Help! Right Math, Wrong Output

verd · 03-13-2005, 02:47 PM

Hey everybody,

I'm having trouble again- I'm a beginning programming student in a C For Engineers class.... And right now we're working on for loops... But for some reason, the program isn't doing what it's supposed to do.

The problem statement is overly complicated, but basically, the program is supposed to take a couple of inputs, (initial volume, final volume, volume increment, temperature, and moles of CO2), apply them to a formula, and find the pressure of a gas.

I'll include the problem as it was given to me:

The pressure of gas changes as the volume and temperature of the gas vary. Write a program that uses the Van der Waals equation of state for a gas,

[Click here to see an image of the formula]

to display in tabular form the relationship between the pressure and the volume of n moles of carbon dioxide at a constant absolute temperature (T). P is the pressure in atmospheres and V is the volume in liters. The Van der Waals constants for carbon dioxide are a = 3.592 L2 * atm/mol2 and b = 0.0427 L/mol. Use 0.08206 L * atm/mol * K for the gas constant R. Inputs to the program include n, the Kelvin temperature, the initial and final volumes in milliliters, and the volume increment between lines of the table. Your program will display a table that varies the volume of the gas from the initial to the final volume in steps prescribed by the volume increment. Here is a sample run:

Please enter at the prompts the number of moles of carbon dioxide, the absolute temperature, the initial volume in milliliters, the final volume, and the increment volume between lines of the table.

Quantity of carbon dioxide (moles) > 0.02
Temperature (kelvin) > 300
Initial volume (milliliters) > 400
Final volume (milliliters) > 600
Volume increment (milliliters) > 50

0.0200 moles of carbon dioxide at 300 kelvin

Volume (ml) Pressure (atm)

400 1.2246
450 1.0891
500 0.9807
550 0.8918
600 0.8178

I've manipulated everything correctly algebraically, so the math is correct... I just can't get it to display a correct output.

Here is what I've done:
(Please, help me find my errors... I have no idea what I'm doing wrong)

Code:

#include <stdio.h>

#define a 3.592
#define b .0427
#define R .03206
#define mL_L .001
#define L_mL 1000.0

int
main()
{
      double initialVol, finalVol, Vol, Volincrement, pressure, nmoles;
      int temp, exit;
      
      printf("Please enter at the prompts the number of moles 
of\ncarbon dioxide, the absolute temperature, the initial\nvolume
 in milliliters, teh final volume, and the increment volume between
 lines of the table.\n\n");
      printf("Quantity of carbon dioxide (moles)> ");
      scanf("%lf", &nmoles);
      printf("\nTemperature (kelvin)> ");
      scanf("%d", &temp);
      printf("\nInitial volume (milliliters)> ");
      scanf("%lf", &initialVol);
      printf("\nFinal Volume (milliliters)> ");
      scanf("%lf", &finalVol);
      printf("\nVolume increment (milliliters)> ");
      scanf("%lf", &Volincrement);
      
      printf("\n\n%.4f moles of carbon dioxide at %d kelvin", nmoles, temp);
      
      printf("Volume  (mL)          Pressure (atm)");
      
            
      for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement)
      {
              printf("   %.2f", Vol);
              
              Vol *= mL_L;
              pressure = (nmoles * R * temp)/(Vol - b * nmoles) - (a * nmoles * nmoles)/(Vol * Vol);
              
                           
              printf("           %.4f\n", pressure);
              
              }
              
              printf("Exit program? (y/n)> ");
              scanf("lf", exit);
              
              
              return (0);
      
      }

Dave Evans · 03-13-2005, 03:08 PM

Quote:

Originally Posted by verd

Hey everybody,

I'm having trouble again-

For starters, use something else for your loop counter:

Code:

  for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement)
      {
              printf("   %.2f", Vol);
              
              Vol *= mL_L;  /* Loop counter is Vol */

Then put in some debug statements to print out all of your constants and user input values before you enter the loop to make sure the formula is working on what you think it should be.

Regards,

Dave

verd · 03-13-2005, 05:48 PM

Quote:

Originally Posted by Dave Evans

For starters, use something else for your loop counter:

Alright, I'll try that... However, I don't quite understand what that means... Are you telling me not to use the variable Vol?

I don't understand- sorry, I'm quite a beginner...

Quote:

Originally Posted by Dave Evans

Then put in some debug statements to print out all of your constants and user input values before you enter the loop to make sure the formula is working on what you think it should be.

I don't know what a debug statement is either- I've tried pulling all of the print statements out of the loop, and on my own, I substituted the variables that I thought would end up in the formula into the formula, and I got the right answer.

