string join does not work for char arrays?

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I have a column list of numbers like the below
M = [2; 3; 5; 7; 16];
What I want is to create a string array like this
"2, 3, 5, 7, 16"
Why can't I use the below to get what I want? What is the correct way to do this?
b = join(num2str(M), ', ')
Here is the error:
"First argument must be a string array, character vector, or cell array of character vectors."

Accepted Answer

Star Strider
Star Strider on 28 Aug 2020
One approach:
M = [2; 3; 5; 7; 16];
M2Str = sprintf("%d, ",M(1:end-1)) + string(M(end))
producing:
M2Str =
"2, 3, 5, 7, 16"
.

More Answers (3)

Walter Roberson
Walter Roberson on 28 Aug 2020
strjoin(string(M),', ')
or
strjoin(cellstr(num2str(M)),', ')
You could also consider
num2str(M.', '%g, ')
but that will leave an extra trailing comma
  1 Comment
Leon
Leon on 28 Aug 2020
Many thanks! This is probably the cleanest way to do this.

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Steven Lord
Steven Lord on 28 Aug 2020
>> M = [2; 3; 5; 7; 16];
>> join(string(M), ", ")
ans =
"2, 3, 5, 7, 16"
num2str on M makes a char matrix which is not a "string array, character vector, or cell array of character vectors" and so is not a valid input to join. [Technically the error message should probably state 'character row vector', actually.]
  1 Comment
Leon
Leon on 28 Aug 2020
Thank you. I didn't know it was as simple as that. I should use the function string instead of num2str.

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Jeremy Perez
Jeremy Perez on 28 Aug 2020
Edited: Jeremy Perez on 28 Aug 2020
Hi Leon,
Type:
help num2str
> num2str: Convert numbers to character representation.
You're not using strings but character arrays, which are different datatypes in MATLAB.
Try not to use character arrays as much as possible when you mean to use strings.
You can use:
join(string(M), ', ')
strjoin(string(M), ', ')
You can also test this:
sprintf('%d, ', M)
It'll add a comma at the end of your string that you can remove but it's a very useful function.

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