Meow, come, come: do you really know if, if else

Catalog

1.if The implementation steps of

2.if(0) Can be used as a comment

3.if The wrong way of writing

4.else Nearby principle

5.else if Two ways of writing

6.if else And if   if The difference between

7. adopt “ Print 1000 To 2000 Years between leap years ” Examples of different codes for comparison

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Meow blogger today above 7 Let's talk about our if And else：

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One 、if The implementation steps of

First, in the C In language ,0 For false , Not 0 It's true .

1. Execute first （） The expression in or function , Get true and false results

2. Conditions Decision function

3. Conduct Branch function

Next, let's take a look at the code , What's the print result ：

#include<stdio.h>
int main()
{
	int a = 0;
	if (a = 0)
	{
		printf("hehe");
	}
	if (a == 0)
	{
		printf("haha");
	}
	return 0;
}

The result is ：

Are you right ？

if(a = 0) take 0 Assign a value to a,a = 0 -> if(a) ->if(0), It is false, so it is not executed .

if(a == 0),a Initialize to at the beginning 0, therefore a It is equal to 0, So it's true , Print haha.

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Two 、if(0) Can be used as a comment

When the code is not needed , It can be used if(0) notes , Of course, meow bloggers do not recommend taking if(0) notes , Just let everyone see if others use if(0), Know when it's used as a comment .

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3、 ... and 、if The wrong way of writing

#include<stdio.h>
int main()
{
	int age = 10;
	if (18 <= age < 30)
	{
		printf(" youth \n");
	}
	return 0;
}

as a result of ：18<=age For false =0 < 30 It's true , So to enter if

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Four 、else Nearby principle

What is the result of the following code ？

#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;
}

result ：

Nothing printed .

  as a result of ：else look for if Nearby principle  

Actually ： The code is equivalent to

#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;
}

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5、 ... and 、else if Two ways of writing

// The first way to write it 
#include<stdio.h>
int main()
{
	int age = 17;
	if (age < 18)
		printf(" teenagers \n");
	else if (age >= 18 && age < 30)
		printf(" youth \n");
	else if (age >= 30 && age < 50)
		printf(" middle-aged \n");
	else
		printf(" The elderly \n");

	return 0;
}

// The second way 
int main()
{
	int age = 17;
	if (age < 18)
		printf(" teenagers \n");
	else if ( age < 30)
		printf(" youth \n");
	else if (age < 50)
		printf(" middle-aged \n");
	else
		printf(" The elderly \n");

	return 0;
}
// Meow bloggers recommend the first , Better code readability 

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6、 ... and 、if else And if   if The difference between

Look at the result of the following code ：

// The first code 
#include<stdio.h>
int main()
{
	int age = 17;
	if (age < 18)
		printf(" teenagers \n");
	else if (age >= 16 && age <= 35)
		printf(" youth \n");
	else if (age >= 30 && age < 50)
		printf(" middle-aged \n");
	else
		printf(" The elderly \n");

	return 0;
}

// The second code 
#include<stdio.h>
int main()
{
	int age = 17;
	if (age < 18)
		printf(" teenagers \n");
    if (age >= 16 && age <= 35)
		printf(" youth \n");
	if (age >= 30 && age < 50)
		printf(" middle-aged \n");
	if (age >= 50)
		printf(" The elderly \n");

	return 0;
}

                    

if else Combination is to make one judgment without making other judgments ;if if Go through each judgment to see whether it is satisfied

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7、 ... and 、 adopt “ Print 1000 To 2000 Years between leap years ” Examples of different codes for comparison

How to judge leap years ：

1、 Can be 4 to be divisible by , But can't be 100 to be divisible by ;

2、 Can be 400 to be divisible by ;

The following code you think is correct , What is wrong ：

//A.
#include<stdio.h>
int main()
{
	int y = 0;
	for (y = 1000; y <= 2000; y++)
	{
		if ((y % 4 == 0 && y % 100 != 0) || (y % 400 == 0))
		{
			printf("%d ", y);
		}
	}
	return 0;
}

//B.
#include<stdio.h>
int main()
{
	int y = 0;
	for (y = 1000; y <= 2000; y++)
	{
		if (y % 4 == 0 && y % 100 != 0)
		{
			printf("%d ", y);
		}
		if (y % 400 == 0)
		{
			printf("%d ", y);
		}
	}
	return 0;
}

//C.
#include<stdio.h>
int main()
{
	int y = 0;
	for (y = 1000; y <= 2000; y++)
	{
		if (y % 4 == 0)
		{
			if (y % 100 != 0) {
				printf("%d ", y);
			}
		}
		if (y % 400 == 0)
		{
			printf("%d ", y);
		}
	}
	return 0;
}

//D.
#include<stdio.h>
int main()
{
	int y = 0;
	for (y = 1000; y <= 2000; y++)
	{
		if (y % 4 == 0)
		{
			if (y % 100 != 0) {
				printf("%d ", y);
			}
		}
		else if (y % 400 == 0)
		{
			printf("%d ", y);
		}
		else
		{
			;
		}
	}
	return 0;
}

//E.
#include<stdio.h>
int main()
{
	int y = 0;
	for (y = 1000; y <= 2000; y++)
	{
		if (y % 4 == 0 && y % 100 != 0)
		{
			printf("%d ", y);
		}
		else if (y % 400 == 0)
		{
			printf("%d ", y);
		}
		else
			;
	}
	return 0;
}

  result ：

A、

B、

 C、

 D、

 E、

  As shown in the figure D It's wrong. .                    Meow bloggers use leap years 2000 Explain with chestnuts in

A and B You know , Meow blogger will not explain here ;C：2000 Enter the first if Judge 2000%4==0 Into it if Judge 2000%100==0 So don't print y. because if The characteristic of each if All judgments , So go to the next if Judge ,2000%400==0, So print 2000;D： The explanation is shown in the figure ：

;

E: The explanation is shown in the figure ：

  That's all the content of this blog , Hopefully that helped , If there is a mistake in the text , Just discuss and solve it with the blogger meow , We learn from each other and make progress .