I'm sorry, I don't understand... (Really, I'm sorry, this is all really new to me)

verd · 03-13-2005, 05:52 PM

It obviously seems like I'm doing something wrong with my loop...

Code:

for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement)

This is what the above code, as far as I know, translates into:

| When Vol is equivalent to whatever the initial volume is | And Vol is less than or equal to whatever the final volume is |

The statement in the brackets will loop

| At the end of the loop, Vol will be raised whatever Volincrement is equal to |

Am I wrong?

Dave Evans · 03-13-2005, 08:10 PM

Quote:

Originally Posted by verd

It obviously seems like I'm doing something wrong with my loop...

Code:

for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement)

This is what the above code, as far as I know, translates into:

| When Vol is equivalent to whatever the initial volume is | And Vol is less than or equal to whatever the final volume is |

The statement in the brackets will loop

| At the end of the loop, Vol will be raised whatever Volincrement is equal to |

Am I wrong?

But inside your loop you change the value of Vol to use in your formula! This royally screws up your loop.

One way to fix it is to change the loop variable name to something else, maybe like this:

Code:

      double loopVol; /* near the beginning of main() */

Now you would change your loop to something like this

Code:

      for(loopVol = initialVol; loopVol <= finalVol; loopVol += Volincrement)
      {
        printf("   %.2f", loopVol);
        Vol = loopVol * mL_L;
        pressure = (nmoles * R * temp)/(Vol - b * nmoles) - 
                   (a * nmoles * nmoles)/(Vol * Vol);
        
                     
        printf("           %.4f\n", pressure);
        
        }

Regards,

Dave

verd · 03-13-2005, 09:39 PM

Wow. That fixed everything

Thank you...

Now what exactly was happening, I don't quite understand.

The user inputted milliliters for Vol, and in order to make the calculation, Vol needed to be in liters... So the mL_L was just a converstion. How did loopVol and Vol fix the problem?

Karthur · 03-13-2005, 10:04 PM

You started into the loop ok, (should have printed the first line), but while you were there you multiplied Vol by 1000, then left the inside of the loop, incremented by your 50 and tested to see whether to do it again - an obvious NO! You can't mess with your loop controls unless you know what you are doing and want the strange effect ( maybe an early exit, or a start over, or a newly calculated limit value, or you just like obfuscated code ).

quzah · 03-13-2005, 10:07 PM

Also, your scanf call at the end is wrong. You need the address-of operator before non-pointer variables for scanf to work correctly. Not to mention the fact that it doesn't actually control the program's outcome in any way.

At any rate, reread Dave's post. You were trying to control the loop with a value check against something that you constantly change every time through the loop, plus you increment it each pass also:

Code:

for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement )
      {
              printf("   %.2f", Vol);
              
              Vol *= mL_L;
              pressure = (nmoles * R * temp)/(Vol - b * nmoles) - (a * nmoles * nmoles)/(Vol * Vol);
              
                           
              printf("           %.4f\n", pressure);
              
              }

Quzah.

verd · 03-14-2005, 08:22 PM

Alright, cool... So I fixed the scanf, and ran the code through a windows compiler (I was running it through the gcc compiler on a mac). The program runs and quits.... I've had this problem before, but usually the "End program" fixes it. I can't see the output, it closes too quickly.

Does anyone know how to avoid this problem? Or is the program crashing because of an error that I made?
(I'm using Dev-C++)

Thanks again for all of the help!

verd · 03-14-2005, 08:23 PM

I forgot to include my new code, sorry:

Code:

#include <stdio.h>

#define a 3.592
#define b .0427
#define R .03206
#define mL_L .001

int
main()
{
      double initialVol, finalVol, Vol, Volincrement, pressure, nmoles, loopVol;
      int temp, exit;
      
      printf("Please enter at the prompts the number of moles
 of\ncarbon dioxide, the absolute temperature, the initial\nvolume in
 milliliters, teh final volume, and the increment volume between lines
 of the table.\n\n");
      printf("\nQuantity of carbon dioxide (moles)> ");
      scanf("%lf", &nmoles);
      printf("Temperature (kelvin)> ");
      scanf("%d", &temp);
      printf("Initial volume (milliliters)> ");
      scanf("%lf", &initialVol);
      printf("Final Volume (milliliters)> ");
      scanf("%lf", &finalVol);
      printf("Volume increment (milliliters)> ");
      scanf("%lf", &Volincrement);
      
      printf("\n\n%.4f moles of carbon dioxide at %d kelvin", nmoles, temp);
      
      printf("Volume  (mL)          Pressure (atm)");
      
