Detailed explanation of C language P2 selection branch statement

Why does circular structure appear , Because life is repeating the same thing day after day , There are similar things , But I can't go back to the beginning anyway . If you want to make new discoveries in every moment of your life , I'm afraid it's just a beautiful vision .
Why does the selection structure appear , Because every day we as like as two peas do not go through the same way. , for example , If it rains today , You have to bring an umbrella .

1. Branch statement

1.1 if sentence

if Basic syntax of statements

if( Judging expressions )// Take branch expression with non-zero result , Otherwise, execute branch 2 ;
{
    
     Branch one ;
}
{
    
     Branch two ;
}

The above is the case of single branch , Of course if Statement can also implement multiple branches as follows ：

if( expression )
{
    
     Branch one ;
}
else if ( expression )
{
    
     Branch two ;
}
else
{
    
     Branch three ;
}

Of course, you can write more branches , But when there are many branches, we are used to it switch This statement will be introduced later .
Knowledge point 1： In the air else problem

#include <stdio.h>
int main()
{
    
int a = 0;
int b = 2;
if(a == 1)
  if(b == 2)
    printf("hehe\n");
  else
    printf("haha\n");
return 0;
}

What is the result of printing here ？ The answer is “hehe”, So why not ？
there else Not with the first if combination ,else Only with the nearest if combination , This is hanging else problem .
Knowledge point 2：if and else It's actually a statement
The following procedure proves if and else It's actually a statement

#include<stdio.h>

int main()
{
    
    int n=0;
    scanf("%d",&n);
    if(n>0)
        if(n%2==0)
            printf(" The number is even \n");
        else
            printf(" The number is odd \n");
    else
        printf(" The number you entered is not positive ");   
    return 0;             

}

The above code is actually easy to misunderstand , Although there is indentation, it will make people misunderstand , Now let's improve ;

#include<stdio.h>

int main()
{
    
    int n=0;
    scanf("%d",&n);
    if(n>0)
    {
    
        if(n%2==0)
        {
    
            printf(" The number is even \n");
        }
        else
        {
    
            printf(" The number is odd \n");
        }
    }
    else
    {
    
        printf(" The number you entered is not positive "); 
    }
    return 0;
}

In this way, it seems that it is much clearer , In fact, the results of the above two codes are the same , It also shows that , stay c In language if And matching else Taken together as a statement .
Knowledge point 3： Short circuit evaluation problem
In the process of writing branch statements, we will certainly write judgment statements , So short circuit evaluation means ：
When using logical and operators, for example ：
expression 1&& expression 2
If the expression 1 Your judgment is false , Then the result of the whole expression is false , expression 2 You won't judge again . Of course , It's OK to use it at ordinary times , however
When the expression 2 by st++; There will be problems when this kind of self increasing statement , Because I skipped the expression 2, So the expression 2 The value of will not increase by itself .
The same logic or operator is the same ：
expression 1 | | expression 2
If the expression 1 Judge as true , So the expression 2 You won't judge ;

1.2 switch sentence

switch Basic syntax of statements

#include<stdio.h>
int ch=0;
scanf("%d",&ch);

switch(ch)
{
    
    case 1:
    printf("hellow world");
    break;
    case 2:
    printf("hellow world");
    break;
    case 3:
    printf("hellow world");
    break;
    ......
}

among ch Must be an integer constant expression ,case The following must be constants ;
switch in break The role of
There is no break Of switch Statements do not implement multiple branches , Because when ch=1 The program jumps to case 1; Execute the following statement at this tag , encounter break, Jump out of switch sentence , If it were not so break Then continue case 2; Below the label , This is it. break The role of .
switch In the sentence default The role of
Let's write the above program completely ：

#include<stdio.h>
int ch=0;
scanf("%d",&ch);

switch(ch)
{
    
    case 1:
    printf("hellow world");
    break;
    case 2:
    printf("hellow world");
    break;
    case 3:
    printf("hellow world");
    break;
    default :
    printf("input is error\n");
    break;
}

If we give ch Input 8, There is no case 8： This tag , Will execute default The code under the statement ;default Can be placed in switch Anywhere in the statement , It's best to put it at the end .

---

We have finished the above switch Statement and if When to use if When to use switch Well ？
In general , We go through Single expression When controlling program branching , Use switch The effect of the statement is better than if The effect of the statement is better .

2. Loop statement

2.1 while sentence

Let me introduce while Basic syntax , Let's take a look at an example ,
Enter an integer value , Value every number it decrements to zero .

#include<stdio.h>

int main()
{
    
    int n=0;
    scanf("%d",&n);
    while(n>=0)
    {
    
        printf("%d",n);
        n--;
    }
    return 0;
}

This is a very simple while loop , stay while Inside the curly braces is the loop body , After one execution, go up and judge n>=0 If set up , Continue to cycle , until n Less than 0;
Knowledge point 1：while In the loop break

#include<stdio.h>

int main()
{
    
    int n=0;
    scanf("%d",&n);
    while(n>=0)
    {
    
        printf("%d",n);
        n--;
        if(2==n)
        {
    
            break;
        }
    }
    return 0;
}

I modified the above code , If n be equal to 2 When you do it break This will jump directly out of the loop , therefore break The function of is to jump out of while（ Pay attention to if while nesting ,break You can only jump one level ）
Knowledge point 2：while Medium continue
A new keyword appears here ,continue, The old rule is to look at the code ：

#include<stdio.h>
int main()
{
    
    int n=0;
    scanf("%d",&n);
    while(n>=0)
    {
    
        printf("%d",n);
        
        if(2==n)
        {
    
            continue;
        }
        n--;

    }
    return 0;
}

You can stop and think about the execution result of this code
If we type 5, So it prints 5 4 3 2 2 2 2 2······ Then it's a dead cycle , So here continue Is that , End this cycle and jump directly to the judgment part , That is to skip continue Later code , Because the adjustment part is skipped, it causes an endless loop , When actually writing code, you should pay attention to .

