PLEASE HELP, Beginner Problem with different results every time!!

edward_fraser · #1 03-May-2005, 06:05

Hey everytime i run my c++ program to implement an Improved Eulers Method numerical loop to calculate the response of something (dont worry about the maths behind it i know the equations are right!) i get different answers each time i do it. Ive tried setting each of the variables in the equation to 0 at the beginning of each loop but this didnt help?? I wont bother posting the results because they are each 1000 entry long text files, but they change slightly everytime!? Any ideas would be greatly appreciated?? Thanks alot,

Ed Fraser

CPP / C++ / C Code:


define variables n open libraries;

int main()
{
        m = 290, 
        k = 16182,
        c = 1000,     
        y0 = 0, 
        v0 = 0,
        t0 = 0;
 
 
 
 
    again=1;      
    while (again==1)
        {
          
        y = y0, v = v0, t = t0,
        K1 = 0, K2 = 0,
        L1 = 0, L2 = 0,
        dt = 0, t = 0, speed = 0;
        
        
       
        
        
        
        display menu;
                   
        
        if (menuchoice==1)
        {
        
                FILE *ifile;
                printf("\n\nPlease enter road profile filename: ");
                scanf("%s",filename);
                ifile=fopen(filename,"r");
        
                system("cls");
                printf("\n\nPlease enter the speed of the vehicle (mph): ");
                scanf("%f",&speed);
                        
                
                /* Convert speed from m.p.h. to ms-1 */
                
        
                
                speed = (speed * 0.44704);                        
                total_time = 150 / speed;              
                num_steps = 1000;                 
                dt = total_time / num_steps;
        
      
       
                        for(loop=0; loop<=num_steps; loop++)
                        {
                        fscanf(ifile, "%f\n", &rt[loop]);
                        }
           
                fclose(ifile);
                
        
                FILE *foutput;                
                foutput = fopen("output.txt", "w");
        
                        for (count=1; count<=num_steps; count++)
                        {       
                                
                                
                                
                                K1 = (dt*(c*(rt[count]-rt[count-1])/(dt*1000)+(k*rt[count]/1000)-(k*y)-(c*v)))/m;
                                K2 = dt*((v + K1)/(dt + t));                
                                L1 = dt*v;
                                L2 = dt * ((y + L1)/(dt + t));
                                v = v + 0.5*(K1 + K2);
                                y = y + 0.5*(L1 + L2);
                                t = t + dt;
                                
                                tout[count] = t;
                                yout[count] = y;
                                             
                                fprintf(foutput, "\n%f,", t);
                                fprintf(foutput, " %f", y);
                
                                      
                        } 
                        
                fclose(foutput);
                
                system("cls");
                printf("\n\nThe results for the response of the vehicle can be found \nin the output.txt file!");                      
                         
                        
                
                            
                }       
                
        else 
        
        if (menuchoice==2)
        {
        
                float x; 
                              
                system("cls");
                printf("\n\nPlease enter the speed of the vehicle (mph): ");
                scanf("%f",&speed);
        
                /* Convert speed from m.p.h. to ms-1 */
                                
                speed = (speed * 0.44704);                        
                total_time = 3 / speed;              
                hump_steps = 100;                 
                dt = total_time / hump_steps;
                
                         
                        for (i=1; i<=num_steps; i++)
                        {
                                if (i<=hump_steps)
                                {
                                x = 3.1415 * ( t / total_time ); 
                                rt[i] = sin(x)*0.08; 
                                t = t + dt;                                              
                                }
                                
                                else 
                                {
                                rt[i]=0;
                                }
                        }   
                        
                        FILE *foutput;                
                        foutput = fopen("output.txt", "w");
        
                        for (count=1; count<=num_steps; count++)
                        {       
                                
                                
                                
                                K1 = (dt*(c*(rt[count]-rt[count-1])/(dt*1000)+(k*rt[count]/1000)-(k*y)-(c*v)))/m;
                                K2 = dt*((v + K1)/(dt + t));                
                                L1 = dt*v;
                                L2 = dt * ((y + L1)/(dt + t));
                                v = v + 0.5*(K1 + K2);
                                y = y + 0.5*(L1 + L2);
                                t = t + dt;
                                
                                tout[count] = t;
                                yout[count] = y;
                                             
                                fprintf(foutput, "\n%f,", t);
                                fprintf(foutput, " %f", y);
                