            
            for(loopVol = initialVol; loopVol <= finalVol; loopVol += Volincrement)
               {
                   printf("   %.2f", loopVol);
                   Vol = loopVol * mL_L;
                   pressure = (nmoles * R * temp)/(Vol - b * nmoles) - (a * nmoles * nmoles)/(Vol * Vol);
        
                     
                   printf("           %.4f\n", pressure);
        
               }
              
        printf("Exit program? (y/n)> ");
        scanf("lf", &exit);
              
              
        return (0);
      
      }

Dave Evans · 03-15-2005, 08:48 AM

Quote:

Originally Posted by verd

I forgot to include my new code, sorry:

Code:

        scanf("lf", &exit);

Change the above to

Code:

       scanf("%lf", &exit);

(Did you notice that the answers from your program do not agree with the sample run that you showed in your first post?)

Regards,

Dave

Dave Evans · 03-15-2005, 08:53 AM

Quote:

Originally Posted by verd

I don't know what a debug statement is either- I've tried pulling all of the print statements out of the loop, and on my own, I substituted the variables that I thought would end up in the formula into the formula, and I got the right answer.

I'm sorry, I don't understand... (Really, I'm sorry, this is all really new to me)

Put printf statements at strategic places to make sure all constants and variables for the formula have the right values.

Or at least proofread your program to make sure the constants are correct.

Regards,

Dave

verd · 03-15-2005, 07:49 PM

Yeah... So... I had the correct answer on the mac, and then hopped onto the pc. Something happened, for some reason, the c file got messed up in transit, and I pretty much had to rewrite it. What was messing up that output was my misreading a handwritten "8" as a "3" in one of the constants. I fixed that, and now the program runs just fine.

Haha, thanks though, for all of the help!
It's much appreciated!!! Really... it is. To me, C is like turning around for a second, only to find that when you turn back around, you're in some foreign country with weird customs and a language you've never heard of before...

I'm getting the hang of it, but there are plenty more stupid errors I'm going to have to make before I actually have a decent understanding of the language. (i.e. the "%" before the "lf" in the exit prompt...)

Thanks again for all of your help!!

03-13-2005, 02:47 PM	#1
verd Registered User Join Date: Jan 2005 Posts: 28	Help! Right Math, Wrong Output Hey everybody, I'm having trouble again- I'm a beginning programming student in a C For Engineers class.... And right now we're working on for loops... But for some reason, the program isn't doing what it's supposed to do. The problem statement is overly complicated, but basically, the program is supposed to take a couple of inputs, (initial volume, final volume, volume increment, temperature, and moles of CO2), apply them to a formula, and find the pressure of a gas. I'll include the problem as it was given to me: The pressure of gas changes as the volume and temperature of the gas vary. Write a program that uses the Van der Waals equation of state for a gas, [Click here to see an image of the formula] to display in tabular form the relationship between the pressure and the volume of n moles of carbon dioxide at a constant absolute temperature (T). P is the pressure in atmospheres and V is the volume in liters. The Van der Waals constants for carbon dioxide are a = 3.592 L2 * atm/mol2 and b = 0.0427 L/mol. Use 0.08206 L * atm/mol * K for the gas constant R. Inputs to the program include n, the Kelvin temperature, the initial and final volumes in milliliters, and the volume increment between lines of the table. Your program will display a table that varies the volume of the gas from the initial to the final volume in steps prescribed by the volume increment. Here is a sample run: Please enter at the prompts the number of moles of carbon dioxide, the absolute temperature, the initial volume in milliliters, the final volume, and the increment volume between lines of the table. Quantity of carbon dioxide (moles) > 0.02 Temperature (kelvin) > 300 Initial volume (milliliters) > 400 Final volume (milliliters) > 600 Volume increment (milliliters) > 50 0.0200 moles of carbon dioxide at 300 kelvin Volume (ml) Pressure (atm) 400 1.2246 450 1.0891 500 0.9807 550 0.8918 600 0.8178 I've manipulated everything correctly algebraically, so the math is correct... I just can't get it to display a correct output. Here is what I've done: (Please, help me find my errors... I have no idea what I'm doing wrong) Code: #include <stdio.h> #define a 3.592 #define b .0427 #define R .03206 #define mL_L .001 #define L_mL 1000.0 int main() { double initialVol, finalVol, Vol, Volincrement, pressure, nmoles; int temp, exit; printf("Please enter at the prompts the number of moles of\ncarbon dioxide, the absolute temperature, the initial\nvolume in milliliters, teh final volume, and the increment volume between lines of the table.\n\n"); printf("Quantity of carbon dioxide (moles)> "); scanf("%lf", &nmoles); printf("\nTemperature (kelvin)> "); scanf("%d", &temp); printf("\nInitial volume (milliliters)> "); scanf("%lf", &initialVol); printf("\nFinal Volume (milliliters)> "); scanf("%lf", &finalVol); printf("\nVolume increment (milliliters)> "); scanf("%lf", &Volincrement); printf("\n\n%.4f moles of carbon dioxide at %d kelvin", nmoles, temp); printf("Volume (mL) Pressure (atm)"); for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement) { printf(" %.2f", Vol); Vol = mL_L; pressure = (nmoles R * temp)/(Vol - b * nmoles) - (a * nmoles * nmoles)/(Vol * Vol); printf(" %.4f\n", pressure); } printf("Exit program? (y/n)> "); scanf("lf", exit); return (0); }
verd is offline

