arrays- two dimensional

im working on a project on where i have to come up with Van der Waals equation and im having trouble with arrays....need help on how to call for the constants! i have no idea how to make it work correctly.....once, i have this correct, i would have to make a seperate func to calculate the equation.....but i want to work on the arrays first.

Code:

---

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

void elements(float*,float*);
void vd_constants(constants[6][2]);

int main()
{
  FILE *output = NULL;

      float p,        /*Pressure*/
            v,        /*Volume of Container*/
            T,        /*Absolute Temperature*/
            n,        /*Input Number of Moles*/
            user_element;

      const float R=8.314472;        /*Gas Constant*/

      float *a,      /*Inputted by user defined element*/
            *b,      /*Inputted by user defined element*/
            eq1,
            eq2,
            eq;

  if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
  else
  {
      printf("Van der Waals Equation:\n\n");
      printf("    an^2\n"
            "(P + -----)(V-nb)=nRT\n"
            "      V^2\n\n");

      elements(a,b);

      printf("\n");
      printf("a=%f b=%f\n\n",*a,*b);

      printf("Please enter the amount of pressure, volume, and moles: "
            "(separated by a space)\n\n");
      scanf("%f %f %f",&p,&v,&n);
      printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);

      eq1=p+(*a*(n*n/v*v));
      eq2=v-(n**b);
      eq=(eq1*eq2)/(n*R);

      printf("Temperature is equal to %f Kelvin\n\n",eq);

      fprintf(output,"Since a=%f and b=%f\n"
                  "Pressure=%f Volume=%f and Moles=%f\n\n"
                  "Temperature is equal to %f Kelvin\n\n",*a,*b,p,v,n,eq);
      fclose(output);
  }
system("PAUSE");
return 0;
}

void elements(float *a, float *b)
{

      int user_element;
      vd_constants(constants[6][2]);

      printf("Which element do you want to solve for T?\n\n");
      printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen chloride\n"
            "4-Oxygen\n5-Sulfur dioxide\n6-Water\n\n");
      scanf("%d",&user_element);

      switch(user_element)
      {
        case 1:
                *a=constants[1][1];
                *b=constants[1][2];
                break;

        case 2:
                *a=constants[2][1];
                *b=constants[2][2];
                break;

        case 3:
                *a=constants[3][1];
                *b=constants[3][2];
                break;

        case 4:
                *a=constants[4][1];
                *b=constants[4][2];
                break;

        case 5:
                *a=constants[5][1];
                *b=constants[5][2];
                break;

        case 6:
                *a=constants[6][1];
                *b=constants[6][2];
                break;
      }

}
void vd_constants(constants[6][2])
{
constants[6][2]={{3.64, .04267},
                {.03457, .0237},
                {3.716, .04081},
                {1.378, .03183},
                {6.803, .05636},
                {5.536, .03049}};
}

---

You could just declare constants local to function elements.

Code:

---

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

void elements(float*,float*);

int main(void)
{
  FILE *output = NULL;

      float p,        /*Pressure*/
            v,        /*Volume of Container*/
            T,        /*Absolute Temperature*/
            n,        /*Input Number of Moles*/
            user_element;

      const float R=8.314472;        /*Gas Constant*/

      float a,      /*Inputted by user defined element*/
            b,      /*Inputted by user defined element*/
            eq1,
            eq2,
            eq;

  if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
  else
  {
      printf("Van der Waals Equation:\n\n");
      printf("    an^2\n"
            "(P + -----)(V-nb)=nRT\n"
            "      V^2\n\n");

      elements(&a,&b);

      printf("\n");
      printf("a=&#37;f b=%f\n\n",*a,*b);

      printf("Please enter the amount of pressure, volume, and moles: "
            "(separated by a space)\n\n");
      scanf("%f %f %f",&p,&v,&n);
      printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);

      eq1=p+(*a*(n*n/v*v));
      eq2=v-(n**b);
      eq=(eq1*eq2)/(n*R);

      printf("Temperature is equal to %f Kelvin\n\n",eq);

