic,n1001 = str2num(t(1:1001,:));toc
% Elapsed time is 0.000285 seconds.
% Elapsed time is 0.011493 seconds.
%results are identical
all(n1000 == n1001(1:1000))
%ans =
% 1

Answers (1)

Walter Roberson
Walter Roberson on 1 Jun 2016
You should use timeit() from the File Exchange to get more accurate timing. (timeit was eventually made part of MATLAB.)
I do see more of a difference than I would expect in R2016a on OS-X.
I recommend you switch to str2double()
  2 Comments
Walter Roberson
Walter Roberson on 2 Jun 2016
f = @() str2double(t1000); t7 = timeit(f)
f = @() str2double(t1001); t8 = timeit(f)
On my system it is not as fast as t1 or t2 but it is faster than any of the others.

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