7. Functions of C language, function definitions and the order of function calls, how to declare functions, prime examples, formal parameters and arguments, and how to write a function well

The order of function calls and function definitions

If the function call is written in front of the function definition , You must add a function pre declaration

Function pre declaration function ：

1. Tell compiler , The several letters that may appear soon represent a function
2. Tell the compiler about the possible formal parameters and return values of the function represented by several letters
3. A function declaration is a statement , You must add a semicolon at the end
4. The library function is declared through #include< The name of the file where the library function is located .h> To achieve

#include <iostream>
#include <math.h>
#include <stdio.h>
using namespace std;
void judge_Prime(int val);
int main(int argc, char const *argv[])
{
    printf(" Please enter the range you want to test :\n");
    int test_number;
    scanf("%d", &test_number);
    printf("1~%d The prime between is :\n");
    for (int i = 1; i <= test_number; i++)
    {
        judge_Prime(i);
    }
    return 0;
}

// Judge whether a function is prime 
void judge_Prime(int val)
{
    int i;
    for (i = 2; i <= sqrt(val); i++)
    {

        if (val % i == 0)
        {
            break;
        }
    }
    if (i <= sqrt(val))
    {
        printf("%d is not a Prime\n", val);
    }
    else
    {
        printf("%d is a Prime\n", val);
    }
}

For example, this example of judging prime numbers , We write judge_Prime(int val) function , because main Function in front , We're executing main Function , I do not know! judge_Prime(int val) What is this function , So you need to state in advance .

Formal parameters and actual parameters

Formal parameters are when we define functions , Parameters written in parentheses

The argument is when we call the function , Incoming parameter

The number of formal and actual parameters must be the same , The positions should correspond one by one , Data types should be compatible

void test（int a）{
    printf("%d",a);
}

int main(){
    test(100);
    return 0;
}

For example, this program ,test function （） Medium a Is the parameter ,main Function test（100）,100 It's a real parameter

The function of formal parameter and actual parameter is to transfer data . When a function call occurs ,  The main function transfers the value of the real parameter to the formal parameter of the called function, so as to realize the data transfer from the main function to the called function .

How to write a function ？

The function of a function should be as independent as possible , A single , To ensure the portability of functions