How to convert a char array field in a struct array to a string field in a vectorized fashion.

I have a struct array in which one of the fields is a character array. I need to convert it to a string scalar for all stuctures in the array. I have been unable to find a way to do this in a vectorized manner.

Example:

myArray = [struct("code", 'CO128'), struct("code", 'TX457'];

This is a toy data structure -- the real thing has dozens of fields in each element and tens of thousands of elements in the array. I have no control over the array format up to this point.

What I am trying to accomplish is the same thing that is performed by the following loop:

for i = 1:size(myArray,2)

myArray(i).code = string(myArray(i).code);

end

I am restricted to the functionality of the basic MATLAB R2021a installation (no optional toolboxes).

Answer 1

[Stephen23]

S = struct('code',{'CO128','TX457'});
S.code % checking

ans = 'CO128'

ans = 'TX457'

tmp = num2cell(string({S.code}));
[S.code] = tmp{:};
S.code % checking

ans = "CO128"

ans = "TX457"

[William Bahn]

Thank you!

For others that come across this, here is my explanation of how this works:

myCellArray = {S.code};

S.code evaluates to a comma separated list and the curly braces then pack this into a cell array with each cell containing the contents of the corresponding the code field of the corresponding element of S.

myStringArray = string(myCellArray);

Converts each cell into a string. Requires that all cells contain content that can be converted to a string scalar, including char arrays and strings, as well as numerical and logical scalars.

myStringCellArray = num2cell(myStringArray);

Converts an array of ANY data type to a cell array, preserving the data type in the corresponding cell.

[S.code] = myStringCellArray{:};

The RHS produces a comma-separated list.

The LHS is a comma-separate list of target locations.

Now for my observations:

This is clever, and tricky, and very non-intuitive (which is not to say that it isn't the cleanest and simplist way to accomplish this task).

This seems like such a basic and common task that there should be support for it in a much more direct way, such as simple casting or perhaps:

convert(S.code like "string")

The big stumbling block for me in figuring something out was the very misleading naming convention that is so common in MATLAB functions. I didn't even look at num2cell because I wasn't converting numbers to anything, so it didn't seem to apply.

I managed to get to an array of strings (i.e., string({S.code}) ) but couldn't figure out how to get that array copied over into the structure array. I kept getting error messages suggesting that I use a comma-separated list, but could never find out how to convert an array into a comma separated list. I knew I could evaluate a cell array (or a structure field) as a comma-separated list, but I couldn't figure out how to convert an array of strings to a cell array of strings. The cellstr() function was almost what I needed, except that it insists on converting things not to strings, as the name would imply, but to character arrays, which was precisely the opposite of what I was trying to do and merely took me back right where I started.

Answer 2

[Fangjun Jiang]

It can be done but not sure if it is more efficient than the simple for-loop

myArray=struct('code',{'a','ab','abc'})
myString=string({myArray.code})
myCell=mat2cell(myString,ones(1,size(myString,1)),ones(1,size(myString,2)))
myArray=struct('code',myCell)