Bisection with typedef declaration

**idecline** · 11-20-2010

Hi All,

I'm working on this C programming assignment, which requires the using of bisection method to find roots of a function. However, I only have a dim idea of how the structure should be. Like how should I declare the function, where should I place the declaration, and if im looking for the root of function, lets say f(x)=x^2+e^x, how do i set it as an argument? *f?

I try to browse around for an example of this, but couldn't find any helpful ones.

Any help will be awesome!

Thanks

**anduril462** · 11-20-2010

I'm not sure where typedef fits into all of this. This is basically an assignment about algorithm implementation. It seems a somewhat advanced problem to tackle if you don't even have the basics of C programming down (i.e. function declaration, argument passing, etc).

Presumably you've been going to your lectures and doing previous assignments, so you should have lecture notes, a textbook, and some old assignments for reference on general C programming. For more C tutorials, check here: Cprogramming.com - Programming Tutorials: C++ Made Easy and C Made Easy. For an explanation of the algorithm, along with some pseudo code, check here: Bisection method - Wikipedia, the free encyclopedia.

When you've attempted a solution, come back and show us the work you did (using code tags) and let us know what specific problems you're having. Then we can better help you.

**Char*Pntr** · 11-20-2010

Originally Posted by idecline

Hi All,

I'm working on this C programming assignment, which requires the using of bisection method to find roots of a function. However, I only have a dim idea of how the structure should be. Like how should I declare the function, where should I place the declaration, and if im looking for the root of function, lets say f(x)=x^2+e^x, how do i set it as an argument? *f?

I try to browse around for an example of this, but couldn't find any helpful ones.

Any help will be awesome!

Thanks

Have you tried writing any C code so far? What have you tried?

If not, check out some useful links here:

Pseudocode Examples

Root finding methods

or even here?

**idecline** · 11-20-2010

Originally Posted by Char*Pntr

Have you tried writing any C code so far? What have you tried?

If not, check out some useful links here:

Pseudocode Examples

Root finding methods

or even here?

Yes, I thought I was pretty ok with C before the pointer and rootfinding methods were introduced, here s what I have done with C, which is to simulate a hockey card collection:

Code:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>
#include <time.h>

/*a function that checks the duplicate cards in one  deck (for bonus question)*/
int dupcheck(int random_deck[], int i)
{
	int j;
	for(j=0;j<i;++j)
	{
		if(random_deck[i]== random_deck[j])
		{
			return (1);
		}
	}
	return(0);
}
/* a function that keeps track of how many identicals cards you have*/
int checkcards(int cardsinhand[], int temp, int numcards)
.
.
.
.
.
	mindeck=Tdeckpurchased[0];
	printf("Max is %d, Min is %d, Ave is %d\n", maxdeck,mindeck,avedeck);
	printf("Ave of Max number of duplicate card is %d\n", avedup);

	return (0);

}

**idecline** · 11-20-2010

Originally Posted by anduril462

I'm not sure where typedef fits into all of this. This is basically an assignment about algorithm implementation. It seems a somewhat advanced problem to tackle if you don't even have the basics of C programming down (i.e. function declaration, argument passing, etc).

Presumably you've been going to your lectures and doing previous assignments, so you should have lecture notes, a textbook, and some old assignments for reference on general C programming. For more C tutorials, check here: Cprogramming.com - Programming Tutorials: C++ Made Easy and C Made Easy. For an explanation of the algorithm, along with some pseudo code, check here: Bisection method - Wikipedia, the free encyclopedia.

When you've attempted a solution, come back and show us the work you did (using code tags) and let us know what specific problems you're having. Then we can better help you.

Thanks for the links,
but I learn better with examples,
like how the function f(x) should be placed,
and how in what form should i take the function into root finding function?

**anduril462** · 11-20-2010

I think Char*Pntr was asking if you have written any code in an attempt to solve your root finding problem. I'm guessing no, but based on your card program, you at least understand basic flow control, loops, functions and arrays, which is good.

At the very least, you will need a function that calculates f(x); call it f and have it take the parameter x. You will also need a function that finds the root, call it find_root, which will need parameters a and b. If you need to, you can pass in the function f using function pointers (Function Pointers in C and C++ - Cprogramming.com), but I'm not sure you need to do that. It would help if we had the full assignment description.

The wikipedia article I provided has a nice, clear pseudo-code example for you to imitate in C. They practically gave you the solution there, so just translate that pseudo code to C. You really need to take a stab at writing this program before you get any more help. Come back and post your code when you've done that, and we'll see what we can do.

**idecline** · 11-20-2010

Originally Posted by anduril462

I think Char*Pntr was asking if you have written any code in an attempt to solve your root finding problem. I'm guessing no, but based on your card program, you at least understand basic flow control, loops, functions and arrays, which is good.

At the very least, you will need a function that calculates f(x); call it f and have it take the parameter x. You will also need a function that finds the root, call it find_root, which will need parameters a and b. If you need to, you can pass in the function f using function pointers (Function Pointers in C and C++ - Cprogramming.com), but I'm not sure you need to do that. It would help if we had the full assignment description.