      fprintf(output,"Since a=%f and b=%f\n"
                  "Pressure=%f Volume=%f and Moles=%f\n\n"
                  "Temperature is equal to %f Kelvin\n\n",*a,*b,p,v,n,eq);
      fclose(output);
  }
system("PAUSE");
return 0;
}

void elements(float *a, float *b)
{

constants[6][2]={{3.64, .04267},
                {.03457, .0237},
                {3.716, .04081},
                {1.378, .03183},
                {6.803, .05636},
                {5.536, .03049}};
      int user_element;

      printf("Which element do you want to solve for T?\n\n");
      printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen chloride\n"
            "4-Oxygen\n5-Sulfur dioxide\n6-Water\n\n");
      scanf("%d",&user_element);

      switch(user_element)
      {
        case 1:
                *a=constants[1][1];
                *b=constants[1][2];
                break;

        case 2:
                *a=constants[2][1];
                *b=constants[2][2];
                break;

        case 3:
                *a=constants[3][1];
                *b=constants[3][2];
                break;

        case 4:
                *a=constants[4][1];
                *b=constants[4][2];
                break;

        case 5:
                *a=constants[5][1];
                *b=constants[5][2];
                break;

        case 6:
                *a=constants[6][1];
                *b=constants[6][2];
                break;
      }

}

---

And your equation shows:
v - (n**b)
Should it be:
v - (n*b)?

well, the thing is...for my project, i must have 4 functions including Main, so i wanted to create two other functions, one is equation and the other is the constants, and im trying to make it so that it works, but i donno how, as for the n**b part, its supposed to be N TIMES *b since *b is a pointer variable....that's why it looks like n**b

> i must have 4 functions including Main,
main calls
- input data
- calculate
- print results

Quote:

---

Originally Posted by SuPaNooB

...as for the n**b part, its supposed to be N TIMES *b since *b is a pointer variable....that's why it looks like n**b

---

Maybe you should do

Code:

---

v - ( n * (*b) )

---

Because it looks more readable to me and tells me that b is, indeed, a pointer and not some silly mistake.
BTW, you don't even need paranthesis around the multiplication since it goes first. Voíla:

Code:

---

v - n * (*b);

---

Or maybe even give it a better name, such as pB, at the very least.

ok ok, i got my program to work, i decided to change a lot of stuff, but i ran into problems...here's my code so far:

my first problem is that for some reason, once u run the program, it freezes in a loop right when it invokes the EQUATION func.....y? also, i wanted to know, how can i make it so that, as a new func, i can make it so that the user has an option to rerun MAIN() by pressing Y or N....?

Code:

---

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

void elements(float*,float*);
float equation(float);
void tryagain();
int main()
{
    FILE *output = NULL;

        float p,                  /*Pressure*/
              v,                  /*Volume of Container*/
              n,                  /*Inputted Number of Moles*/
              *a,                /*Inputted by user defined element*/
              *b,                /*Inputted by user defined element*/
              eqfinal;            /*Van der Waals equation*/

    if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
    else
    {
      printf("Van der Waals Equation:\n\n");
      printf("    an^2\n"
              "(P + -----)(V-nb)=nRT\n"
              "      V^2\n\n");

      elements(a,b);

      printf("\n");
      printf("a=%f b=%f\n\n",*a,*b);

      printf("Please enter the amount of pressure (atm), volume (L), and "
              "moles (mol):\n(Separated by a Space)\n\n");
      scanf("%f %f %f",&p,&v,&n);
      printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);

      equation(eqfinal);

      printf("Temperature is equal to %.3f Kelvin\n\n",eqfinal);

      fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",*a,*b,
                      p,v,n,eqfinal);
      fclose(output);
    }

system("PAUSE");
return 0;
}

void elements(float *a, float *b)
{
    int user_element;

    float constants[6][2]={
                          {3.640, .04267},
                          {.03457, .0237},
                          {3.716, .04081},
                          {1.378, .03183},
                          {6.803, .05636},
                          {5.536, .03049}
                          };

