~ in Matlab function

I have some Matlab function call as

[Hz,~] = freqz(num, den, f, fsA)

What "~" means or what does it represent?

Answer 1

[dpb]

It is a placeholder for the optional second argument but indicates to not assign to a variable -- in the above call, there is no reason to use it; it is the same as if had written

 Hz = freqz(num, den, f, fsA);

the second output (and any/all subsequent) will just go to the bit bucket.

There would be reason to use it, and its raison d'etre would be if the expression were instead

  [~,w] = freqz(num, den, f, fsA);

there it is the placeholder for the frequency response first output variable but for some reason that wasn't needed; only the second output, the angular frequency values were required. It saves creating unwanted/unneeded variable(s) in the workspace.

BTW -- This is a badly-named return variable in the original code; the first return argument from freqz is the response magnitude; 'Hz' implies a frequency; the third output variable is, in fact, the frequency vector in hertz.

PS. I know the syntax is in the doc somewhere, but one can't search for the single character punctuation and it isn't mentioned at all in the basics of function creation...so I never came across where that is.

[Steven Lord]

Related to the PS: searching for ~ in the online documentation lists the documentation page for the not, ~ function/operator as the first hit. The Tips section of that page states that you can also use ~ as a placeholder and links to a page that discusses ignoring inputs in function definitions.

It should probably also link to the Ignore Function Outputs page that's a Related Topic for the Ignore Inputs in Function Definitions page; I'll note that to the documentation staff.

[Walter Roberson]

https://www.mathworks.com/help/matlab/matlab_prog/ignore-function-outputs.html

also mentioned in the tips for https://www.mathworks.com/help/matlab/ref/not.html and in the description of tilde in https://www.mathworks.com/help/matlab/matlab_prog/matlab-operators-and-special-characters.html#bvg3oy_-5

[dpb]

"searching for ~ in the online documentation..."

But the @doc link in Answers doesn't bring up anything is where one is in the forum...it's a lot more effort to have to go delve into the main doc and search there just to add a little to an answer when/if pressed for time.

[Walter Roberson]

Note that there are some functions where it makes a difference whether you output to a single output or if you specify an output list with ~ as the trailing outputs. For example

A = rand(5,7,4);
b = size(A); %b is 5 7 4
[b, ~] = size(A); %b is 5
[b, c] = size(A); %b is 5 c is 28
[b, c, ~] = size(A); %5 and 7
[b, c, d] = size(A); %5, 7, and 4

Also: it is not possible for a function to detect that ~ has been used. Functions can only detect the number of output positions.

[Walter Roberson]

the @doc facility misses a number of things. For example you cannot even reference toolboxes by name.

[dpb]

It also often lists many toolbox overloaded functions before the base product.

It's annoying it only works if click on with the mouse, typing more than the @ sign goes to name match and then tab won't/doesn't select the matched but have to go click on it, too...but it's still better than having to go open the full doc directly, just could be better...