2.2 do while sentence

do while, seeing the name of a thing one thinks of its function , Come up and do it first , Anyway, let's start with a cycle , And then judge , If the conditions are met, continue , If you are not satisfied, it will end .
Look at the chestnuts first , Enter an integer to show whether it is odd or even , Then repeat .
Code up ！

#include<stdio.h>

int main()
{
    
    int retry=0;
    do
    {
    
        int n=0;
        printf(" Please enter an integer ：");
        scanf("%d",&n);
        if(n%2==0)
        {
    
            printf(" even numbers \n");
        }
        else
        {
    
            printf(" Odd number \n");
        }
        printf(" repeat ：yes\1  no\0:);
        scanf("%d",&retry);
    }while(retry);
    return 0;
}

As above, execute it first , Then determine whether to continue to repeat . Others and while Similar, I won't repeat much .

2.3 for sentence

for Sentence syntax
Write a program output 0-12 Number between
Go straight to the code

#include<stdio.h>

int main()
{
    
    int n=0;
    for(n=0;n<=12;n++)
    {
    
        printf("%d ",n);
    }
    return 0;
}

Above is for Basic syntax ,n=0; For initialization statements ,n<=12 Is the judgment statement ,n++ To adjust the statement .
Knowledge point 1：for Statement break
Like while The cycle is the same ,for Inside the loop break Will jump out of the loop ;
Knowledge point 2：for Statement continue
Although the role and while Medium continue It's the same, but ,for In the loop continue, It doesn't jump directly to the judgment part of the program , Instead, it jumps to the adjustment part of the program , So subroutines don't appear like ,while The dead cycle in .
Knowledge point 3：for A variation of the cycle
stay for In circulation （） The three expressions in can be omitted , If the second expression is omitted, that is, the judgment expression , That would be a dead circle , It is not recommended to omit

---

Attention should be paid to the use of circular statements , Try not to modify the control quantity of the loop inside the loop , Prevent circulation from getting out of control .

3.goto sentence

About goto, The following passage is me copy Of , Let's just have a look and understand ,goto It's easy to use
C The language provides a language that can be abused at will goto Statement and mark the label of jump .
In theory, goto Statements are not necessary , In practice, there is no goto Statements can also easily write code .
But on some occasions goto Statements are still useful , The most common use is to terminate the processing of structures nested in some depth
cheng .
for example ： Jump out of two or more loops at a time .
Multilayer loops are used in this case break You can't achieve your goal . It can only exit from the innermost loop to the upper loop .
Code demonstration ：

for(...)
    for(...)
    {
    
        for(...)
        {
    
            if(disaster)
            goto error;
        }
    }
…
error:
    if(disaster)
//  Handling error situations 

This is it. goto The most common scene , Jump out of multiple nested loops , You can save writing multiple break;

---

4. Exercises

practice 1
Write code , Demonstrate the movement of multiple characters from both ends , Converging in the middle

#include<stdio.h>
#include<string.h>
#include<Windows.h>
int main()
{
    
    char arr[] = {
     "***************" };
    char brr[] = {
     "hellow world!!!" };
    int sz = strlen(arr);
    int left = 0;
    int right = sz - 1;
    while (left <= right)
    {
    
        arr[left] = brr[left];
        arr[right] = brr[right];
        right--;
        left++;
        printf("%s\n", arr);
        Sleep(1000);

        system("cls");

    }
    printf("%s\n", arr);


    return 0;
}

practice 2
Dichotomy search （ Here is the simplest binary search , The details will be introduced later ）

#include<stdio.h>

int main()
{
    
	int arr[] = {
     1,2,3,4,5,6,7,8,9,10 };
	int findnum = 8;
	int left = 0;
	int right = sizeof(arr) / sizeof(arr[0]);
	while (left <= right)
	{
    
		int mid = left + (right - left) / 2;
		if (arr[mid] < findnum)
		{
    
			left = mid + 1;
		}
		else if (arr[mid] > findnum)
		{
    
			right = mid - 1;
		}
		else
		{
    
			printf(" eureka , Subscript to be  %d ", mid);
			break;
		}
	}

	return 0;
}

practice 3
Guess the number game

#include<stdio.h>
#include<time.h>
void menu()
{
    
	printf("***********************\n");
	printf("****** 1 play ******\n");
	printf("****** 0 exit ******\n");
	printf("***********************\n");

}

void play()
{
    
	int ret = rand()%100;
	while (1)
	{
    
		printf(" Please start guessing numbers ！\n");
		int sc = 0;
		scanf("%d", &sc);
		if (sc < ret)
		{
    
			printf(" Guess a little \n");
		}
		else if (sc > ret)
		{
    
			printf(" Guess the \n");
		}
		else
		{
    
			printf(" congratulations , Guessed it ！！！\n");
			break;
		}

	}
}

int main()
{
    
	srand((unsigned int)time(NULL));
	int input = 0;

	do
	{
    
		menu();
		scanf("%d", &input);
		if (input == 1)
		{
    
			play();
		}
	} while (input);
	return 0;
}

practice 4
Shut down the applet , use goto Statements for

#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int main()
{
    
	char arr[10] = {
     0 };
	system("shutdown -s -t 60");
again:
	printf(" Please enter that I am a pig , Otherwise your computer will be 60s Internal shutdown \n");
	printf(" Please enter ：");
	scanf("%s", &arr);
	if (0==strcmp(arr," I am a pig "))
	{
    
		printf(" Cancel shutdown \n");
		system("shutdown -a");
	}
	else
	{
    
		goto again;
	}

	return 0;
}