MATLAB Answers

Preallocating str for speed

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Ernest Adamtey
Ernest Adamtey on 11 Aug 2021
Commented: dpb on 12 Aug 2021
I have this code. I want it to run faster. And I keep getting the warning to preallocate str for speed.
clc
str=[];
for T='956754674275'
if T=='0'
str = [str sprintf('A')];
elseif T=='1'
str = [str sprintf('B')];
elseif T=='2'
str = [str sprintf('C')];
elseif T=='3'
str = [str sprintf('D')];
elseif T=='4'
str = [str sprintf('E')];
elseif T=='5'
str = [str sprintf('F')];
elseif T=='6'
str = [str sprintf('G')];
elseif T=='7'
str = [str sprintf('H')];
elseif T=='8'
str = [str sprintf('I')];
elseif T=='9'
str = [str sprintf('J')];
end
end
a=str
Please how do I preallocate str for speed. Please help.
  5 Comments
Wan Ji
Wan Ji on 12 Aug 2021
@Stephen Cobeldick@Ernest Adamtey, Yes, @dpb 's answer is pretty efficient and faster!

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Answers (3)

Henry Barth
Henry Barth on 11 Aug 2021
correspondingChars = 'ABCDEFGHIJ';
str = correspondingChars(arrayfun(@str2double,T,'UniformOutput',true));

dpb
dpb on 11 Aug 2021
No loop needed here -- using relationship to ASCII coding and that MATLAB char() strings are just arrays, write
T='956754674275';
% the engine
C='A'-'0'; % difference offset from '0' to 'A' in code--MATLAB returns 17
str=char(T+C); % convert to char() from double of input + offset
As long as the substitution pattern is ordered sequentially, the above works no matter what the initial coding point is simply by changing the target reference in computing the offset constant, C. If it could be a variable, then create a lookup table and index into it instead.
Above produces--
>> T='956754674275';
>> C='A'-'0'
C =
17
>> char(T+C)
ans =
'JFGHFEGHECHF'
>> all(ans==str)
ans =
logical
1
>>

Wan Ji
Wan Ji on 12 Aug 2021
According to @dpb's answer, here I provide a more general answer!
T='956754674275';
encode = 'ABCDEFGHIJ';
str = encode(double(T)-double('0')+1)
then
str =
'JFGHFEGHECHF'
  1 Comment
dpb
dpb on 12 Aug 2021
Yes, that's the lookup table version mentioned.
NB: that the above can be more succinctly written as
str=encode(T-'0'+1);
or
str=encode(T-'/');

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