Resolve Error: coder.varsize Not Supported for Strings

Issue

Code generation does not support defining string variables as variable-size by using the coder.varsize (MATLAB Coder) directive. If you call coder.varize on a string variable, this error occurs during code generation:

coder.varsize() is not supported for variables of string type.

Possible Solutions

Consider this MATLAB^® function, which returns a string. Code generation fails for this function because the code generator defines s as a fixed-size string with length 0 and cannot increase the size of the string inside the for-loop. You cannot use coder.varsize to define s as variable size because s is a string.

function out = varSizeStringError(n)
s = "";
for i = 1:n
    s = s + ":) ";
end
out = s;
end

Avoid Using Strings

If your application does not require strings, you can use a variable-size character row vector instead of a string. For example:

function out = varSizeStringExample1(n)
s = '';
coder.varsize('s');
for i = 1:n
    s = append(s,':) ');
end
out = s;
end

Convert Variable-Size Character Vectors to Strings

Because code generation supports defining variable-size character arrays using the coder.varsize directive, you can define a variable-length character row vector and then convert that vector to a string. For example:

function out = varSizeStringExample2(n)
chars = '';
coder.varsize("chars");
s = convertCharsToStrings(chars);
for i = 1:n
    s = s + ":) ";
end
out = s;
end

Use string(blanks(coder.ignoreConst(0))) to Construct an Empty Variable-Length String

You can use string(blanks(coder.ignoreConst(0))) to instruct the code generator to create an empty, variable-length string in the generated code. You can then change the length of this string without explicitly defining the string as variable size. For example:

function out = varSizeStringExample3(n)
s = string(blanks(coder.ignoreConst(0)));
for i = 1:n
    s = s + ":) ";
end
out = s;
end