Preallocating str for speed

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Ernest Adamtey
Ernest Adamtey 2021-8-11
评论： dpb 2021-8-12
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 个评论
显示 3更早的评论
Stephen23
Stephen23 2021-8-11
编辑：Stephen23 2021-8-11
"...how do I accept this comment as an answer."
Or take look at dpb's much better approach.
(assuming that you want neat, simple, very efficient code using basic character code operations)
Wan Ji
Wan Ji 2021-8-12
@Stephen Cobeldick@Ernest Adamtey, Yes, @dpb 's answer is pretty efficient and faster!

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回答（3 个）

Henry Barth
Henry Barth 2021-8-11
correspondingChars = 'ABCDEFGHIJ';
str = correspondingChars(arrayfun(@str2double,T,'UniformOutput',true));
dpb
dpb 2021-8-11
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 2021-8-12
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 个评论
dpb
dpb 2021-8-12
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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