Functions. Cedric Saule

Transcription

1 Cedric Saule

2 or procedures? In algorithmic (and some programming languages), we use two kinds of unconditional branchings : Procedures : execute computations and do not return nothing. Exemple : printing instructions, memory allocations Functions : execute computations and return a result. Exemples : log(), sin() 2

3 or procedures? In C, there is only functions. All unconditional branchings return a result. These results can be a type (classical functions). ''Procedures'' return the type ''void''. ''void'' is a real type, it cannot be omitted. 3

4 or procedures? A new datatype : ''void'' The ''void'' type is not ''nothing''. The returned value of such functions is ''void'' ''void'' can be used as an abstraction for generic pointers (Cf. next lesson) 4

5 Functions are composed of : Header : returned_type name(type1 variable_name1, type1 variable_name 2,...) Block : { Instructions. If the returned type is not ''void'', the function finishes by returning the variable compatible with the returned type as definied in the header. Examples : int computeaddition(int x, int y){return x+y; void printvalue(int value){printf(''the value is : %d\n'', value); 5

6 Function should be declared (ANSI C) Functions headers have to be found in header files. int computeaddition(int x, int y); void printvalue(int value); If no returned data type, the compiler will consider the returned type is ''int''. If no parameters, the ''void'' type should be used. void myfunction( void ); // no parameters. void myfunction(); // means the compiler will not check for parameters types. 6

7 Function call : int add(int x1, int x2) /* Declaration */ { int y = x1 + x2; return y; int myint = add(4, 5); /* Function call */ The call affects 4 to the local variable #1 and 5 to the local variable #2. 7

8 Classical mistakes : Functions Wrong number of arguments. Wrong argument type (be careful with implicit transtyping). Implicit conversions when possible. Impossible to decide the order of affectation. int i = 0; int x = myfunction(i++,i++); // We cannot predict what will happen. Is it myfunction(0,1) or myfunction(1,0)? 8

9 A surprising mistake : #include <stdio.h> int main() { double x1 = 1.2, x2 = 3.5; int i = myoddfunction(x1, x2); char myoddfunction(int a, int b) { char c; if(a < b) c = 'A'; else c = 'B'; return c; 9

10 A surprising mistake : #include <stdio.h> int main() { double x1 = 1.2, x2 = 3.5; int i = myoddfunction(x1, x2); char myoddfunction(int a, int b) { char c; if(a < b) c = 'A'; else c = 'B'; return c; main.c:12:6: error: conflicting types for 'myoddfunction' main.c:7:10: note: previous implicit declaration of 'myoddfunction' was here Explanation? 10

11 A surprising mistake : #include <stdio.h> int main() { double x1 = 1.2, x2 = 3.5; int i = myoddfunction(x1, x2); char myoddfunction(int a, int b) { char c; if(a < b) c = 'A'; else c = 'B'; return c; main.c:12:6: error: conflicting types for 'myoddfunction' Implicit declaration : int myoddfunction(double, double) New declaration : char myoddfunction(int, int) main.c:7:10: note: previous implicit declaration of 'myoddfunction' was here Explanation? 11

12 A surprising mistake : #include <stdio.h> char myoddfunction(int a, int b) { char c; if(a < b) c = 'A'; else c = 'B'; return c; int main() { double x1 = 1.2, x2 = 3.5; int i = myoddfunction(x1, x2); return 0; Lost of precision because of type conversion in the function. 12

13 A surprising mistake : #include <stdio.h> #include "main.h" int main() { double x1 = 1.2, x2 = 2.5; int i = myoddfunction(x1, x2); Main.h #ifndef MAIN_H #define MAIN_H char myoddfunction(int a, int b); #endif return 0; char myoddfunction(int a, int b) { char c; if(a < b) c = 'B'; else c = 'A'; return c; 13

14 A surprising mistake : #include <stdio.h> #include "main.h" int main() { double x1 = 1.2, x2 = 2.5; int i = myoddfunction(x1, x2); return 0; Main.h #ifndef MAIN_H #define MAIN_H char myoddfunction(int, int); #endif Type is enough char myoddfunction(int a, int b) { char c; if(a < b) c = 'B'; else c = 'A'; return c; 14

15 A surprising mistake : #include <stdio.h> #include "main.h" int main() { double x1 = 1.2, x2 = 2.5; int i = myoddfunction(x1, x2); return 0; Main.h #ifndef MAIN_H #define MAIN_H char myoddfunction(int, int); #endif Do not forget the '';'' char myoddfunction(int a, int b) { char c; if(a < b) c = 'B'; else c = 'A'; return c; 15

16 A surprising mistake : #include <stdio.h> #include "main.h" int main() { double x1 = 1.2, x2 = 2.5; int i = myoddfunction(x1, x2); Main.h #ifndef MAIN_H #define MAIN_H extern char myoddfunction(int, int); #endif return 0; char myoddfunction(int a, int b) { char c; ''extern'' functions are declared in another file. if(a < b) c = 'B'; else c = 'A'; return c; 16

17 In C language, everything is a function : main() is a function. All functions are global. No operation can be done out of a function (except global variable definition and affectation). Each type is passed by value (otherwise we have to specify it) The value of an argument is a copy of the value of the variable given as parameter. The original value is not modified. 17

18 void addone(int x) { Functions printf(''x = %d\n'', x); x++; printf(''x = %d\n'', x); Outputs: i = 0 int main { int i = 0; printf(''i = %d\n'', i); addone(i); printf(''i = %d\n'', i); return 0; x = 0 x = 1 i = 0 18

19 Unchecked number of arguments. How to use an undetermined number of variables in a clean way? ''Max'' example. #include <stdarg.h> int max(short n, int x1,...) { va_list magik; int x, i, m; m = x1; va_start(magik, x1); for (i = 2; i <= n; i++) { if ((x = va_arg(magik, int)) > m) m = x; return m; Undefined list of arguments Variable type defined in stdarg.h Functions defined in stdarg.h 19

20 Quick preview How to modify a variable inside a function? How to use a variable without knowing its type? (like arguments in ''printf'' ) How to define a generic sort function? Language C use a specific datatype : pointers. 20