    printf("Which element do you want to solve for T?\n\n");
    printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen Chloride\n"
          "4-Oxygen\n5-Sulfur Dioxide\n6-Water\n\n");
    scanf("%d",&user_element);

    switch(user_element)
    {
      case 1:
              *a=constants[0][0];
              *b=constants[0][1];
              break;
      case 2:
              *a=constants[1][0];
              *b=constants[1][1];
              break;
      case 3:
              *a=constants[2][0];
              *b=constants[2][1];
              break;
      case 4:
              *a=constants[3][0];
              *b=constants[3][1];
              break;
      case 5:
              *a=constants[4][0];
              *b=constants[4][1];
              break;
      case 6:
              *a=constants[5][0];
              *b=constants[5][1];
              break;
    }
}
float equation(float eqfinal)
{
    float *a,          /*Inputted by user defined element*/
          *b,          /*Inputted by user defined element*/
          p,
          v,
          n,
          eq1,
          eq2;

    const float R=8.314472;        /*Gas Constant*/

    eq1=p+(*a)*((n*n)/(v*v));
    eq2=v-n*(*b);
    eqfinal=(eq1*eq2)/(n*R);

    return eqfinal;
}
void tryagain()
{
 char ans;
 do
  {
    main();

    printf("\n\nDo you want to execute again? (Y or N)\n\n");
    scanf("%c%c",&ans,&ans);

    }while(toupper(ans) == 'Y');
}

---

Quote:

---

Originally Posted by SuPaNooB

my first problem is that for some reason, once u run the program, it freezes in a loop right when it invokes the EQUATION func.....y?

---

Your app seems to be a ticking time bomb to me.

Quote:

---

also, i wanted to know, how can i make it so that, as a new func, i can make it so that the user has an option to rerun MAIN() by pressing Y or N....?

---

Do a loop around main, at the end ask if the user wants to re-run and if 'y', re-loop basically. Or make a function that returns a bool and returns true if the user wants to re-run the equation or false if not. Then base your loop around that.

As for your app, you're using non-allocated, non-initalized pointer. Plus in your finalequation function, you're making new variables, with the same name, which too, are un-allocated and unitialized. And I suppose you want that function to take your data from the first function.
Think a little about that. Pass the argument to the function like you do the first and allocate memory in your pointers.
Heck, you don't even need pointers. You can make local variables and pass them by pointers to the function.

well, i somewhat understand what ur saying, but can u give me some examples on how to user it properly then? as for boolean, i have never used that cmd b4, how can i use it?

Some examples:

Ticking time bomb problem:

Code:

---

void foo(int* p)
{
        *p = 100;
}

int main()
{
        int* my_var;
        int my_var2;
        foo(my_var); /* This is bad. Why? Because my_var points to a random memory location this is NOT YOURS! */
        foo(&my_var2); /* Now this is okay, because we're passing the address of a local variable that we own */
}

---

Loop if user wants to exit:

Code:

---

int WantToExit()
{
        char cAnswer = 0;
        while (cAnswer != 'Y' && cAnwser != 'N')
        {
                printf("Want to quit ([Y]es/[N]o)?\n");
                scanf("%c", &cAnswer);
                cAnswer = toupper(cAnswer);
        }
        return (cAnswer == 'Y');
}

int main()
{
        do
        {
                /* Some code here */
        }
        while ( WantToExit() == 0 )
        /* Do some cleanup here and exit program */
}

---

Of course, it's up to you to understand this little piece of code and implent it into your own. If in doubt, I can explain (or someone else).
(Which brings a better question: does C support bool?)

this is what i have, can some1 correct it for me? its not passing a and b correctly to the equation func....

