Repeating function in c++

[spurraldo]

Hi,
I am trying to create a c++ program that will intake an original value of x_0 to begin a function that uses the previous value of x to calculate the one in question.
I.e.- f(x_0) = x_1, f(x_1) = x_2 and so on.
I understand that I will need to set up a loop using x = f(x) but am unsure how this is done exactly. The function being used is as follows: f(x) = a*sin(pi*x) where a and x_0 are the inputs. Any help would be great, thanks.

[Zhuge]

Try making f() a function that takes x and a as parameters and returns a*sin(pi*x). Prompt the user for x and a, then run a loop that assigns f(x, a) to x.

[hanst99]

Pseudocode:

x = x_0
loop
x =f(x);
endloop

That is what you described, though I got a feeling that's not what you meant.

[spurraldo]

hanst99- yes this is what i meant, i wish to input x and a then run the loop but before doing so I need to create the function and this is what I am having trouble doing.

[Zhuge]

Did you try simply writing the function as it was given to you? It should work.
int f(int x, int a) { return a*sin(PI*x); }

[hanst99]

PS: sin is defined in cmath, so you need to include that header.

#include <cmath> 

Also, PI is usually not defined, so you will have to do that yourself. Or just write 3.14.

[spurraldo]

Zhuge- I had the function included in the same way you suggested but when building I get an error saying a function-definition is not allowed here before "{" token.

[Zhuge]

Sounds like something wrong with more than just the function. Try posting your entire code if it isn't too long.

[spurraldo]

1 int main (){
2 int a,x;
3 cout<<"a=";
4 cin>>a;
5 cout<<"x=";
6 cin>>x;
7
8 int f(int x, int a) {
9 return a*sin(pi*x);
10 }
11
12 for (i=0; i<=20; i++){
13 x=f(x);
14 return f(x);
15 }
16 }

the errors occuring when compiling are:
a function-definition is not allowed here before "{" token (line 8)
"i" was not declared in the scope (line 12)
"f" was not declared in the scope (line 13)

[mookial]

Try using M_PI instead of PI, and remember #include <cmath>

[matsom]

@spurraldo:
Lines 8 to 10 should be outside int main() -you cannot define a function inside main();
ref: http://www.cplusplus.com/doc/tutorial/functions/

line 13: x = f(x) is not the correct way to call f() -it needs two integer paramaters
line 14: probably not what you intended, read the ref above.

[spurraldo]

matsom - thanks I moved lines 8 and 10 out of the main but it is now telling me that there are too few arguments in the part of the code that I moved?

[hanst99]

What we told you earlier. You defined the function not as f(x), but as f(x,a), so you also need to pass it the second parameter a.