Matlab,problem with concatenation in an iteration? How do I fix it?

Question

The problem I have is that it only works partially, but what can I add to make it work?

A2 = [20 4 6 8 5];
A3 = [10 2 3 4 6];
Str=[];
formatSpec = 'P%d (%d,%d)';
for i=1:length(A2)
    str = char(sprintf (formatSpec, i, A2(i),A3(i)));
    Str=[Str;str];
end
set(handles.text2,'string',Str);

Answer 1

You are not concatenating strings but char-arrays. Thinking it this way, it already answers your question: If you have a two-digit number, the char-array is one element longer than the char-array of a single-digit number... and you cannot concatenate two arrays of different size vertically.

The solution is fairly simple: use actual strings (introduced somewhere around R2016a). Strings are indicated with "" instead of '', which are chars. So replace your charwith string and it works fine. (Even better: provide the formatSpecas ""-string and it sprintf() will return a string right away)

Side note:

BTW, you should always allocate memory if your are looping. That is why the Str has an orange squiggly underline. This is because MATLAB stores arrays in RAM contiguously and has to copy it to a larger section it it outgrows the current one. So instead of Str=[], write Str = strings(length(A2),1), and index Str(i) = ... in the loop.

Personally, I like num2str more that sprintf but I cannot give a good reason for this, except for that it also works without providing a format.