Code:

---


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

void elements(float*,float*);
float equation(float);
int Finish();

int main()
{
  do
  {
    FILE *output = NULL;

        float p,                  /*Pressure*/
              v,                  /*Volume of Container*/
              n,                  /*Number of Moles*/
              a,                /*Inputted by user defined element*/
              b,                /*Inputted by user defined element*/
              eqfinal;            /*Van der Waals equation*/

    if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
    else
    {
        printf("Van der Waals Equation:\n\n");
        printf("    an^2\n"
                "(P + -----)(V-nb)=nRT\n"
                "      V^2\n\n");

        elements(&a,&b);

        printf("\nElement has a and b defined as:"
                "\na=%f b=%f\n\n",a,b);

        printf("Please enter the amount of pressure (atm), volume (L), and "
                "moles (mol):\n(Separated by a Space)\n\n");
        scanf("%f %f %f",&p,&v,&n);

        printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);

        equation(eqfinal);

        printf("Temperature is equal to %.3f Kelvin\n\n",&eqfinal);

        fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",a,b,
                      p,v,n,&eqfinal);
        fclose(output);
    }

  }while ( Finish() == 0 );

  system("PAUSE");
  return 0;
}

void elements(float *a, float *b)
{
  int user_element;

  float constants[6][2]={
                        {3.640, .04267},
                        {.03457, .0237},
                        {3.716, .04081},
                        {1.378, .03183},
                        {6.803, .05636},
                        {5.536, .03049}
                        };

  printf("Which element do you want to solve for T?\n\n");
  printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen Chloride\n"
        "4-Oxygen\n5-Sulfur Dioxide\n6-Water\n\n");
  scanf("%d",&user_element);

  if(user_element<1 || user_element>6)
  {
      printf("\nError, Rerunning Choices!\n\n");
      elements(a,b);
  }
  else
  {
      switch(user_element)
      {
        case 1:
              *a=constants[0][0];
              *b=constants[0][1];
              break;
        case 2:
              *a=constants[1][0];
              *b=constants[1][1];
              break;
        case 3:
              *a=constants[2][0];
              *b=constants[2][1];
              break;
        case 4:
              *a=constants[3][0];
              *b=constants[3][1];
              break;
        case 5:
              *a=constants[4][0];
              *b=constants[4][1];
              break;
        case 6:
              *a=constants[5][0];
              *b=constants[5][1];
              break;
      }
  }
}
float equation(float eqfinal)
{
  float  p,v,a,b,n,
        eq1,        /*Split Equation into parts-*/
        eq2;        /*-To Prevent Errors in Calcuation*/

  const float R=8.314472;        /*Gas Constant*/

  eq1=p+a*((n*n)/(v*v));
  eq2=v-n*b;
  eqfinal=(eq1*eq2)/(n*R);

  printf("Testing a=%f, b=%f\n",a,b);

  return eqfinal;
}
int Finish()
{
  char cAnswer = 0;
  while (cAnswer != 'Y' && cAnswer != 'N')
  {
      printf("Want to quit ([Y]es/[N]o)?\n");
      scanf("%c", &cAnswer);
      cAnswer = toupper(cAnswer);
  }
  return (cAnswer == 'Y');
}

---

Oh, why oh why doesn't it work, I wonder?

Quote:

---

Code:

---

equation(eqfinal);

---

Warning 21 warning C4700: uninitialized local variable 'eqfinal' used

Code:

---

eq1=p+a*((n*n)/(v*v));

---

Warning 22 warning C4700: uninitialized local variable 'n' used
Warning 23 warning C4700: uninitialized local variable 'v' used
Warning 24 warning C4700: uninitialized local variable 'a' used
Warning 25 warning C4700: uninitialized local variable 'p' used

Code:

---

eq2=v-n*b;

---

Warning 26 warning C4700: uninitialized local variable 'b' used

---

Lots of uninitialized variables used.
Lots of conversion warnings too. Consider fixing those warnings, especially the uninitialized ones, that's bad.

what compiler r u using? im not getting none of those errors, it compiles all the way without errors...??? im using dev-c++ from bloodshed.

Visual Studio 2008 Team Edition with lvl 4 warnings.
There are probably ways to enable such warnings. I'm sure someone around here knows.

here's a new one...changed a few things, doesnt print out eqfinal at all....