03-13-2005, 05:52 PM	#4
verd Registered User Join Date: Jan 2005 Posts: 28	It obviously seems like I'm doing something wrong with my loop... Code: for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement) This is what the above code, as far as I know, translates into: \| When Vol is equivalent to whatever the initial volume is \| And Vol is less than or equal to whatever the final volume is \| The statement in the brackets will loop \| At the end of the loop, Vol will be raised whatever Volincrement is equal to \| Am I wrong?
verd is offline

03-13-2005, 09:39 PM	#6
verd Registered User Join Date: Jan 2005 Posts: 28	Wow. That fixed everything Thank you... Now what exactly was happening, I don't quite understand. The user inputted milliliters for Vol, and in order to make the calculation, Vol needed to be in liters... So the mL_L was just a converstion. How did loopVol and Vol fix the problem?
verd is offline

03-13-2005, 10:04 PM	#7
Karthur Registered User Join Date: Mar 2005 Posts: 36	You started into the loop ok, (should have printed the first line), but while you were there you multiplied Vol by 1000, then left the inside of the loop, incremented by your 50 and tested to see whether to do it again - an obvious NO! You can't mess with your loop controls unless you know what you are doing and want the strange effect ( maybe an early exit, or a start over, or a newly calculated limit value, or you just like obfuscated code ).
Karthur is offline

03-13-2005, 10:07 PM	#8
quzah +++ OK NO CARRIER Join Date: Oct 2001 Posts: 10,643	Also, your scanf call at the end is wrong. You need the address-of operator before non-pointer variables for scanf to work correctly. Not to mention the fact that it doesn't actually control the program's outcome in any way. At any rate, reread Dave's post. You were trying to control the loop with a value check against something that you constantly change every time through the loop, plus you increment it each pass also: Code: for(Vol = initialVol; Vol <= finalVol; Vol += Volincrement ) { printf(" %.2f", Vol); Vol = mL_L; pressure = (nmoles R * temp)/(Vol - b * nmoles) - (a * nmoles * nmoles)/(Vol * Vol); printf(" %.4f\n", pressure); } Quzah. __________________ Hundreds of thousands of dipshits can't be wrong. Are you up for the suck?
quzah is offline

03-14-2005, 08:22 PM	#9
verd Registered User Join Date: Jan 2005 Posts: 28	Alright, cool... So I fixed the scanf, and ran the code through a windows compiler (I was running it through the gcc compiler on a mac). The program runs and quits.... I've had this problem before, but usually the "End program" fixes it. I can't see the output, it closes too quickly. Does anyone know how to avoid this problem? Or is the program crashing because of an error that I made? (I'm using Dev-C++) Thanks again for all of the help!
verd is offline

03-15-2005, 07:49 PM	#13
verd Registered User Join Date: Jan 2005 Posts: 28	Yeah... So... I had the correct answer on the mac, and then hopped onto the pc. Something happened, for some reason, the c file got messed up in transit, and I pretty much had to rewrite it. What was messing up that output was my misreading a handwritten "8" as a "3" in one of the constants. I fixed that, and now the program runs just fine. Haha, thanks though, for all of the help! It's much appreciated!!! Really... it is. To me, C is like turning around for a second, only to find that when you turn back around, you're in some foreign country with weird customs and a language you've never heard of before... I'm getting the hang of it, but there are plenty more stupid errors I'm going to have to make before I actually have a decent understanding of the language. (i.e. the "%" before the "lf" in the exit prompt...) Thanks again for all of your help!!
verd is offline

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