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

Learn about function prototypes, scope, and function definition in structured programming language, with examples and header files.

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

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  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. #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; }

  5. Function Prototype #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; }

  6. #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Function Definition

  7. #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Function Call (Must be after prototype, but can be before definition)

  8. #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Header files (filename.h) contain function prototypes and global variable declarations

  9. #include <stdio.h> 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; } Example: isNegative.c int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } 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. #include <stdio.h> #include "myFunctions.h" #include "myFunctions.c" int main (void) { int number; printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative(number)) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; } Example: isNegative.c • Note: • " "around file name for user-defined files • < > for standard system files isNegative() is declared in myFunctions.h and defined in myFunctions.c,not in this file!

  12. Example: myFunctions.h int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Example: myFunctions.c 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. #include <stdio.h> int isNegative ( int n ) { int result; if (number<0) { result=1; } else { result = 0; } return result; } Example: isNegative.c 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. #include <stdio.h> int isNegative ( int n ) { int result; if (number<0) { result=1; } else { result = 0; } return result; } Example: isNegative.c 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. #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Example: isNegative.c 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. #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Example: isNegative.c 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. #include <stdio.h> int isNegative ( int n ) { int result; if ( n<0 ) { result=1; } else { result = 0; } return result; } Example: isNegative.c 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. #include <stdio.h> int number; int isNegative ( void ) { int result; if ( number <0 ) { result=1; } else { result = 0; } return result; } Example: isNegativeGlobal.c int main (void) { printf ("Enter an integer: "); scanf ("%d",&number); if (isNegative()) { printf("Negative\n"); } else { printf("Positive\n"); } return 0; }

  21. #include <stdio.h> int number; int isNegative ( void ) { int result; if ( number <0 ) { result=1; } else { result = 0; } return result; } Example: isNegativeGlobal.c 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 prototypebefore its use • Be careful about the scope of variables

  25. Readings • King: 9.1 – 9.4 • D&D: 5.6 – 8.5, 5.12 • Kernighan & Ritchie: 4.1, 4.2, 4.4, 4.11

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