Function input

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?

Thanks a lot in adavnce!

Answer 1

[per isakson]

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?

[Nina]

Thank you! Unfortunately, I don't know why, when I tried this, I still got the error message Undefined function or variable 'x'. Did you define x somehow before?

[Walter Roberson]

Are you using a very old MATLAB, before there was support for anonymous functions?

Please show your current code.

[Nina]

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]

string_input_by_user = input( ....., 's')

Answer 2

[mohammad reza]

now that you have got a user defined function, how can u integrate the function in the script? (with out limit)

[Walter Roberson]

Indefinite integration requires the Symbolic Toolbox int() function