linking sub programs to main()

[cuba06]

I have to make a program that will calcuate the efficiency of an engine. The efficiency is called eta, and it uses the variables CompressionRatio and the specific heat capacity (k).

I need to have a sub program set for simply asking the user to input a value of k, one sub program to calculate eta and one program to print a table of results. I need to link all of these in main.

Here is what I have so far, and I don't know what I am doing wrong. As it stands, when executed, the program will run the sub program get_k, but then it does nothing. It will simply run the program get_k again, but not calculate anything or print any sort of a table.

I made the compression ratios an array, and I put eta as an array as well because I need to keep each result of the calculation and print it in the table. Is there a different way of doing that, and would any of you be able to show me how to do it in code?

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

/*  A function to introduce the program and return a specific heat ratio.  */
double get_k()
{
       int option;
       double SpecificHeat[4] = {1.416, 1.398, 1.314, 1.35}, k = 0;
       
       /*  Introduction  */
       printf("Welcome to the Engine Efficiency Calculator! To begin, enter the number to \n");
       printf("show a list of engine efficiencies for the gas selected. If you wish to use \n");
       printf("a different specific heat ratio than those shown, enter 5. \n");
       /*  List options.  */
       printf(" 1. Hydrogen (k = 1.416)\n 2. Oxygen (k = 1.398)\n 3. Carbon Dioxide (k = 1.314)\n 4. Air/Fuel Mixture (k = 1.35)\n 5. Other\n 6. Terminate Program\n");
       printf("Enter your option:\n");
       scanf("%d", &option);
       
       if (option >=1 && option <= 4)
       {
                  k = SpecificHeat[option - 1];
       }
       else if (option == 5)
       {
                  printf("Please enter a different specific heat ratio between 1.0 and 2.0: \n");
                  scanf("%f", &k);
                  while (k<1 || k>2)
                  {
                        printf("Error! You must enter a specific heat ratio between 1.0 and 2.0!\n");
                        printf("Please enter a different specific heat ratio:\n");
                        scanf("%lf", &k);
                  }
       }
       else if (option == 6)
       {
                 printf("GOODBYE! (press any key to quit)\n");
                 return k = -1;
       }
       return k;       
}

/*  A funtion to calculate eta using an array.  */
double calculate_eta(double eta[], int i, double k)
{       
       int CompressionRatio[7] = {1, 2, 3, 5, 10, 100, 1000};
       
       for (i=0; i<7; i++)
       {
           eta[i] = 1 - (1/pow(CompressionRatio[i], (k-1)));
       }
       return eta[i];
}
/*  A function to print a table with the results  */
int print_table(double eta[7], double k)
{
       printf("-----------------------------------------------------\n");
       printf("Compression Ratio  Specific Heat Capacity  Efficiency\n");
       printf("1                 | %lf                  |   %lf\n", k, eta[0]);
       printf("2                 | %lf                  |   %lf\n", k, eta[1]);
       printf("3                 | %lf                  |   %lf\n", k, eta[2]);
       printf("5                 | %lf                  |   %lf\n", k, eta[3]);
       printf("10                | %lf                  |   %lf\n", k, eta[4]);              
       printf("100               | %lf                  |   %lf\n", k, eta[5]);       
       printf("1000              | %lf                  |   %lf\n", k, eta[6]);
       printf("-----------------------------------------------------\n");
}


int main()
{
    /*  Declare Variables used in main  */
    int  i, eta[7], z = 1;  
    double k = get_k();
    double calculate_eta(int eta[i], int i, double k);
    int print_table(double k, int eta[7]);
    
    while (z == 1)
    {
          k = get_k();
          if (k == -1)
          {
                break;
          }
          else
          {
              calculate_eta(eta[i], i, k);
              printf("eta is %lf", eta[i]);
              print_table(k, eta[i]);
          }
}
    system("PAUSE");
    return 0;
}

[zacs7]

You can group printf() statements, it makes it easier to read, ie:

   printf(  "Welcome to the Engine Efficiency Calculator! To begin, enter the number to \n"
            "show a list of engine efficiencies for the gas selected. If you wish to use \n"
            "a different specific heat ratio than those shown, enter 5. \n"

            " 1. Hydrogen (k = 1.416)\n 2. Oxygen (k = 1.398)\n 3. Carbon Dioxide (k = 1.314)\n 4. Air/Fuel Mixture (k = 1.35)\n 5. Other\n 6. Terminate Program\n"
            "Enter your option:\n");

    double calculate_eta(int eta[i], int i, double k);
    int print_table(double k, int eta[7]);

Look like function prototypes (in main), if they are indeed prototypes, make them global.

What is 'z', won't it always be 1? In that case you can write,

while(1)
{

/* or */
for(;;)
{

Where exactly does it repeat? Try using a debugger or add some 'printfs'. You seem to be calling get_k() twice... once at initialization of 'k' in main;

double k = get_k();

and again in the loop, why? Should get_k() only run once?

[cuba06]

so you are saying that i should make these things global, i.e. outside of main?

    double calculate_eta(int eta[i], int i, double k);
    int print_table(double k, int eta[7]);

and which place would it be better to call get_k? in the loop im guessing?

EDIT:

okay, using printf statements, I found that the place where the code starts to fail is in main, in the else statement, with the line

int print_table(double eta[i], double k);

its either that, or the calculation... i need to store all of the calculated values in eta...

[matsp]

A few other nigglies:

       double SpecificHeat[4] = {1.416, 1.398, 1.314, 1.35}, k = 0;

Don't initialize an array and a simple variable on the same line - it's terribly hard to read (it's easy to miss the single variable at the end). Assuming that the above array is constant, you should also make it "static const" so that:
- it only needs to be initialized once [by making it static]
- you get a compiler error if you by mistake attempt to modify the values [by making it const].
[This obviously forces it to be a separate line too]

                 return k = -1;

Whilst this is technically correct - why? Either you want to return -1, or you want to set k = -1 and then return k a line or two further down.

And you have a problem if someone enteres 0 or a number greater than 7 - k is now zero, which will lead to interesting computations in your calculation [k-1 -> -1, meaning the 1/CompressionRatio].

/*  A funtion to calculate eta using an array.  */
double calculate_eta(double eta[], int i, double k)
{       
       int CompressionRatio[7] = {1, 2, 3, 5, 10, 100, 1000};
       
       for (i=0; i<7; i++)
       {
           eta[i] = 1 - (1/pow(CompressionRatio[i], (k-1)));
       }
       return eta[i];
}

You probably don't want a return there - make the function void and let if "fall through the end". As it is, you are indexing out of bounds, because when the loop ends, i is 7, whcih is the eighth element of your seven element array. Where you are calling the function, it doesn't use the return value anyways.

--
Mats