Code:

---

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

void elements(float*,float*);
int Finish();
float equation(double p, double v, double a, double b, double n);

int main()
{
  do
  {
    FILE *output = NULL;

        float p,                  /*Pressure*/
              v,                  /*Volume of Container*/
              n,                  /*Number of Moles*/
              a,                /*Inputted by user defined element*/
              b,                /*Inputted by user defined element*/
              eqfinal=equation(p,v,a,b,n);          /*Van der Waals equation*/

    if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
    else
    {
        printf("Van der Waals Equation:\n\n");
        printf("    an^2\n"
                "(P + -----)(V-nb)=nRT\n"
                "      V^2\n\n");

        elements(&a,&b);

        printf("\nElement has a and b defined as:"
                "\na=&#37;f b=%f\n\n",a,b);

        printf("Please enter the amount of pressure (atm), volume (L), and "
                "moles (mol):\n(Separated by a Space)\n\n");
        scanf("%f %f %f",&p,&v,&n);

        printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);

        eqfinal;

        printf("Temperature is equal to %.3f Kelvin\n\n",eqfinal);

        fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",a,b,
                      p,v,n,eqfinal);
        fclose(output);
    }

  }while ( Finish() == 0 );

  system("PAUSE");
  return 0;
}

void elements(float *a, float *b)
{
  int user_element;

  float constants[6][2]={
                        {3.640, .04267},
                        {.03457, .0237},
                        {3.716, .04081},
                        {1.378, .03183},
                        {6.803, .05636},
                        {5.536, .03049}
                        };

  printf("Which element do you want to solve for T?\n\n");
  printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen Chloride\n"
        "4-Oxygen\n5-Sulfur Dioxide\n6-Water\n\n");
  scanf("%d",&user_element);

  if(user_element<1 || user_element>6)
  {
      printf("\nError, Rerunning Choices!\n\n");
      elements(a,b);
  }
  else
  {
      switch(user_element)
      {
        case 1:
              *a=constants[0][0];
              *b=constants[0][1];
              break;
        case 2:
              *a=constants[1][0];
              *b=constants[1][1];
              break;
        case 3:
              *a=constants[2][0];
              *b=constants[2][1];
              break;
        case 4:
              *a=constants[3][0];
              *b=constants[3][1];
              break;
        case 5:
              *a=constants[4][0];
              *b=constants[4][1];
              break;
        case 6:
              *a=constants[5][0];
              *b=constants[5][1];
              break;
      }
  }
}

float equation(double p, double v, double a, double b, double n)
{
  float eqfinal,
        eq1,
        eq2;

  const float R=8.314472;

  eq1=p+a*((n*n)/(v*v));
  eq2=v-n*b;
  eqfinal=(eq1*eq2)/(n*R);

  printf("Testing a=%f, b=%f\n", a, b);

  return eqfinal;
}
int Finish()
{
  char cAnswer = 0;
  while (cAnswer != 'Y' && cAnswer != 'N')
  {
      printf("Want to quit ([Y]es/[N]o)?\n");
      scanf("%c", &cAnswer);
      cAnswer = toupper(cAnswer);
  }
  return (cAnswer == 'Y');
}

---

One big fat warning:

Code:

---

printf("Temperature is equal to %.3f Kelvin\n\n",eqfinal);

---

Warning 22 warning C4700: uninitialized local variable 'eqfinal' used

You're mixing a lot of doubles or floats in your program. Use one type ONLY. If you want double, then change every float to double, otherwise, change any arguments and variables to float. It's not good to mix them (at least it's not sane).
equation returns the variable eqfinal, and you're forgetting to assign it after the function, hence the warning above.

alright, i pretty much got it now, i believe i got the right answer now, imma double check by using pen and paper =] .........looky here:

Code:

---

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

void elements(float*,float*);
int Finish();
float equation(double p, double v, double a, double b, double n);

int main()
{
  do
  {
    FILE *output = NULL;

        float p,                  /*Pressure*/
              v,                  /*Volume of Container*/
              n,                  /*Number of Moles*/
              a,                /*Inputted by user defined element*/
              b,                /*Inputted by user defined element*/
              eqfinal;          /*Van der Waals equation*/

    if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
    else
    {
        printf("Van der Waals Equation:\n\n");
        printf("    an^2\n"
                "(P + -----)(V-nb)=nRT\n"
                "      V^2\n\n");

        elements(&a,&b);

        printf("\nElement has a and b defined as:"
                "\na=%f b=%f\n\n",a,b);

        printf("Please enter the amount of pressure (atm), volume (L), and "
                "moles (mol):\n(Separated by a Space)\n\n");
        scanf("%f %f %f",&p,&v,&n);

        printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);



        printf("Temperature is equal to %.3f Kelvin\n\n",equation(p,v,a,b,n));

        fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",a,b,
                      p,v,n,eqfinal);
        fclose(output);
    }

  }while ( Finish() == 0 );

  system("PAUSE");
  return 0;
}

void elements(float *a, float *b)
{
  int user_element;

  float constants[6][2]={
                        {3.640, .04267},
                        {.03457, .0237},
                        {3.716, .04081},
                        {1.378, .03183},
                        {6.803, .05636},
                        {5.536, .03049}
                        };

  printf("Which element do you want to solve for T?\n\n");
  printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen Chloride\n"
        "4-Oxygen\n5-Sulfur Dioxide\n6-Water\n\n");
  scanf("%d",&user_element);

  if(user_element<1 || user_element>6)
  {
      printf("\nError, Rerunning Choices!\n\n");
      elements(a,b);
  }
  else
  {
      switch(user_element)
      {
        case 1:
              *a=constants[0][0];
              *b=constants[0][1];
              break;
        case 2:
              *a=constants[1][0];
              *b=constants[1][1];
              break;
        case 3:
              *a=constants[2][0];
              *b=constants[2][1];
              break;
        case 4:
              *a=constants[3][0];
              *b=constants[3][1];
              break;
        case 5:
              *a=constants[4][0];
              *b=constants[4][1];
              break;
        case 6:
              *a=constants[5][0];
              *b=constants[5][1];
              break;
      }
  }
}

float equation(double p, double v, double a, double b, double n)
{
  float eqfinal,
        eq1,
        eq2;

  const float R=8.314472;

  eq1=p+a*((n*n)/(v*v));
  eq2=v-n*b;
  eqfinal=(eq1*eq2)/(n*R);

  printf("Testing a=%f, b=%f\n", a, b);

  return eqfinal;
}
int Finish()
{
  char cAnswer = 0;
  while (cAnswer != 'Y' && cAnswer != 'N')
  {
      printf("Want to quit ([Y]es/[N]o)?\n");
      scanf("%c", &cAnswer);
      cAnswer = toupper(cAnswer);
  }
  return (cAnswer == 'Y');
}

---

Nope. Same problems.

Code:

---

        fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",a,b,
                      p,v,n,eqfinal);

---

eqfinal is uninitialized.
You're mixing doubles & floats.

Btw, I'm not sure if you missed this, but to use toupper, you need to include #include <ctype.h>

yea i fixed that, i believe everything is working correctly now, all i have to do now is just make it look a lil nicer and add a few more printf statements, add some extra elements and im good to go...thank you for your help. here's the end result:

Code:

---

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

void elements(float*,float*);
int Finish();
float equation(double p, double v, double a, double b, double n);

int main()
{
  do
  {
    FILE *output = NULL;

        float p,                  /*Pressure*/
              v,                  /*Volume of Container*/
              n,                  /*Number of Moles*/
              a,                /*Inputted by user defined element*/
              b,                /*Inputted by user defined element*/
              eqfinal;          /*Van der Waals equation*/

    if((output=fopen("c:answer.txt","w")) == NULL)
      printf("Error opening data file.\n");
    else
    {
        printf("Van der Waals Equation:\n\n");
        printf("    an^2\n"
                "(P + -----)(V-nb)=nRT\n"
                "      V^2\n\n");

