What wrong with my code

[spring2014]

Hi,
I want to write a code that calculate the dy/dx=u (which u is a constant number) by euler method. But there is error in line 51
When I write for example 10 instead of u in line 51 (and delete line 18) and enter 40 for MAX it works and the final answer is near 400 but when I want to change the code in that way that it ask me 10 in line 18 and put to u and calculate it in does not work. why? how can I correct it?

  #include <iostream>
#include <cmath>
using namespace std;
const double dist=0.001;               /* stepsize in t*/

int N=1;
void euler(double x, double y[], double step); /* Euler function */

double f(double x, double y[], int i);          /* function for derivatives */

int main()
{
double t, y[N];
int j;
double MAX;
double u;
cin>>MAX;
cin>>u;
               /* max for t */

y[0]=0;                                       /* initial condition */


for (j=0; j*dist<=MAX ;j++)                     /* time loop */
{
   t=j*dist;
   cout<<"j="<<j<<"\t"<<"t="<<t<<"\t"<<"y[t]="<< y[0]<<"\n";
   euler(t, y, dist);

}

}

void euler(double x, double y[], double step)
{
double  s[N];      /* for euler */
int i;
{
for (i=0;i<N;i++)
 {     s[i]=step*f(x, y, i);
}
}

{
for (i=0;i<N;i++)
     y[i]+=s[i];
}
}
double  f(double x, double y[], int i)
{
      return(u);                 /* derivative of first equation */
}

[keskiverto]

1. Your indentation is not informative. It is hard to see the scopes.

2. You say "there is error". I bet you mean: "compiler returns an error", but you don't tell what the compiler actually says. It is very useful to learn to understand what the compiler tells you.

[anup30]

can you show another similar simple example without calculus? i have forgotten my college differentiation (a long time ago!)

[spring2014]

The compiler says :

14:15:44 **** Incremental Build of configuration Debug for project tesy10 ****
Info: Internal Builder is used for build
g++ -O0 -g3 -Wall -c -fmessage-length=0 -o main10.o "..\\main10.cpp"
..\main10.cpp: In function 'double f(double, double*, int)':
..\main10.cpp:58:14: error: 'u' was not declared in this scope
return(u); /* derivative of first equation */
^
..\main10.cpp:59:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^

It says u is not declared in this scope at line 51

[SECMoG1T]

You dint declare u as a parameter nor is u a global variable so that is why you get the error

[spring2014]

How can I fix it?

[SECMoG1T]

By passing u as a parameter

 double f(double x, double y[], int i , double u) /// u is a parameter
   { 
        return(u); /* derivative of first equation */
    }

/// then again I noted you are not using parameters x, y, and i what is their need. 

[SECMoG1T]

Hold on a second , what is u ssupposed to be maybe a result of some calculation within the function
If so then you can make a local variable u

  double f(double x, double y[], int i ) 
     { 
         double u;
         u= // some calculations here.
         return(u); /* derivative of first equation */
     }