The wikipedia article I provided has a nice, clear pseudo-code example for you to imitate in C. They practically gave you the solution there, so just translate that pseudo code to C. You really need to take a stab at writing this program before you get any more help. Come back and post your code when you've done that, and we'll see what we can do.

I sat in front of the computer for the whole day, but didnt know how to start. I wrote some codes down on the paper, but did not seem to work at all...
Upload the assignment is kinda cheating? I still wanna try it out myself, so I just try to understand the parts im confused at.

and yes, i think a function for calculating the f(x), and a function find_root are what I need. But the equation I have actually has more variables that are to be defined by users, the equation is :
f(T)=T/w*cosh(wx/T) + ymin - T/w - y

where all values will be given by user except T.

I'll try to do my best and post it up, but...
should I write a function "to calculate f(x)" first? like : f(x,y,ymin,w){...}
and use the bisection to find the root in this same function?

THANKS

**anduril462** · 11-20-2010

I sat in front of the computer for the whole day, but didnt know how to start. I wrote some codes down on the paper, but did not seem to work at all...

We've all been there before. It really helps if you break your problem up into smaller pieces. Start with writing your input routines, and test that. Then write your f(x) function and test that against some hand calculations or known values to make sure you're still on track. Then write your find_root function. You could even break that up into smaller pieces, like calculate the error and midpoint of a and b, then evaluate f(x) for a, b and midpoint, printing out the values as you go. Verify this against hand calculations or known results and make sure you're on the right track. Then decide whether to pick the left half of your range (a, midpoint) or right half (midpoint, b) to discard, and set your new a or b accordingly, again printing out your new values to make sure you're on track. Then put it all in a loop and see if it converges.

Upload the assignment is kinda cheating?

You should check with your professor first. In my personal opinion, if you have made a serious effort and done most of the assignment yourself, then coming here for help is no different than discussing your problems with a classmate, TA or professor, and asking them to help you debug. We can help you sort out syntax and logic errors, general design questions, and just about any other pertinent question you have, we just can't write the code for you.

should I write a function "to calculate f(x)" first? like : f(x,y,ymin,w){...}

Yes, then test it with a few values for which you know the f(T) and make sure it's correct.

and use the bisection to find the root in this same function?

No. You don't want to call the find_root function from within f, you want to call f from the find_root function.

**idecline** · 11-20-2010

I did the best I can and stuck at the scanf part.
I thought float and double share the %f, but when i ran the test, it told me:
'warning: float format, double arg'

and I tried to use typedef to include my f() as an argument, but seems like a failure.

Help PLEASE,
Thank you

Code:

#include <math.h>
#include <stdio.h>

double f(double T0, double x, double y, double ymin, double w)
{
	return ( (T0 / w) * cosh(w * x / T0) + ymin - T0 / w - y);
}

typedef double (*DfD) (double);
double find_root(DfD f, double Ta, double Tb, double tol)
{
	double middle = (Ta + Tb)/2.0;
	if ((middle - Ta)< tol)
		return middle;
	else if (f(middle,x,y,ymin,w)* f(Ta,x,y,ymin,w)<0.0)
		return find_root(f,Ta,middle,tol);
	else
		return find_root(f,middle,Tb,tol);
}

int main(void)
{
	double x, y, ymin, w;
	double newy;
	scanf("%f%f%f%f",&x,&y,&ymin,&w);
	newy = &find_root;
	return(0);
}

**idecline** · 11-20-2010

You should check with your professor first. In my personal opinion, if you have made a serious effort and done most of the assignment yourself, then coming here for help is no different than discussing your problems with a classmate, TA or professor, and asking them to help you debug. We can help you sort out syntax and logic errors, general design questions, and just about any other pertinent question you have, we just can't write the code for you.

Or maybe just to help you to understand what im trying to do.
I have it attached.
Attachment 10179

**idecline** · 11-20-2010

Made some correction, hope this is better than what i had ?

Code:

#include <math.h>
#include <stdio.h>


double x, y, ymin, w, T0;

double f(double T0, double x, double y, double ymin, double w)
{
	return ( (T0 / w) * cosh(w * x / T0) + ymin - T0 / w - y);
}

typedef double (*DfD) (double);
double find_root(DfD f, double Ta, double Tb, double tol)
{
	double middle = (Ta + Tb)/2.0;
	if ((middle - Ta)< tol)
		return middle;
	else if (f(middle)* f(Ta)<0.0)
		return find_root(f,Ta,middle,tol);
	else
		return find_root(f,middle,Tb,tol);
}

int main(void)
{
	scanf("%f%f%f%f",&x,&y,&ymin,&w);
	f(T0,x,y,ymin,w);
	return(0);
}

**idecline** · 11-21-2010

Thank you guys,
I finally figure it out and got it done!
THANKS FOR THE PUSH!!!
YOU GUYS ARE AWESOME....

**anduril462** · 11-21-2010

Glad to hear, and you're very welcome!

**Char*Pntr** · 11-21-2010

Originally Posted by anduril462

Bisection method - Wikipedia, the free encyclopedia.

Well I quess we all learned something here! I particularly enjoyed anduril462's link.
Bookmarked. :-)

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