C Program for Regula Falsi (False Position) Method

This program implements false position (Regula Falsi) method for finding real root of nonlinear equation in C programming language.

In this C program, x0 & x1 are two initial guesses, e is tolerable error and f(x) is non-linear function whose root is being obtained using false position method.

C Source Code: False Position Method

``````
/* Program: Finding real roots of nonlinear
equation using Regula Falsi or False Position Method
Author: CodeSansar
Date: November 18, 2018 */
#include<stdio.h>
#include<conio.h>
#include<math.h>
/* Defining equation to be solved.
Change this equation to solve another problem. */
#define   f(x)   x*log10(x) - 1.2

int main()
{

float x0, x1, x2, f0, f1, f2, e;
int step = 1;
clrscr();
/* Inputs */
up:
printf("\nEnter two initial guesses:\n");
scanf("%f%f", &x0, &x1);
printf("Enter tolerable error:\n");
scanf("%f", &e);
/* Calculating Functional Values */
f0 = f(x0);
f1 = f(x1);
/* Checking whether given guesses brackets the root or not. */
if( f0*f1 > 0.0)
{
printf("Incorrect Initial Guesses.\n");
goto up;
}
/* Implementing Regula Falsi or False Position Method */
printf("\nStep\t\tx0\t\tx1\t\tx2\t\tf(x2)\n");
do
{
x2 = x0 - (x0-x1) * f0/(f0-f1);
f2 = f(x2);
printf("%d\t\t%f\t%f\t%f\t%f\n",step, x0, x1, x2, f2);

if(f0*f2 < 0)
{
x1 = x2;
f1 = f2;
}
else
{
x0 = x2;
f0 = f2;
}
step = step + 1;

}while(fabs(f2)>e);

printf("\nRoot is: %f", x2);
getch();
return 0;
}
``````

Output: False Position Method

```Enter two initial guesses:
2
3
Enter tolerable error:
0.000001

Step        	x0             x1          x2             f(x2)
1           2.000000        3.000000      2.721014        -0.017091
2           2.721014        3.000000      2.740206        -0.000384
3           2.740206        3.000000      2.740636        -0.000009
4           2.740636        3.000000      2.740646        -0.000000

Root is: 2.740646
```