Preallocating str for speed

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.

[Wan Ji]

just do

a = '956754674275';
str = a;

Then

ct = 0;
for T='956754674275'
    ct = ct+1;
        if T=='0'
        str(ct) = 'A'; % following you just do in this form
    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

[Ernest Adamtey]

Please it gives an output

'956754674275JFGHFEGHECHF'

instead of 'JFGHFEGHECHF'

[Ernest Adamtey]

Thank you Wan Ji. how do I accept this comment as an answer.

[Stephen23]

"...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]

@Stephen Cobeldick@Ernest Adamtey, Yes, @dpb 's answer is pretty efficient and faster!

Answer 1

[Henry Barth]

correspondingChars = 'ABCDEFGHIJ';
str = correspondingChars(arrayfun(@str2double,T,'UniformOutput',true));

Answer 2

[dpb]

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
>> 

Answer 3

[Wan Ji]

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'

[dpb]

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-'/');