C language -- 11 branch statement if else

1、 Preface

（1）C Language is a structured programming language .C The three basic program structures of the language are ：

• Sequential structure ： Follow the program sequence
• Selection structure ： Select the branch direction according to the judgment result
• Loop structure ： Refers to having a loop body , The number of cycles can be determined according to the judgment conditions
  （2） Branch statement （ Select statement ） And loop statements
  
  （3） What is a sentence ？
  C There is a semicolon in language (;) Separated by a statement .

2、if The grammatical structure of a sentence

There are roughly three kinds of ：

if( expression )
	 sentence ;

if( expression )
	 sentence 1;
else
	 sentence 2;

// Multiple branches 
if( expression 1)
	 sentence 1;
else if( expression 2)
	 sentence 2;
else
	 sentence 3;

3、 About if else Examples of statements

Example 1 ： understand if else structure

#include <stdio.h>
int main()
{
    
	int age = 10;
	if (age >= 18)
		printf(" adult \n");
	return 0;
}

Run the program , There will be no printouts ; If you add

	else
		printf(" A minor \n");

Then the running result is ：

 A minor 

Example 2 ：if or else By default, only the following sentence is executed
The age in example 1 10 Change it to 20, And then else Add a sentence to the sentence of , View the run results .

#include <stdio.h>
int main()
{
    
	int age = 20;
	if (age >= 18)
		printf(" adult \n");
	else
		printf(" A minor \n");
		printf(" You can't go to Internet cafes \n");
	return 0;
}

Running results ：

 adult 
 You can't go to Internet cafes 

Obviously, this result is unreasonable , What we want is if “ A minor ”, To print “ You can't go to Internet cafes ”, But the age entered is 20, Show “ adult ” That's all right. , It still shows “ You can't go to Internet cafes ”. This proves if/else The default is to execute only one statement below it . When I type this code , In fact, this compiler is already very smart , Can reflect if/else The default is to execute only one statement below it One manifestation of this property is ：else The second statement under it is automatic and else The alignment of the , To see if the program can only print “ adult ”, This code is specially given to else The second item under printf Before the statement tab key .
So how can we make the program realize the function we want ？
take else The two statements to be executed inside are enclosed in curly braces , One “{}” It's just a code block , It is a logic .

{
    
	 Statement list ;
}

The code is revised as follows ：

#include <stdio.h>
int main()
{
    
	int age = 20;
	if (age >= 18)
	{
    
		printf(" adult \n");
	}
	else
	{
    
		printf(" A minor \n");
		printf(" You can't go to Internet cafes \n");
	}
	return 0;
}

Example 3 ：if else Multi branch case of
The wrong sample ：

int main()
{
    
	int age = 60;
	if (age < 18)
		printf(" juvenile \n");
	else if (18 <= age < 26)
		printf(" youth \n");
	return 0;
}

After running the program , prints “ youth ”. Why does this result ？

So though 60 It's not in 18 To 26 Between , Will also print “ youth ”.
Write it correctly ：

int main()
{
    
	int age = 60;
	if (age < 18)
		printf(" juvenile \n");
	else if (age >= 18 && age < 26)
		printf(" youth \n");
	else if (age >= 26 && age < 40)
		printf(" Prime of life \n");
	else if (age >= 40 && age < 60)
		printf(" Middle and old age \n");
	else 
		printf(" The elderly \n");
	return 0;
}

The running result is : The elderly
Example 4 ： In the air else

int main()
{
    
	int a = 0;
	int b = 2;
	if (a == 1)
		if (b == 2)
			printf("hi\n");
	else
		printf("hello\n");
	return 0;
}

After running the code , No results in the print window . Why? ？
Let's see if I don't delete it manually tab interval , How does the compiler recognize this code .（ I'll add to it {} Make the program more readable ）

int main()
{
    
	int a = 0;
	int b = 2;
	if (a == 1)
	{
    
		if (b == 2)
		{
    
			printf("hi\n");
		}
		else
		{
    
			printf("hello\n");
		}
	}
	return 0;
}

You can see ,else With the one closest to him if Match the , because a The value of is 0, So we won't enter the first if, Naturally, there will be no printed results . So code separation is very important , add {} Separating the code can improve the readability of the program .

4、if else Contrast of writing forms

Code one and code two actually implement the same function . Let's use a piece of code to explain why code one and code two are actually the same .

int test()
{
    
	if (1)
		return 0;
	printf("haha\n");
	return 1;
}
int main()
{
    
	test();
	return 0;
}

When running the program , Find that nothing can be printed . Press the fn+f10 Debugging code , You can see that the execution is finished test Medium return 0 Just skip it printf(“haha\n”);return 1; These two sentences .
Modify the code as follows ：

int test()
{
    
	if (0)
		return 0;
	return 1;
}
int main()
{
    
	test();
	return 0;
}

You can see that the code goes to if (0) after , Go on and you will arrive at return 1 了 .

Code three and code four are the same , Why the num == 5 Written as 5 == num Well ？ In order to avoid a situation

int main()
{
    
	int num = 1;
	if (num = 5)
	{
    
		printf("hello\n");
	}
}

take == It has been written. =, Will become 5 Assign a value to num, At this time, the judgment condition is true , It's not judgment num Whether it is 5 了 , The program will eventually print hello. But in code 4, if you will 5 == num Written as 5 = num The program reports an error .

5、 Example

Output 1-100 Between the odd numbers

int main()
{
    
	int i;
	for(i = 0; i < 100; i++)
		if (i % 2 == 1)
		{
    
			printf("%d\t", i);
		}
		else
		{
    
			printf("");
		}


	return 0;
}

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