Pre-specify user input to function

This is the first time I've posted here, so apologies if it's in the wrong place.

I have a function which takes a user input and I would like to be able to pre-specify the input before the function is called. A basic example is below

function c = myfunc(a,b)

    d = input('Enter value for d: ');
    c = a + b + d;

end

My function is called inside a loop and I would like to not have to manually enter my 'd' value each time. I also cannot add d to the list of function arguments, because the function is called in lots of other places in the code and I don't want to go through and change them all.

Essentially, I would like the functionality that this bash script (script.sh) has

#!/bin/bash

echo "Specify d: "
read d

echo $d

when called as

./script.sh <<< 5

Is this possible?

Thanks.

Answer 1

[Stephen23]

You could use a persistent variable:

function c = myfunc(a,b)
persistent d
if isempty(d)
    d = input('Enter value for d: ');
end
c = a + b + d;
end

and calling:

>> myfunc(1,2)
Enter value for d: 7
ans =
    10
>> myfunc(1,3)
ans =
    11
>> myfunc(1,4)
ans =
    12

To reset the d value could either add some code (e.g. set d=[] when the function is called with no inputs) or use clear:

>> clear myfunc
>> myfunc(1,5)
Enter value for d: 3
ans =
     9
>> myfunc(1,6)
ans =
     10

[Scott Morgan]

Thanks for the quick reply! This is exactly what I wanted.

Answer 2

[Guillaume]

Yes, this is absolutely the right place.

No, it is not possible to programmatically feed an input to the input function.

I don't get the "the function is called in lots of other places in the code and I don't want to go through and change them all.". Using your hypothetical syntax, you would still have to modify each call to prefeed the value.

If you only want to manually input a value once, and reuse the same value thereafter, you can modify the function to use a persistent variable:

function c = myfunc(a,b)
    persistent d;
    if isempty(d)  %only prompt on first call
       d = input('Enter value for d: ');
    end
    c = a + b + d;
end

Note: to reinitialise the persistent variable to empty:

clear myfunc

Answer 3

[Steven Lord]

I would use nargin or separate the computational piece from the interface piece.

nargin based approach

function c = myfunc(a, b, d)
  if nargin < 3
    d = input('Enter value for d: ');
  end
  c = a + b + d;
end

separation of concerns approach

function c = myfunc(a,b)
  d = input('Enter value for d: ');
  c = myfuncImpl(a, b, d);
end

Those places in your code where you need to prompt the user for a value of d would call myfunc.

function c = myfuncImpl(a, b, d)
  c = a + b + d;
end

Those places for which d is already known and the code does not need to prompt the user would call myfuncImpl instead of myfunc.