c语言程序设计课件ppt（英文）c program language 之5 function and program structure

LOGOFunction and Program Structure Chapter Four ContentsBasics of Functions 1return statements2Local variables and externavariables3Recursion4Scope Rules 5Header Files6Static Variables7Register Variables89 The C PreprocessorFunction and Program Structure 老师让几个同学编程， A负责编写求两个数的最大公约数； B负责两个数的最小公倍数； C负责判断这两个数是不是亲密数； D负责计算这两个数中有多少既能被 3整除又能被 7整除的数 老师说出了两个数， A,B,C,D立即把自己程序运行的结果告诉老师；老师又说了另外两个数， A,B,C,D又给出新的结果 AB CDmain( )( int a,b; scanf(“%d,%d”,funa(a,b); funb(a,b); func(a,b); fund(a,b);36, 120Function and Program Structure Amain( ) int a,b,h;scanf(“%d,%d”,if(ab) a=a+b;b=a-b;a=a-b; for(h=a;h=2;h-)If( a%h=0 printf(“%d,%d,%d”,a,b,h);funa( int a , int b) int h ;if(ab) a=a+b;b=a-b; a=a-b; for(h=a;h=2;h-)If( a%h=0 printf(“%d,%d,%d”,a,b,h);return ;main( )( int a,b; scanf(“%d,%d”,funa(a,b); funb(a,b); func(a,b); fund(a,b);36, 120Function and Program Structure C program source1source2source3.sourcenThe C Preprocessorfunction1function2.functionnFunction is the smallest unit in C program.Function definitions can appear in any order,and in one source file or severalA program may reside in one or more source files. Source files may be compiled separately and loaded together, along with previously compiled functions from librariesFunctionDeclaration StatementExecutive StatementVariablesFunction prototypeVariables typeVariable nameBlank statementFunction call statementControl statementBlock structureExpression statementFunction and Program Structure A program is just a set of definitions of variables and functions. Communication between the functions is by arguments and values returned by the functions, and through external variables. DefinitionPrototypeCallReturn ArgumentsBasics of Functions1 The types of functions Type Standard functions（ library）Need use head files#includeUser- defined functionsEach function definition has the form 2 Definition of a function return-type function-name ( argument declarations ) declarations statements The type of the result that the function returns parameter types and names If return type is omitted, int is assumed A minimal function is empty function,which does nothing and returns nothing .Basics of Functionsfunc( ) printf(“ * n”); main( ) int i;main( ) for(i=1; iy? x: y ; return( n ); Basics of Functionsinput： 6.8， 4.5 m=?there is no function declaration in caller.return type is float， the actual type of return value is int.float m;%freturn statement1 return statement return expression ; The return statement is the mechanism for returning a value from the called function to its caller. A function neednt return a value, the expression can be omitted, or no return statement.A return statement with no expression passes a no useful value to the caller . Sometimes it is a trouble.when defining function type to “void” , no any value returned .It is allowed that several return statements occur in a function.void function name( ) float f(float x, float y) float z; z=x+y*y;if( ! y) return(0.0);return(cos(z) );1. return(ab?a:b);2. return(0);3. return (x);4. retrun;5. no return statement6. void return statementmain( ) int a;void disp( );printf (“Enter an integer:n”);scanf (“ % d ”, disp( a );If y is zero， return 0Otherwise return cos(z) void disp( int x ) if( x 0 ) printf(“Positive.n”);else if( x = = 0 ) printf(“Zero.n”);else printf(“Negative.n”); 5 nesting -function.return statement while calling a function, another function is called. fun1( ).fun2( ).fun3( ). fun1( ).fun2( ). nesting-function does not mean nesting-definition. void beijing( );void shanghai ( );void tianjin( );main( ) printf(“I m in main.n”); beijing( );printf(“Im finally back in main( ).n”); void beijing( ) printf(“I m in beijing.n”); shanghai( );printf(“Here Im back in beijing.n”); void shanghai( ) printf(“Now Im in shanghai.n”); tianjin( ); printf(“Now I m back in shanghai.n”); void tianjin( ) printf( “I m in tianjin now.n”); main( ) call func A;func A call func B；func B return statementLocal variables and external variables externalLocalinnerFunctionBlockOut of int x,y;main( ) int a,b,c;fun(a,b);.int fun( int m)shareLocal variables and external variables 1 . local variables Declared within a function or a block . Can only be used in the function or the block which define them. They are created a new each time the function is called, and destroyed on return from the function or outside the block. They are not visible for other functions . Local variables in different functions can share the same name The formal arguments are treated like local variablesfloat f(int a) intb,c; a,b,c are local variablesmain( ) int x=10; int x=20; printf(“%d “, x); printf(“%d“,x ); Local variables and external variables main( ) int x=1;void f1( ),f2( );f1( );f2(x);printf(“%cn”,x );void f1( ) int x=3;printf(“%d”,x );void f2( int x) printf(“%d ”, +x);2 . External variables Defined outside any function Their value is retained and is available to any other function which accesses them. Can be initialized only once Can share the same name with local variable, but the latter is useful within the naming function. should be declared if use it before its definition.Local variables and external variables extern int a;main( ) int I; for(I=1;I=5;I+) +a; printf（ ”%dn”,a）； s ( ) ; int a=10;s ( ) int a=100;+a; printf(“%dn”, a); Declare an external variableDefine and initialize itDefine and initialize a local variableint x=500;main( ) int x=300; f( ); ff( ); printf(“x=%dn”, x); f ( ) x+=100; printf(“x=%dn”,x); ff ( ) int x=10; printf(“x=%dn”,x); Local variables and external variables int k=1;main( ) int i=4;fun( i );printf(“%d, %d”, i ,k); fun( int m) m+=k; k+=m; char k=B;printf(“%d “, k-A);printf(“%d, %d “, m, k);type name ( arguments) statements 1 definition 2 return:obtain the result by calling function3 declaration:type name( )4 formal arguments and actual arguments6 function nesting call 5 function call: name(actual arugments)C functions may be used recursively; a function may call itself either directly or indirectly.recursion： a function can be describe as a combination of the independent variable and itself .RecursionFunctions that call themselves Can only solve a base caseDivide a problem up intoWhat it can doWhat it cannot doWhat it cannot do resembles original problemThe function launches a new copy of itself (recursion step) to solve what it cannot doEventually base case gets solvedGets plugged in, works its way up and solves whole problemRecursive functions Recursionfactorials 5! = 5 * 4 * 3 * 2 * 1Notice that5! = 5 * 4!4! = 4 * 3! .Can compute factorials recursively Solve base case (1! = 0! = 1) then plug in2! = 2 * 1! = 2 * 1 = 2;3! = 3 * 2! = 3 * 2 = 6;RecursionRecursionExecute processfacto(int n) if (n=0)return(1)；else return(n*facto(n-1);facto(int n)