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Preallocating str for speed

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Ernest Adamtey
Ernest Adamtey il 11 Ago 2021
Commentato: dpb il 12 Ago 2021
I have this code. I want it to run faster. And I keep getting the warning to preallocate str for speed.
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')];
Please how do I preallocate str for speed. Please help.
  5 Commenti
Stephen23 il 11 Ago 2021
Modificato: Stephen23 il 11 Ago 2021
" 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 il 12 Ago 2021
@Stephen Cobeldick@Ernest Adamtey, Yes, @dpb 's answer is pretty efficient and faster!

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Risposte (3)

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

dpb il 11 Ago 2021
No loop needed here -- using relationship to ASCII coding and that MATLAB char() strings are just arrays, write
% 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 =
>> char(T+C)
ans =
>> all(ans==str)
ans =

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

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