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CSI-121 Structured Programming Language Lecture 14 Functions (Part 2)

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CSI 121 Structured Programming Language Lecture 14 Functions (Part 2) * * Topics Prototypes Scope Prototyping of Functions Must declare functions before use (like ... – PowerPoint PPT presentation

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Title: CSI-121 Structured Programming Language Lecture 14 Functions (Part 2)


1
CSI-121Structured Programming LanguageLecture
14Functions (Part 2)
2
Topics
  • Prototypes
  • Scope

3
Prototyping of Functions
  • Must declare functions before use (like
    variables)
  • Declaration is called a prototype
  • Specifies the name, parameters and return type of
    the function, but not the code

4
Example isNegative.c
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
5
Example isNegative.c
Function Prototype
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
6
Example isNegative.c
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
Function Definition
7
Example isNegative.c
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
Function Call (Must be after prototype, but can
be before definition)
8
Example isNegative.c
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
Header files (filename.h) contain function
prototypes and global variable declarations
9
Example isNegative.c
include ltstdio.hgt int isNegative (int) int
main (void) int number printf ("Enter an
integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
stdio.h contains function prototypes for
printf(), scanf(), and other I/O functions
10
Header files
  • You can make your own header files with
    prototypes of frequently used functions
  • include "myFunctions.h"
  • Put the functions in a corresponding C file, and
    include those too
  • include "myFunctions.c"

11
Example isNegative.c
include ltstdio.hgt include "myFunctions.h" inclu
de "myFunctions.c" int main (void) int
number printf ("Enter an integer ") scanf
("d",number) if (isNegative(number)) pri
ntf("Negative\n") else printf("Positive\
n") return 0
  • Note
  • " " around file name for user-defined files
  • lt gt for standard system files

isNegative() is declared in myFunctions.h and
defined in myFunctions.c, not in this file!
12
Example myFunctions.c
Example myFunctions.h
int isNegative ( int n ) int result if (
nlt0 ) result1 else result
0 return result
int isNegative ( int )
13
Scope
  • Where can you use a variable which is declared in
    a function?
  • In that function only
  • Not in a calling function
  • Not in a called function

14
Scope Local Variables
  • Formal parameters only accessible whilst
    function executing
  • Variables declared in a function body only
    accessible whilst function executing
  • In fact, this is true of every block in a program

15
Example isNegative.c
include ltstdio.hgt int isNegative ( int n
) int result if (numberlt0) result1
else result 0 return result
int main (void) int number printf ("Enter
an integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
16
Example isNegative.c
include ltstdio.hgt int isNegative ( int n
) int result if (numberlt0) result1
else result 0 return result
int main (void) int number printf ("Enter
an integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
ERROR! Number is local to the main function, not
accessible here
17
Example isNegative.c
include ltstdio.hgt int isNegative ( int n
) int result if ( nlt0 ) result1 el
se result 0 return result
int main (void) int number printf ("Enter
an integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
Use the parameter n which is local to the
function isNegative()
18
Example isNegative.c
include ltstdio.hgt int isNegative ( int n
) int result if ( nlt0 ) result1 el
se result 0 return result
int main (void) int number printf ("Enter
an integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
n is the formal parameter
number is the actual parameter
19
Example isNegative.c
include ltstdio.hgt int isNegative ( int n
) int result if ( nlt0 ) result1 el
se result 0 return result
int main (void) int number printf ("Enter
an integer ") scanf ("d",number) if
(isNegative(number)) printf("Negative\n")
else printf("Positive\n") return
0
result n local to isNegative() number local
to main()
20
Example isNegativeGlobal.c
include ltstdio.hgt int number int isNegative
( void ) int result if ( number lt0
) result1 else result
0 return result
int main (void) printf ("Enter an integer
") scanf ("d",number) if
(isNegative()) printf("Negative\n") else
printf("Positive\n") return 0
21
Example isNegativeGlobal.c
include ltstdio.hgt int number int isNegative
( void ) int result if ( number lt0
) result1 else result
0 return result
int main (void) printf ("Enter an integer
") scanf ("d",number) if
(isNegative()) printf("Negative\n") else
printf("Positive\n") return 0
number is now GLOBAL - declared outside any
function, accessible in all functions (after the
declaration)
22
Scope Global Variables
  • Global variables are accessible in any function
    after their declaration to the end of that source
    file
  • They're useful, but risky
  • if any and every function can modify them, it can
    be difficult to keep track of their value
  • Better to use local variables and parameter
    passing if possible

23
Scope Functions
  • Functions are also accessible in any function
    after their declaration to the end of that source
    file

24
Summary
  • Include function prototype before its use
  • Be careful about the scope of variables

25
Readings
  • King 9.1 9.4
  • DD 5.6 8.5, 5.12
  • Kernighan Ritchie 4.1, 4.2, 4.4, 4.11
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