Preallocating str for speed
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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.
回答(3 个)
Henry Barth
2021-8-11
correspondingChars = 'ABCDEFGHIJ';
str = correspondingChars(arrayfun(@str2double,T,'UniformOutput',true));
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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
>>
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