                                      
                        } 
                        
                fclose(foutput);
                
                system("cls");
                printf("\n\nThe results for the response of the vehicle can be found \nin the output.txt file!");
                
                
        }                  
                              
             
        leaving menu;               
        if (again==1)
        {
                 printf("Great!\n\n");
                 system("PAUSE");   
        }
       
        else
      
        if (again==2)
        {
                printf("\n\nThank you for using the road profile response  emulator.\n\n");   
        }
                       
                   
                                            
        }

        return 0;   
        
}

cable_guy_67 · #2 03-May-2005, 06:45

Ed, welcome to GIDForums. Glad to see your code in tags. Really.

But, you have not declared a single variable in your code (c not c++). Some just need a little push:

Quote:

Originally Posted by edward_fraser

CPP / C++ / C Code:

int main()
{
        m = 290,
        k = 16182,
        c = 1000,
        y0 = 0,
        v0 = 0,
        t0 = 0;

becomes:

CPP / C++ / C Code:

int main()
{
   int  m = 290,
        k = 16182,
        c = 1000,
        y0 = 0,
        v0 = 0,
        t0 = 0;

and so on. You will also need some includes to get things like:

Quote:

Originally Posted by edward_fraser

CPP / C++ / C Code:

    FILE *ifile;
    printf("\n\nPlease enter road profile filename: ");
    scanf("%s",filename);
    ifile=fopen(filename,"r");

to work. Work on that, post the errors you are getting. Right now, that dog won't fly.

Once you get to that point let us know what happens,

Quote:

Originally Posted by example

When I use 0 as my input my computer starts on fire. I am compiling with (insert compiler here) on (insert os here). This only happens with this input. This is what I am seeing.

Code:

>gcc -Wall myprog.c -o myexe
>myexe.exe

Welcome to my program
Enter the number : 0
BOOM!

Well, you get the idea...

edward_fraser · #3 03-May-2005, 10:45

Hey, i've improved the program and added all the inlcude's etc... the program does the calculation fine and outputs it to a .txt file, although if i dont close the program and start it again, will give me different values everytime i do it. This makes me think i need to reset the values somehow but im not sure where or how?? Any ideas would be really great cause im ripping my hair out over it!!

Cheers,

E
D

CPP / C++ / C Code:


/* Assignment 2 - H3BCNT */
/* Program By Edward Fraser */
/* Written 20/04/05 */  

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

float rt[1000], yout[1000], tout[1000];
float t, y, v, dt, k, m, c, 
      L1, K1, K2, speed, 
      y0, v0, t0, L2,
      total_time;
char filename[20];
int count, num_steps,
    again, menuchoice,
    i, hump_steps, loop;

int main()
{
        m = 290, 
        k = 16182,
        c = 1000,     
        y0 = 0, 
        v0 = 0,
        t0 = 0;
 
 
 
 
    again=1;      
    while (again==1)
        {
          
        y = y0, v = v0, t = t0,
        K1 = 0, K2 = 0,
        L1 = 0, L2 = 0,
        dt = 0, t = 0, speed = 0;
        
        
       
        
        
        
        system("cls");
        printf("Welcome to the Road Emulator v1.0");
        printf("\n\nThis program will simulate a road surface to");
        printf("\nanalyse the response of a car driving along it");
        printf("\n\nWould you like to:");
        printf("\n1. View response to a specified road profile");
        printf("\n2. View response to a speed bump\n");
        scanf("%d",&menuchoice);
        
        while ((menuchoice!=1)&&(menuchoice!=2))
        {
          system("cls");
          printf("\n\nSorry you must choose either 1 for road profile response");
          printf("\nor 2 for the road hump response!! \n\nPlease re-enter: ");
          scanf("%d",&menuchoice);
        }       
                 
        
        
        if (menuchoice==1)
        {
        
                system("cls");
                FILE *ifile;
                printf("\n\nPlease enter road profile filename: ");
                scanf("%s",filename);
                ifile=fopen(filename,"r");
        
                        while(ifile==NULL)
                        { 
                         system("cls");
                         printf("\n\n  Sorry improper filename, please re-enter: ");
                         scanf("%s",filename);
                         ifile=fopen(filename,"r");
                         }   
        
                system("cls");
                printf("\n\nPlease enter the speed of the vehicle (mph): ");
                scanf("%f",&speed);
        
