LECTURE 15: Scope of Names (Read Section 6.3, p.296-297)
Example 1.
#include<stdio.c>
int main(void)
{
int a, b;
printf(“(%d,%d)\n”,a,b); // will display 2 junk values
a = 5;
b = 10;
printf(“(%d,%d)\n”,a,b); // will display (5,10)
a = strange_function(a,b);
printf(“(%d,%d)\n”,a,b); // will display (0,10)
}
int strange_function(int a, int b)
{
printf(“(%d,%d)\n”,a,b); // will display (5,10)
b = b – 2*a;
printf(“(%d,%d)\n”,a,b); // will display (5,0)
return(b);
}
Explanation:
-
upon declaration
of a and b in main function, memory is reserved to store values of
variables a and b;
-
the values of a and b are based on the current
content of these memory locations, which is some junk value;
-
upon call to strange_function, two new memory locations are reserved to hold values
of input arguments a and b. Important: these memory locations are going to be reserved until
the program exits the strange_function; any changes to values of a and b within strange_function will occur only in these two reserved memory locations
– values of a and b from main function will remain unchanged!
-
values of the
input arguments a and b are 5 and 10;
-
value of b that is returned by strange_function (b = 0) will be stored in
variable a from main function; therefore,
the final display will show (0,10)!!
Example 2.
#include<stdio.c>
int main(void)
{
int a, b;
printf(“(%d,%d)\n”,a,b); // will display 2 junk values
a = 5;
b = 10;
printf(“(%d,%d)\n”,a,b); // will display (5,10)
a = strange_function(a,b);
printf(“(%d,%d)\n”,a,b);
}
int strange_function(int c, int d) // the only
difference from Example 1
{
printf(“(%d,%d)\n”,a,b); // Run time error, program will crash!!
b = b – 2*a;
printf(“(%d,%d)\n”,a,b);
return(b);
}
Note:
program crashed because strange_function could see only input arguments c and d; variables a and b created in main are not visible by strange_function!!
Example 3.
#include<stdio.c>
int main(void)
{
int a, b;
printf(“(%d,%d)\n”,a,b); // will display 2 junk values
a = 5;
b = 10;
printf(“(%d,%d)\n”,a,b); // will display (5,10)
a = strange_function(a,b);
printf(“(%d,%d)\n”,a,b); // will display (-15,10)
}
int strange_function(int b, int a) // the only
difference from Example 1
{
printf(“(%d,%d)\n”,a,b); // will display (10,5)
b = b – 2*a;
printf(“(%d,%d)\n”,a,b); // will display (10,-15)
return(b);
}
Note: values
of b and a
in strange_function will be assigned the corresponding values from the
function call in the main function (look at p.133-134 from the textbook:
Argument list correspondence)!