Definition and declaration of functions

1. Function definition ： A function is a code module that performs a specific function , Usually there are parameters , There can be no parameters ; Usually there is a return value , It can also be null .

2. General form ：< data type >< The name of the function >（< Formal parameter description >）

The function name is an identifier , Conform to the naming rules （ In numbers and letters _ form , But don't start with a number , It can't be a keyword ）

3. The data type is the type of the return value of the whole function , When no return value is needed , Data types are available void;

4. The advantages of functions ： Code reuse , modularization , Easy to maintain , read .

5. Formal parameters can be variables of any type , The parameters are separated by commas , The actual values of these formal parameters will be given when making a function call .

6. Functions cannot be defined repeatedly , And cannot nest definitions .

7. The called function must be a declared function , Or the position of the called function is before the calling function .

8. If the function is not declared before calling it , Then the compiling system will put the function form encountered for the first time （ Function definition or function out of use ） Declaration as a function , And the type of the return value of the function is defaulted to int;

9. Function call ： Nested calls , Recursively call （ Pay attention to the ending conditions ）.

10. The disadvantages of global variables ： Easily modified , Long life cycle , Occupy memory , Easily shielded （ Reassigning values in functions results in overwriting ）.

11 Scope ：1.{} The inside is local ;2 overall situation ： Define to the end of the program

The return value of the function

1. The return value of a function can only be passed through return Statement returns the main function , And can only return one value ;

2. The type of the function return value should be consistent with the data type defined by the function , If it's not the same , Then the type in the function definition shall prevail , Automatic type conversion , no return value , Can be defined as void type .

Some examples

1. Find with function 1+2+.......+n And

#include <stdio.h>
int sum(int n)
{
    if(n==0)
        return 0;
    return n+sum(n-1);// Recursively call

}

int main(int argc, char *argv[])
{ 
   int a;
    a=sum(100);
    printf("%d",a);
    return 0;
} 

2. seek X^n Value

#include <stdio.h>
double work(double x,int n)
{
    int i;
    double m=x;
    for(i=1;i<n;i++)
       {
           x=m*x;
       }
    return x;

}
int main(int argc, char *argv[])
{ 
    double s;
    double c;
    int d;
    scanf("%lf%d",&c,&d);
    s=work(c,d);
    printf("%f",s);
    return 0;
}

3. Input the total number of chickens and rabbits and the total number of feet , Find out how many chickens and rabbits there are ？

#include <stdio.h>
void number(int x,int y)
{
    int i,j;
    for(i=0;i<=x;i++)
    {
        for(j=0;j<=x;j++)
        {
            if(i+j==x&&2*i+4*j==y)
            {
                printf(" The number of Rabbits =%d Number of chickens =%d\n",j,i);
                return ;
            }
            
        }
    }
    printf("invail!\n");
    return;

}
int main(int argc, char *argv[])
{ 
    int a,b,m;
    printf(" Please enter the total number of pieces and feet \n");
    scanf("%d%d",&a,&b);
    number(a,b);
    return 0;
} 

4. Write a function to judge leap years

#include <stdio.h>
int leapyear(int x)// The function of judging leap years
{
    if(x%4==0&&(x%100!=0||x%400==0))
        return 1;
}
int main(int argc, char *argv[])
{ 
    int x,i;
    printf("please input year\n");
    scanf("%d",&x);
    i=leapyear(x);
    if(i==1)
        printf("%d is a leap year\n",x);
    else
        printf("%d is not a leap year\n",x);
    return 0;
} 

5. Write a function to judge prime numbers

#include <stdio.h>
int prime(int x)// Judge the prime function
{
    int i;
    for(i=2;i<x;i++)
    {
        if(x%i!=0)
            {
                return 1;
            }

    }
    return 0;
}
int main(int argc, char *argv[])
{ 
    int x,a;
    printf("please input a number\n");
    scanf("%d",&x);
    a=prime(x);
    if(a==1)
        printf("the number is a prime number\n");
    else
        printf("the number is not a prime number\n");
    return 0;
} 

6. Guess the number game

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main(int argc, char *argv[])
{ 
    int number;
    int guess;
    int i;
    srand((unsigned)time(NULL));
    number=rand()%100;
    number++;
    printf("guess number game!\n");
    printf("the range of number is 1-100\n");
    for(i=0;;i++)
    {
         printf("please input number\n");
         scanf("%d",&guess);
         if(guess>number)
         {
            printf(" big \n");

         }
         else if(guess<number)
            printf(" The small \n");
         else
         {
             printf("victory!\n");
             break;
         }
    }
    return 0;
} 

7. seek N The class of

#include <stdio.h>

int factorial(int n)//FOR Cycle factorial
{
    int i;
    for(i=n;i>1;i--)
    {
        n=n*(i-1);
    }
    return n;
}
int funtion(int n)// Recursively factoring
{
    if(n==1)
        return 1;
    return n*funtion(n-1);
}
int main(int argc, char *argv[])
{ 
    int n;
    int sum;
    printf("please input n\n");
    scanf("%d",&n);
    //sum=factorial(n);
    sum=funtion(n);
    printf("%d The factorial of is %d\n",n,sum);
    return 0;
} 

8. Print 1000-2000 The leap year of the year

#include <stdio.h>
void leapyear(int n)
{
    int i;
    for(i=n;i<=2000;i++)
    {
        if(i%4==0&&(i%100!=0||i%400==0))
            printf("%-8d",i);
    }
    return;
}
int main(int argc, char *argv[])
{ 
    leapyear(1000);
    return 0;
} 

9. Calculation 1！+2！+3！+...........+N!

#include <stdio.h>
int sume(int n)
{
    int i,j;
    int sum=1;
    int S=0;
    for(i=1;i<=n;i++)
    {
        sum=sum*i;
        S=S+sum;
    }
    return S;

}
int main(int argc, char *argv[])
{ 
    int n,sum;
    printf(" Enter the order multiplier you want \n");
    scanf("%d",&n);
    sum=sume(n);
    printf(" The sum of factorials is %d\n",sum);
    return 0;
} 

10. seek 1！-2！+3！-............+N!

#include <stdio.h>
int sume(int n)
{
    int i,j;
    int sum=1;
    int S=0;
    for(i=1;i<=n;i++)
    {
       
        sum=sum*i;
        S=S+sum;
        sum=(-1)*(sum);
    }
    return S;

}
int main(int argc, char *argv[])
{ 
    int n,sum;
    printf(" Enter the order multiplier you want \n");
    scanf("%d",&n);
    sum=sume(n);
    printf(" The sum of factorials is %d\n",sum);
    return 0;
} 

11. Landing system , Exit the system after three errors
#include <stdio.h>
#define password 1234
int system(int n)
{
    int i=1;
    int mm;
    while(i<=n)
    {
        printf("input your password\n");
        scanf("%d",&mm);
        if(mm==password)
        {
            return 1;
        }
         i++;
    }
    return 0;
}

int main(int argc, char *argv[])
{ 
     int a;
     a= system(3);
    if(a==1)
        printf("login succeeded!\n");
    else
        printf("your password is wrong!login out system\n");
    return 0;
} 

12. Find the greatest common divisor of two numbers

#include <stdio.h>
int conven(int x,int y)
{
    int i,r;
    for(i=1;i<=x&&i<=y;i++)
    {
        if(x%i==0&&y%i==0)
               r=i;
    }
    return r;
        
}
int main(int argc, char *argv[])
{ 
    int x,y,a;
    printf("input two number\n");
    scanf("%d%d",&x,&y);
    a=conven(x,y);
    printf("%d and %d The greatest common divisor of is %d",x,y,a);
    return 0;
}