Lesson 18 - Learning Goals

LONGEVITY OF C VARIABLES

• Automatic variables are created when a procedure or block is entered, and destroyed when the procedure or block is exited.
  
• External variables are created when the program begins execution, and are preserved through the entire program execution.
  

FUNCTIONS

IN FORTRAN :

! THIS PROGRAM COMPUTES THE CUBE OF ITS INPUT.
REAL FUNCTION CUBE(X)
   IMPLICIT NONE
   REAL :: X
   CUBE = X * X * X
END FUNCTION CUBE

IN C :

/* This program computes the cube of its input */
real cube (x)
float x;
{
   return (x * x * x);
}

SIMILARITIES :

• A function can return 1 value of specified type. Function calls can be used in expressions
  

DIFFERENCES :

• FORTRAN uses special variable (same name as function) to hold return value
• C uses return statement to specify return value

STATIC VS. AUTOMATIC VARIABLES

STATIC :

• A static variable has a fixed location, and retains its value from one procedure call to the next.
• A static variable may be initialized to a particular value at the start of the program.
• Variables declared outside of all procedures are static.
• A variable declared inside a procedure must have the keyword ''static'' added to make it static.

AUTOMATIC :

• An automatic variable has a transient location which disappears after a procedure terminates.
• An automatic variable may be initialized to a particular value, but this initialization occurs every time the procedure is called.
• A variable declared inside a procedure is automatic unless declared static.

EXAMPLE

#include <stdio.h>

void ephemeral ( ), permanent ( );

main()
{
   ephemeral ( );
   permanent ( );
   ephemeral ( ); /* New value */
   permanent ( ); /* Keep old value */
   printf ("\n");
}

void ephemeral ( ) 
{
   int value = 0;
   printf ("E: %d\t", value++);
}

void permanent ( ) 
{
   static int value = 0;
   printf ("P: %d\t", value++);
}

~~~~~~~~~~~~~~~~~~~~~~

Output :

E: 0 P: 0 E: 0 P: 1 

PROTOTYPE DECLARATION

• If a variable/procedure is declared at the top-level in a source file, any statement below the declaration can access it.
• This means that a function can't call a function that is ''below'' it in the file. In particular, you can't have two functions call each other. It also means the main( ) function has to be on the bottom.
• To get around this, use ''prototype'' declarations. A prototype tells the compiler what the function call looks like without listing the code, which comes later.
• A prototype must give the number of arguments, the type of each argument, and the type of the value returned. It does not need to give names to the arguments.
  
  

PROTOTYPE EXAMPLE

#include <stdio.h>

/* Two Prototype Declarations */

float average (float, float);
void swap (float *, float *); 
        /* Both arguments to swap are  */
        /* pointers to floats.         */

main () {
   float a, b, ave;
   a = 3.396;
   b = 5.2;
   ave = average (a,b);
   swap (&a,&b);
   printf ("The average of %.2f and %.2f is %.2f\n",
      a, b, ave); /* Print 2 sig. figures */
}

float average (float x, float y) {
   return (x+y)/2;
}

void swap (float *x, float *y) {
   float temp;
   temp = *x;
   *x = *y;
   *y = temp;
}

~~~~~~~~~~~~~~~~~~~~~~

OUTPUT :

The average of 5.20 and 3.40 is 4.30

PASSING ARRAYS IN C

• Arrays are always passed call-by-reference because call-by-value would require copying an entire array.

• Procedure array variable is a pointer to array in calling procedure.

• Dimension of input array in procedure can be left unspecified, as in FORTRAN.
  
  

PASSING ARRAYS - EXAMPLES

IN FORTRAN :

PROGRAM ARRAYS
   IMPLICIT NONE
   INTEGER :: A(50), TOP, BIGEST
   ...
   TOP = BIGEST (A, 50)
   ...
   ! RETURN LARGEST OF FIRST N INTS OF B
   INTEGER FUNCTION BIGEST (B, N)
      INTEGER B(*)
      BIGEST = -9999
      DO 10 I=1,N
10       IF (B(I).GT.BIGEST) BIGEST = B(I)
   STOP
END PROGRAM ARRAYS

IN C :

main ( )
{ 
   int a[50], top ;
   ...
   top = bigest (a, 50) ;
   ...
}
/* Return largest of first n ints of b */
int bigest (int b[ ], int n) 
{
   int i, largest;
   for ( i=0 , largest=-9999 ; i<n ; i+=1)
      if (b[i] > largest) largest=b[i];
   return (largest);
}