# Function input

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Nina on 12 May 2012
Commented: Walter Roberson on 16 Jul 2016
Hello everyone,
I know this is simple but I just can't figure it out by myself now.
I'm trying to create a function integrator, which asks the user to type the function to be integrated. I tried to use
g = input ('Enter function to be integrated: ')
f = @(x) g;
but MATLAB refuses it with Undefined function or variable 'x'. But how can we define x before so that f wouldn't get immediately evaluated?

per isakson on 12 May 2012
Try
string_input_by_user = input( .....
string_input_by_user = 'sin';
foo = str2func( ['@(x)', string_input_by_user, '(x)' ] );
or better
foo = str2func( ['@', string_input_by_user ] );
these two variants work
>> foo(4)
ans =
-0.7568
--- 2010-05-13 ---
Divide and conquer. One thing at a time. What exactly does the user input? Assume the user inputs the tree letter string "sin". Thus, the value of
string_input_by_user
is
'sin'
Then you can create a function_handle
foo = str2func( ['@', string_input_by_user ] );
and make a test
>> foo(pi/2)
ans =
1
>> foo([ 0 : pi/4 : pi+eps] )
ans =
0 0.7071 1.0000 0.7071 0.0000
If the user inputs the six letter string "sin(x)" it's a bit more tricky. You can create a function_handle
foo = str2func( ['@(x)', string_input_by_user ] );
The problem is that the user might input "sin(y)".
Expression
In Matlab "47*x" is an expression not a function. Make your code work with functions first. Then return to expressions.
The Question, Increasing of speed in numerical calculation, shows how to use expressions. However, there is some noise.
btw. Do you step through the code with the debugger and inspect the value of the variable?

Show 1 older comment
Walter Roberson on 12 May 2012
Are you using a very old MATLAB, before there was support for anonymous functions?
Nina on 12 May 2012
I'm using R2012a...
But I get the error message after I try
string_input_by_user = input( .....)
after entering the function, no matter is sin(x), 47*x, or anything, I get
Error using input
Undefined function or variable 'x'.
If it did not happen, I would continue with my code with
f = str2func( ['@', string_input_by_user, (x)]);
But it can't get to that point yet
Walter Roberson on 12 May 2012
string_input_by_user = input( ....., 's')

mohammad reza on 12 Jul 2016
now that you have got a user defined function, how can u integrate the function in the script? (with out limit)

#### 1 Comment

Walter Roberson on 16 Jul 2016
Indefinite integration requires the Symbolic Toolbox int() function