Computer Science 1620

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Computer Science 1620

• Function Scope Global Variables

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• Functions and Scope
• recall that a variable's scope does not extend
  past the statement block in which it is defined
• therefore, a variable declared in a function can
  only be used in that function
• this also applies to parameter variables

3

include ltiostreamgt using namespace
std double factorial(int x) double result
1 for (int y 1 y lt x y) result
y return result int main() cout ltlt
"Factorial of 5 " ltlt factorial(5) ltlt endl
cout ltlt "Result of computation " ltlt result ltlt
endl return 0
Variable result's lifetime
Compiler error result is being used outside of
its scope.
4

include ltiostreamgt using namespace
std double factorial(int x) double result
1 for (int y 1 y lt x y) result
y return result int main() cout ltlt
"Factorial of 5 " ltlt factorial(5) ltlt endl
cout ltlt "Result of computation " ltlt x ltlt
endl return 0
Variable x's lifetime
Compiler error x is being used outside of its
scope.
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• Functions and Scope
• since a variable's scope doesn't exist past the
  function it is declared in, two functions can
  have the same variable name
• refers to two different variables

6

include ltiostreamgt using namespace
std double factorial(int n) double result
1 for (int y 1 y lt n y) result
y return result int choose(int n, int r)
double result factorial(n) / (factorial(r)
factorial(n-r)) return static_castltintgt(resul
t) int main() cout ltlt "5 choose 2 " ltlt
choose(5, 2) ltlt endl return 0
These variables are distinct, even though they
have the same name.
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• Global Variables
• all variables so far have been declared within a
  function
• local variables
• also called automatic variables
• what happens if a variable is declared outside of
  a function
• variable becomes global
• its scope is any function defined after its
  declaration
• its lifetime is until the end of the program

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include ltiostreamgt using namespace std
double result double factorial(int x)
result 1.0 for (int y 1 y lt x y)
result y return result int
main() cout ltlt "Factorial of 5 " ltlt
factorial(5) ltlt endl cout ltlt "Result of
computation " ltlt result ltlt endl return
0
Variable result's lifetime
This is ok result is being used within its
scope.
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include ltiostreamgt using namespace std
double factorial(int x) result 1.0
for (int y 1 y lt x y) result y
return result double result int
main() cout ltlt "Factorial of 5 " ltlt
factorial(5) ltlt endl cout ltlt "Result of
computation " ltlt result ltlt endl return
0
Compiler error result being used outside of its
scope
Variable result's lifetime
10

• Global Variables Name Resolution
• suppose you have a global variable and local
  variable with the same name
• which variable takes priority?

Does this output a 1 or a 2?
int a 1 int main() int a 2
cout ltlt "a " ltlt a ltlt endl return 0
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• Global Variables Name Resolution
• Rule local variables hide global variables with
  the same name

This outputs a 2
int a 1 int main() int a 2
cout ltlt "a " ltlt a ltlt endl return 0
12

• Global Variables Name Resolution
• Rule local variables hide global variables with
  the same name
• local variables hide global variables until they
  go out of scope

This outputs a 2
int a 1 int main() int a 2
cout ltlt "a " ltlt a ltlt endl return 0
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• Global Variables Name Resolution
• Rule local variables hide global variables with
  the same name
• local variables hide global variables until they
  go out of scope

This outputs a 1
int a 1 int main() int a
2 cout ltlt "a " ltlt a ltlt endl
return 0
14

• Scope Operator
• unlike a local variable that's hidden, there is a
  way to acquire the value of a hidden global
  variable
• use the scope operator ()

This outputs a 2
int a 1 int main() int a 2
cout ltlt "a " ltlt a ltlt endl return 0
15

• Scope Operator
• unlike a local variable that's hidden, there is a
  way to acquire the value of a hidden global
  variable
• use the scope operator ()
• tells C to look for variable in global space

int a 1 int main() int a 1
cout ltlt "a " ltlt a ltlt endl return 0
This outputs a 1
16

• Global Variables vs. Parameters
• since global variables exist across functions,
  they can be used to pass information from
  function to function
• instead of using parameters and return types,
  just use global variables
• NOT RECOMMENDED

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include ltiostreamgt using namespace
std double factorial(int x) double result
1 for (int y 1 y lt x y) result
y return result int main() cout ltlt
"Factorial of 5 " ltlt factorial(5) ltlt endl
return 0
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include ltiostreamgt using namespace std int
x double result void factorial() result
1.0 for (int y 1 y lt x y) result
y int main() x 5 factorial()
cout ltlt "Factorial of 5 " ltlt result ltlt endl
return 0
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• Global Variables vs. Parameters
• using global variables for parameter passing and
  return values is considered bad programming
  practice
• difficult to follow
• risk of overwriting parameters or return values
• risk of hiding global variable
• sometimes leads to longer code

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include ltiostreamgt using namespace std int
x double result void factorial() result
1.0 for (int y 1 y lt x y) result
y int main() x 5 factorial()
cout ltlt "Factorial of 5 " ltlt result ltlt endl
return 0
No obvious associativity with function call
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include ltiostreamgt using namespace
std double factorial(int x) double result
1 for (int y 1 y lt x y) result
y return result int main() cout ltlt "5
choose 2 " ltlt factorial(5)/
(factorial(2) factorial(5-2)) ltlt endl
return 0
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include ltiostreamgt using namespace std int
x double result void factorial() result
1.0 for (int y 1 y lt x y) result
y int main() cout ltlt "5 choose 2 "
double choose 1.0 x 5 factorial()
choose result x 2 factorial()
choose / result x 3 factorial()
choose / result cout ltlt "5 choose 2 " ltlt
choose ltlt endl return 0
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• Global Variables
• what are global values used for?
• named constants
• named constants are very often declared outside
  of a function, rather than inside of a function
• this way, all the functions can use the named
  constant
• external variables
• to be discussed later (if at all)

24
Write a program that reads the radius of a
sphere, and outputs its widest circumference, its
volume, and its surface area
include ltiostreamgt using namespace std int
main() const double PI 3.14 double
r cout ltlt "Please enter the radius of the
sphere (m) " cin gtgt r cout ltlt
"Circumference (m) " ltlt 2 PI r ltlt endl
cout ltlt "Volume (m3) " ltlt 4.0 / 3.0
PIrrr ltltendl cout ltlt "Surface Area (m2)
" ltlt 4 PI rr ltlt endl return
0
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Write a program that reads the radius of a
sphere, and outputs its widest circumference, its
volume, and its surface area
include ltiostreamgt using namespace std
const double PI 3.14 int main() double
r cout ltlt "Please enter the radius of the
sphere (m) " cin gtgt r cout ltlt
"Circumference (m) " ltlt 2 PI r ltlt endl
cout ltlt "Volume (m3) " ltlt 4.0 / 3.0
PIrrr ltltendl cout ltlt "Surface Area (m2)
" ltlt 4 PI rr ltlt endl return 0