        elements(&a,&b);

        printf("\nElement has a and b defined as:"
                "\na=%f b=%f\n\n",a,b);

        printf("Please enter the amount of pressure (atm), volume (L), and "
                "moles (mol):\n(Separated by a Space)\n\n");
        scanf("%f %f %f",&p,&v,&n);

        printf("\nPressure=%.3f Volume=%.3f Moles=%.3f\n\n",p,v,n);



        printf("Temperature is equal to %.3f Kelvin\n\n",equation(p,v,a,b,n));

        fprintf(output,"AC\nKA\n\n"
                      "Since a=%f and b=%f\n"
                      "Pressure=%f atm Volume=%f L and Moles=%f mol\n\n"
                      "Temperature is equal to %f Kelvin\n\n",a,b,
                      p,v,n,equation(p,v,a,b,n));
        fclose(output);
    }

  }while ( Finish() == 0 );

  system("PAUSE");
  return 0;
}

void elements(float *a, float *b)
{
  int user_element;

  float constants[6][2]={
                        {3.640, .04267},
                        {.03457, .0237},
                        {3.716, .04081},
                        {1.378, .03183},
                        {6.803, .05636},
                        {5.536, .03049}
                        };

  printf("Which element do you want to solve for T?\n\n");
  printf("1-Carbon Dioxide\n2-Helium\n3-Hydrogen Chloride\n"
        "4-Oxygen\n5-Sulfur Dioxide\n6-Water\n\n");
  scanf("%d",&user_element);

  if(user_element<1 || user_element>6)
  {
      printf("\nError, Rerunning Choices!\n\n");
      elements(a,b);
  }
  else
  {
      switch(user_element)
      {
        case 1:
              *a=constants[0][0];
              *b=constants[0][1];
              break;
        case 2:
              *a=constants[1][0];
              *b=constants[1][1];
              break;
        case 3:
              *a=constants[2][0];
              *b=constants[2][1];
              break;
        case 4:
              *a=constants[3][0];
              *b=constants[3][1];
              break;
        case 5:
              *a=constants[4][0];
              *b=constants[4][1];
              break;
        case 6:
              *a=constants[5][0];
              *b=constants[5][1];
              break;
      }
  }
}

float equation(double p, double v, double a, double b, double n)
{
  float eqfinal,
        eq1,
        eq2;

  const float R=8.314472;

  eq1=p+a*((n*n)/(v*v));
  eq2=v-n*b;
  eqfinal=(eq1*eq2)/(n*R);

  return eqfinal;
}
int Finish()
{
  char cAnswer = 0;
  while (cAnswer != 'Y' && cAnswer != 'N')
  {
      printf("Want to quit ([Y]es/[N]o)?\n");
      scanf("%c", &cAnswer);
      cAnswer = toupper(cAnswer);
  }
  return (cAnswer == 'Y');
}

---

eqfinal is now unused. You can remove it.
You're still mixing doubles and floats. Select one type and stay with it.

is mixing them really a pain? heh, well, i been trying to fix this with a friend for days and now that i got some "light", i feel better since the deadline for this project is tomorrow.

am i mixing them only in that output statement, or do u mean mixing them in the entire program?, i've tried lots of diff ways to make it work and so far, this seems to do the job despite it being messy =\ ......

You're mixing them in the program.
There are two things to consider:
If you use double first then floats at other parts, then you're basically truncating a lot of precision.
Otherwise, you are using unnecessary memory with unnecessary precision.
Further, you're also using a lot of doubles in float arrays, thus truncating them and causing compiler warnings. You want to avoid compiler warnings if possible.
Good practice.

Make all variables double OR float.

alright, i'll look into it then, for now imma fix and clean it up a bit, add more and etc......but thanks for the advice.

To get warnings in Dev-C++:

Under Tools -> Compiler Options:
Where it says "Add the following commands when calling compiler:" put -Wall in the box. (And make sure the check box gets checked, it doesn't automatically.)