Outputting problem

[superja]

I have some problem outputting double variable types.

Below is my script:

 #include <stdio.h> 
#include <math.h>
 
double x;
double h;
int n, choice;
 
void Euler(double x, double h, int n);
void RK4(double x, double h, int n);
double f(double x, double y);
double yexact(double x);
 
main()
{ 
printf("Enter the initial value of x: ");
scanf("%lf", &x);
 
printf("\nEnter step-size h: ");
scanf("%lf", &h);
 
printf("\nEnter the number of steps n: ");
scanf("%d", &n);
 
printf("\nEnter 0 for Euler Method or 1 for RK4: ");
scanf("%d", &choice); 
 
 
while(choice < 0 || choice > 1)
{
printf("\nEnter 0 for Euler Method or 1 for RK4: ");
scanf("%d", &choice);
}
 
if(choice == 0)
{
Euler(x, h, n); 
}
if(choice == 1)
{
RK4(x, h, n);
}
 
scanf("%d");
return 0;
}
 
 
/* Function for Euler Method */
 
void Euler(double x, double h, int n)
{ 
 
double Y, y, E;
int ctr; 
Y = 1;
y = yexact(x);
printf("\nx_0 = %lf\nY_0 = 1\ny_0 = %lf\n", x, y);
 
for(ctr = 1; ctr < n+1; ctr++)
{
Y = Y + h*f(x, Y);
 
x = x + h;
y = yexact(x);
 
printf("\nx_%d = %lf\n", ctr, x);
printf("Y_%d = %lf\n", ctr, Y);
printf("y_%d = %lf\n", ctr, y);
E = Y - y;
printf("E_%d = %lf\n\n", ctr, E);
}
}
 
/* Function for RK4 */
 
void RK4(double x, double h, int n)
{
double Y, y, E;
int ctr;
double k_1, k_2, k_3, k_4; 
 
Y = 1;
y = yexact(x);
printf("\nx_0 = %lf\nY_0 = 1\ny_0 = %lf\n", x, y);
 
for(ctr = 1; ctr < n+1; ctr++)
{
k_1 = h*f(x, Y);
k_2 = h*f(x + 0.5*h, Y + 0.5*k_1);
k_3 = h*f(x + 0.5*h, Y + 0.5*k_2);
k_4 = h*f(x + h, Y + k_3);
 
Y = Y + (k_1 + 2*k_2 + 2*k_3 + k_4)/6;
x = x + h;
y = yexact(x);
 
printf("\nx_%d = %lf\n", ctr, x);
printf("Y_%d = %lf\n", ctr, Y);
printf("y_%d = %lf\n", ctr, y);
E = Y - y;
printf("E_%d = %lf\n\n", ctr, E);
}
} 
 
/* Function for f(x,y) */
 
double f(double a, double b)
{
double c;
c = -2*b -0.25*a*a + 0.125;
return c;
}
 
/* Function for y(x) */
 
double yexact(double d)
{
double y_exact;
y_exact = exp(-2*d) - 0.125*d*d + 0.125*d;
return y_exact;
}
 

What is displayed on the screen is only 7 digits, but I need it to be double precision. I can't see the problem, please help.

[DeMan]

%d print an int argument in decimal
%ld print a long int argument in decimal
%c print a character
%s print a string
%f print a float or double argument
%e same as %f, but use exponential notation
%g use %e or %f, whichever is better
%o print an int argument in octal (base 8)
%x print an int argument in hexadecimal (base 16)
%% print a single %

That is, you should use %f for doubles (or %e for scientific notation). You can also format the output to give (among other things) the sepecified number of decimal places so if
n=3.1415926535897

printf("%.2f", n); //If memory serves me correct
would print 3.14


you can similarly force the number of characters before the decimal point eg

printf("%2.2f", n); //or similar
would print 03.14

searching for formatted print on the net could explain better....

[superja]

I've tried forcing 13 digits but it just stops calculating correctly, just a load of zeros. I'm using Bloodshed Dev c++, could it be something to do with the compiler. Although I have tried everything I can think of.

[DeMan]

sorry, tyhe biggest point I was trying to allude to is that to print a double you should be using %f (float, rather than %d (decimal integer) :)

[superja]

I am using %lf for the doubles. But it's ok now it's been dealt with. It must be something picky with the compiler or something.