Replace multiple substrings within a string

I have a string of letters where i want to replace every A with 'BRARB' and every B with 'ALBLA'. I want to do this so that the Bs in 'BRARB' that was replaced by A in the current iteration, are not changed into 'ALBLA'. In other words,

function X=LindIter(N)

    X='A';
    for i=1:N
    %if a letter in the string i A, replace it with BRARB, if it is B, replace with ALBLA.
    X=strrep(X, 'A', 'BRARB');
    end

As the code is now, it only replaces A with BRARB. The output, if N=1 should be BRARB, and if N=2 it should be ALBLARBRARBRALBLA

Answer 1

[Stephen23]

Method one: for loop and indexing:

S = 'A'
for k = 1:3
    C = num2cell(S);
    C(S=='A') = {'BRARB'};
    C(S=='B') = {'ALBLA'};
    S = [C{:}]
end

Giving:

S = A
S = BRARB
S = ALBLARBRARBRALBLA
S = BRARBLALBLALBRARBRALBLARBRARBRALBLARBRARBLALBLALBRARB

Method two: in one line using regexprep, arrayfun, and cell2mat:

>> fun = @(s)cell2mat(arrayfun(@(c)regexprep(c,{'A','(?<!R)B(?!R)'},{'BRARB','ALBLA'}),s,'uni',0));
>> S = 'A';
>> S = fun(S)
S = BRARB
>> S = fun(S)
S = ALBLARBRARBRALBLA
>> S = fun(S)
S = BRARBLALBLALBRARBRALBLARBRARBRALBLARBRARBLALBLALBRARB
...etc

Answer 2

[Roy Kadesh]

function X=LindIter(N)
X='A';
for i=1:N
    %replace A with BRARB, replace B with ALBLA.
    X(X=='A')='1';
    X(X=='B')='2';
    X=strrep(X, '1', 'BRARB');
    X=strrep(X, '2', 'ALBLA');
end

[Guillaume]

This will of course fails if '1' or '2' is present in the original string. You could of course use any other character guaranteed not to be present in the original string, if one exist (with arbitrary input there's isn't such a guarantee) but why bother with the unneeded replacement step. Just query the position and use that.

[Roy Kadesh]

The problem with using position, is that you are replacing it with a longer array, and X(1)='BRARB'; will of course return an error. So how would you use a queried position?

[Guillaume]

Well, yes, you can't use simple indexing and have to do some splitting and reconcatenation instead, but it will be a lot more realiable.

[Stephen23]

"So how would you use a queried position?"

See the first method in my answer.