9. Pointer of C language (3) classic program, exchange the value of two numbers for deep analysis, (easy to understand), are formal parameters and arguments a variable?

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Catalog

Before understanding this content , We need to understand a very important knowledge point first , Formal parameters and actual parameters

Let's verify whether the arguments and formal parameters are variables

Classic program , Exchange the values of two numbers through a function

Method 1： Exchange... Through intermediate variables

Method 2： Exchange by changing the pointer direction of formal parameters

Method 3： Change by address

summary

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Before understanding this content , We need to understand a very important knowledge point first , Formal parameters and actual parameters

  problem 1： What are formal 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

problem 2： Are formal parameters and arguments a variable ？

              No , Formal and actual parameters are never a variable

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Let's verify whether the arguments and formal parameters are variables

#include <iostream>
#include <stdio.h>
using namespace std;
void test(int i)
{
    i = 99;
    printf(" Shape parameter i The address for :%#X\n", &i);
}
int main(int argc, char const *argv[])
{
    int i = 6;
    printf(" Actual parameters i The address for %#X\n", &i);
    test(i);
    printf(" after test Function to get i  by  %d\n", i);
    return 0;
}

i By function test after ,i The value of has not changed , And the shape parameter i And real parameters i The address of is different , This further illustrates that arguments and formal parameters are not variables .

Classic program , Exchange the values of two numbers through a function

Method 1： Exchange... Through intermediate variables

void exchange(int a, int b)
{
    
    int t;// Define intermediate variables 
    t = a;
    a = b;
    b = temp;
    
}
int main(int argc, char const *argv[])
{
    int a = 3;
    int b = 5;
    exchange(a, b);
    printf("----------------------\n");
    printf("a=%d\n", a);
    printf("b=%d\n", b);
    return 0;
}

This method doesn't work , Arguments will a=3,b=5 Pass into formal parameter a,b in , Only formal parameters are exchanged , After the function runs, the space is released , Actual parameters a and b The value of is still the original value , Because arguments and formal parameters are not variables

 

Method 2： Exchange by changing the pointer direction of formal parameters

void exchange_2(int *p, int *q)
{
    int *t;// Define intermediate pointer variables t
    t = p;
    p = q;
    q = t;
}
int main(int argc, char const *argv[])
{
    int a = 3;
    int b = 5;
    exchange_2(&a, &b);// Pass in a and b The address of 
    printf("----------------------\n");
    printf("a=%d\n", a);
    printf("b=%d\n", b);
    return 0;
}

Method 2 Can not be , Because this way

Although it was a and b The address of （ The pointer ）, namely p Save the a The address of ,q preservation b The address of , But what has changed is p and q The direction of

Before change ：

After change ：

  Although what has changed is p,q The direction of , But our output is still a and b

  Method 3： Change by address

void exchange_3(int *p, int *q)
{
    int t;
    // If you want to swap *p and *q Value , be t Must be defined as int, Cannot be defined as int*, Otherwise syntax error 
    t = *p; //p The data type of is int*,*p The data type of is int
    *p = *q;
    *q = t;
}
int main(int argc, char const *argv[])
{
    int a = 3;
    int b = 5;
    exchange_3(&a, &b);
    printf("----------------------\n");
    printf("a=%d\n", a);
    printf("b=%d\n", b);
    return 0;
}

Method 3 feasible

Into a,b The address of ,p and q There is a,b The address of ,*p It's a real parameter a,*q It's a real parameter b, In exchange for *p and *q It's equivalent to exchange a and b. let me put it another way , Is to find through the address a and b Then change their values .

Before change ：

After change ：

 

summary ：

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 1. Formal and actual parameters will never be a variable

 2. To exchange the values of two numbers through a function , Is to change the address . The address of the number to be changed is passed in as a parameter

 3. When writing the p = &a; After this sentence ,p What's stored is a The address of ,*p Namely a

 4. After the change ,a,b Your address will not change