                        while (speed>50)
                        {
                        system("cls");
                        printf("\n\nI'm sorry the speed must be between 0 and 50 mph!");
                        printf("\n\n          Please Re-enter Speed (mph): ");
                        scanf("%f", &speed);
                        }
                
                
                /* Convert speed from m.p.h. to ms-1 */
                
        
                
                speed = (speed * 0.44704);                        
                total_time = 150 / speed;              
                num_steps = 1000;                 
                dt = total_time / num_steps;
        
      
       
                        for(loop=0; loop<=num_steps; loop++)
                        {
                        fscanf(ifile, "%f\n", &rt[loop]);
                        }
           
                fclose(ifile);
                
        
                FILE *foutput;                
                foutput = fopen("output.txt", "w");
        
                        for (count=1; count<=num_steps; count++)
                        {       
                                
                                
                                
                                K1 = (dt*(c*(rt[count]-rt[count-1])/(dt*1000)+(k*rt[count]/1000)-(k*y)-(c*v)))/m;
                                K2 = dt*((v + K1)/(dt + t));                
                                L1 = dt*v;
                                L2 = dt * ((y + L1)/(dt + t));
                                v = v + 0.5*(K1 + K2);
                                y = y + 0.5*(L1 + L2);
                                t = t + dt;
                                
                                tout[count] = t;
                                yout[count] = y;
                                             
                                fprintf(foutput, "\n%f,", t);
                                fprintf(foutput, " %f", y);
                
                                      
                        } 
                        
                fclose(foutput);
                
                system("cls");
                printf("\n\nThe results for the response of the vehicle can be found \nin the output.txt file!");                      
                         
                        
                
                            
                }       
                
        else 
        
        if (menuchoice==2)
        {
        
                float x; 
                              
                system("cls");
                printf("\n\nPlease enter the speed of the vehicle (mph): ");
                scanf("%f",&speed);
        
                /* Convert speed from m.p.h. to ms-1 */
                                
                speed = (speed * 0.44704);                        
                total_time = 3 / speed;              
                hump_steps = 100;                 
                dt = total_time / hump_steps;
                
                         
                        for (i=1; i<=num_steps; i++)
                        {
                                if (i<=hump_steps)
                                {
                                x = 3.1415 * ( t / total_time ); 
                                rt[i] = sin(x)*0.08; 
                                t = t + dt;                                              
                                }
                                
                                else 
                                {
                                rt[i]=0;
                                }
                        }   
                        
                        FILE *foutput;                
                        foutput = fopen("output.txt", "w");
        
                        for (count=1; count<=num_steps; count++)
                        {       
                                
                                
                                
                                K1 = (dt*(c*(rt[count]-rt[count-1])/(dt*1000)+(k*rt[count]/1000)-(k*y)-(c*v)))/m;
                                K2 = dt*((v + K1)/(dt + t));                
                                L1 = dt*v;
                                L2 = dt * ((y + L1)/(dt + t));
                                v = v + 0.5*(K1 + K2);
                                y = y + 0.5*(L1 + L2);
                                t = t + dt;
                                
                                tout[count] = t;
                                yout[count] = y;
                                             
                                fprintf(foutput, "\n%f,", t);
                                fprintf(foutput, " %f", y);
                
                                      
                        } 
                        
                fclose(foutput);
                
                system("cls");
                printf("\n\nThe results for the response of the vehicle can be found \nin the output.txt file!");
                
                
        }                  
                              
             
        printf("\n\nWhat would you like to do now?\n1. Calculate Another Profile");
        printf("\n2. Exit Program\n\n");
        scanf("\n\n%d",&again);
        system("cls");
               
        if (again==1)
        {
                 printf("Great!\n\n");
                 system("PAUSE");   
        }
       
        else
      
        if (again==2)
        {
                printf("\n\nThank you for using the road profile response emulator.\n\n");   
        }
                       
                   
                                            
        }

        return 0;   
        
}

WaltP · #4 03-May-2005, 23:00

Read the Sticky. Take careful note of the guideline Do not start new threads on the same topic

And keep your hair on your side of the keyboard. I just found some in my soup

Generally, reset your variables as you start your pass into the calculations portion of the code. Usually at the beginning of a loop.

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PLEASE HELP, Beginner Problem with different results every time!!

Improved but still doesn't get repeated results!?!?!