question about scope in c++

PTBmilo · 02-09-2003, 11:25 PM

I'm having trouble understanding what is going on with the whole issue of scope in c++. I am writing a program to calculate the velocity of an aircraft depending on some input. I don't want to have global variables, but whenever I put them all in main(), g++ tells me that they are undeclared when they show in the rest of the functions.

This works:

double x,y,z;
int a();
int b();

int main () {
<code>
a();
b();
}

int a(){
x=1
}
int b(){
y=2
}

###########################

but this doesn't:

int main(){
double x,y,z;
int a();
int b();

a();
b();
}
int a(){
x=1
}
int b(){
y=2
}

What's going on here? Does it have to do with the prototypes? I've tried all kinds of different placements, but can only get the top style to work.

Can somebody help me out? I can post the entire code if you want.

GtkUser · 02-10-2003, 01:07 AM

Here is one variant. You needed to place your prototypes outside of any code blocks but the local variables should be inside code blocks.

Code:

//prototypes
int MyFunction();

int main()
{
  //variables defined in main
  int num;

  //function call
  num = MyFunction();
  return 0;
}

//function definition
int MyFunction()
{
  return 10;
}

PTBmilo · 02-10-2003, 01:43 AM

Ok, I tried putting the prototypes outside of the main function, but it still doesn't work. I see the method that you suggest, but I really can't use that at this point in the class.... my teacher will actually TAKE OFF points... he's really trying to get us to use his EXACT style for now (which sucks because I'm taking another c++ class at the same time)

So am I correct in assuming that you must actually declare the variables outside of the main function for them to be global? I know you end up getting around this with pointers, and pass by value or whatever, but we're not there yet, and I haven't had time to investigate too much on my own.

I just thought that if you declare a variable in a block, that all functions called from that block can access them, but I guess that's wrong

Thanks again for the suggestion.

PTBmilo · 02-10-2003, 01:47 AM

PS:

Here's what DOESN'T work

Code:

#include<iostream>
#include<cmath>
using namespace std;

const double K              = 1.4;
const double ROE            = 0.002378;
const double PSI_2_PSF_CONV = 144;      /* (12in/ft)*(12in/ft) */


int psi2psf();       /* Converts input to pounds/(square foot) units */
int evaluate();      /* Calculates the equasion */
int outputResults(); /* Output the results */


int main()
{
        
double  feetPerSecond, 
        milesPerHour,
        totalPressure_psi,   /* total pressure input...pounds/(square inch) */
        totalPressure_psf,   /* total pressure in terms of square feet */
        staticPressure_psi,  /* static pressure input... pounds/(square inch) */
        staticPressure_psf;  /* static pressure in terms of square feet */
        
        
        
        cout << "\nEnter the Total Pressure in psi: ";
        cin  >> totalPressure_psi;
        cout << "\nEnter the Static Pressure in psi: ";
        cin  >> staticPressure_psi;
        
        staticPressure_psi += .000000000001; /* quality control */
        
        psi2psf();
        evaluate();
        outputResults();        

return 0;
}

int psi2psf ()
{
        totalPressure_psf  = totalPressure_psi  * PSI_2_PSF_CONV;
        staticPressure_psf = staticPressure_psi * PSI_2_PSF_CONV;

return 0;
} 
int evaluate ()
{       /* local variables... for computing only (see top) */
        double  val1,
                val2,
                val3,
                velocitySquared; /* code-cleaner... gimme that memory! :) */
        
        val1 = (2*K*staticPressure_psf)/((K-1)*ROE);
        val2 = ((totalPressure_psf - staticPressure_psf)/staticPressure_psf) \
                + 1;
        val3 = (K-1)/K;
        
        velocitySquared = val1 * ( pow(val2,val3) - 1 ); 
        
        feetPerSecond = sqrt(velocitySquared);

        milesPerHour = (feetPerSecond * pow(60,2))/5280;  /* ft/sec -> mi/hr */

return 0;
}

int outputResults() /* extremely watered down */
{
        cout.setf(ios::fixed | ios::showpoint); /* Show decimal point */
        cout.precision(1);                      /* Print correct accuracy */
             <<"***************OUTPUT**************RESULTS******************\n"

return 0;
}

######################################################
and here's what I get from g++:

Code:

/home/milo/ENG_38/dev [244]> g++ -o lab_2 lab_2.cpp
lab_2.cpp: In function `int psi2psf()':
lab_2.cpp:59: `totalPressure_psf' undeclared (first use this function)
lab_2.cpp:59: (Each undeclared identifier is reported only once
lab_2.cpp:59: for each function it appears in.)
lab_2.cpp:59: `totalPressure_psi' undeclared (first use this function)
lab_2.cpp:60: `staticPressure_psf' undeclared (first use this function)
lab_2.cpp:60: `staticPressure_psi' undeclared (first use this function)
lab_2.cpp: In function `int evaluate()':
lab_2.cpp:79: `feetPerSecond' undeclared (first use this function)
lab_2.cpp:81: `milesPerHour' undeclared (first use this function)

PTBmilo · 02-10-2003, 02:25 AM

OK, I got it. the {BLOCKS} send the variable to it's children, but not the functions. I ended up doing these:

Code:

int evaluate(double& totalPressure_psf, double& staticPressure_psf,
             double& feetPerSecond,      double& milesPerHour)

GtkUser · 02-10-2003, 02:49 AM

Quote:

Originally posted by PTBmilo

I just thought that if you declare a variable in a block, that all functions called from that block can access them, but I guess that's wrong

You have to pass local varibles as arguments to function calls because they are allocated on different runtime stacks.

Code:

#include<iostream>

int DoubleVar ( int localVar )
{
  localVar *= 2;
  return localVar;
}

int main()
{
  int localVar = 10;

  //pass localVar by value, it needs to be returned to update main's copy
  localVar = DoubleVar( localVar );
  std::cout << localVar << std::endl;
  return 0;
}

The argument is copied because it is passed by value, so you have to return it it in an assignment in order to double the size of the localVar in main. The main runtime stack is separate from DoubleVar's runtime stack.

Code:

#include<iostream>

void DoubleVar ( int * localVar )
{
  (*localVar) *= 2;
}

int main()
{
  int localVar = 10;

  DoubleVar( &localVar );
  std::cout << localVar << std::endl;
  return 0;
}

On the other hand when you pass by reference, you pass a pointer to DobuleVar's runtime stack to the memory location on mains runtime stack. So updates to the value are occuring at memory locations on main's runtime stack.

Actually C++ has a nicer syntax for passing by reference

Code:

#include<iostream>

void DoubleVar ( int & localVar )
{
  localVar *= 2;
}

int main()
{
  int localVar = 10;

  DoubleVar( localVar );
  std::cout << localVar << std::endl;
  return 0;
}

leed_25 · 02-10-2003, 12:48 PM

You might want to do a web search on the terms 'lexical scope' and 'dynamic scope' if you would lijke a slightly deeper understanding of this issue. C++ is lexically scoped.

PTBmilo · 02-10-2003, 05:59 PM

Thanks to the both of you..... this is some interesting stuff.