当前位置：网站首页>How do different types of variables compare with zero

C In language , Use all kinds of The operator hold Variable Connect , Form a meaningful formula , It's called expression . for example ：a = b + c

1.2 What is a sentence

Add a after the expression A semicolon , There is a sentence . for example ：a = b + c;
Of course, it's not just statements with semicolons after expressions , There are also similar to Input Output Function call wait sentence . for example ：scanf("%d",&a); printf("%d",a); add（a,b）;
only one A semicolon ; It's called. Empty statement
C In language , Use a pair of braces {} The enclosed Multiple statements be called Compound statement , Compound statement Grammatically considered to be A sentence

notes ： Just remember that the sentence ends with a semicolon , Except for compound statements

2. Judgment statement How is it implemented

1.1 if

First, execute expression , The value of the expression is really Just perform if Inside sentence , by false be Don't execute

1.2 Single branch

First, execute the expression , The value of the expression is really Just perform if Inside sentence 1, by false be perform else Inside sentence 2

1.3 Multiple branches

First judgement expression 1, expression 1 The value of is really Just perform if Inside sentence 1, by false Then judge else if Inside expression 2, expression 2 The value of is really execute else if Inside sentence 2, by false execute else Inside sentence 3

1.4 nesting

First judgement expression 1, expression 1 The value of is really Just perform if Inside sentence 1 then Judge Inside expression x , if expression x by really be Execute statement x, expression x by False execution statement y . expression 1 by false Then judge else if Inside expression 2, expression 2 The value of is really execute else if Inside sentence 2, expression 2 by false execute else Inside sentence 3

Conclusion ：
if Statement execution , First execute the value of the completion expression , Get logical results , In making a judgment
C in 0 Said the false , Not 0 Said really

3.bool Variables and " Zero value " Compare

3.1 C Is there any in the language bool type ？

reflection ：C Is there any in the language bool type ？

answer ：c99 Not before bool type , Mainly c90 No, . but c99 Introduced _Bool type （_Bool Is a type , But in the new header file stdbool.h in , It was written again as bool, In order to ensure C/C++ Compatibility ）.

bool Is a true and false type ：true It's true ,false For false

3.2 bool Variables of type occupy several bytes ？

In the test above , We can see that bool Variables of type account for A byte .

My little friend may be confused when he sees this , Why is it capitalized BOOL 、TRUE It can also run , And its size is four bytes ？ Let's turn to its definition

Now we know why capital letters BOOL Variables of type account for Four bytes 了 , Because in the source code , That's the definition ：typedef int BOOL;

notes ： This is all Microsoft I made it myself BOOL value . stay vs Transfer to BOOL Corresponding header file , Turn to the top , You can see Microsoft's copyright information . Use BOOL Need to add #include<windows.h> The header file .

We generally write code not only on the platform we use , It also needs to be available on other platforms , So this involves Cross platform , and Microsoft Defined Exclusive type , Other platforms I won't support it , So it's just Unsupported use

summary ：
priority of use c90, It's the way we've been using before and after （ although c90 Does not support bool type , But it can express true and false by integer ,0 For false 、 Not 0 It's true ）
In case you have to use bool, recommend c99 standard , Not recommended MS Customize .

3.3 bool Value and 0 Compare how to compare ？

#include<stdio.h>
#include<stdbool.h>
int main()
{
	bool a = true;
	if (1 == a)
	{
		printf("1\n");
	}
	if (true == a)
	{
		printf("2\n");
	}
	if (a)// recommend 
	{
		printf("3\n");
	}
	return 0;
}

Why do you recommend No 3 individual if How to write ？

answer ： a Itself is a bool Type value , Is it true or false .

Conclusion ：bool type , Direct determination , Compare with specific values without operators

4.float Variables and " Zero value " Compare

We know that floating point numbers are stored in memory , There may be a Loss of accuracy （ notes ： The loss here , Not blindly reduced , There may be more , When the floating-point number itself is stored , When the calculations are endless , Meeting “ rounding ” Or other Strategy ）

4.1 Whether two floating-point numbers can be compared equally

As you can see from the following image code x=1.0,y=0.1, So for if Expression judgment x - 0.9 It should be equal to 0.1 ,0.1 And y Equality should be printed 1, But it printed 0 , That's why ？

So let's go on , Let's look at printing x-0.9 and 0.1 What exactly is printed ？

In the original memory x - 0.9 Is not the same as 0.1, It's a loss of accuracy , and y Neither 0.1, The accuracy is also lost , So the code above is not printed 1

Conclusion ： Because of the loss of accuracy , Two floating-point numbers , Never use == Make an equality comparison

4.2 So how do two floating point numbers compare ？

A range accuracy comparison should be made

notes ：fabs Is the absolute value of a floating-point number

How to set precision ？
Define your own macro settings
Use system accuracy

1. Use the macro definition to set the precision

2. Use system accuracy

4.3 float Variables and 0 Compare

#include<stdio.h>
#include<float.h>
#include<math.h>
int main()
{
	float x = 0.000000000000001f;// When the value is small enough , Will be judged as 0
	//if (fabs(x) < DBL_EPSILON)
	if (fabs(x-0) < DBL_EPSILON)
	{
		printf("1\n");
	}
	else
	{
		printf("0\n");
	}
	return 0;
}

5. Pointer variables and “ Zero value ” Compare

5.1 Express 0 Methods

We've studied before Numbers 0、 Escape character \0、 empty NULL, They can also mean 0

We can go to NULL Definition , You can see that in fact NULL Namely 0

5.2 Pointer variable is compared with zero

6.else With which if What about pairing ？

1. This code always makes people feel like it's different from the first one if matching , In fact, it is similar to the second one if matching , because else The principle of proximity is adopted , So this code doesn't print anything

#include<stdio.h>
int main()
{
	int x = 0;
	int y = 1;
	if (10 == x)
		if (11 == y)
			printf("hi\n");
	else
		printf("hello\n");
	return 0;
}

2. This is the right code style , It's easy to see else With which if matching

#include<stdio.h>
int main()
{
	int x = 0;
	int y = 1;
	if (10 == x)
	{
		if (11 == y)
        {
            printf("hi\n");
        }
	}	
	else
	{
		printf("hello\n");
	}
	return 0;
}

Why? Not written x==10, The written 10 == x？

answer ： Because writing in this way can avoid == It's written in =, If you write it carelessly =, We know that the left side of the assignment number must be a variable , So this is a grammatical error , The compiler prompts , If it is written carelessly x = 10, This is a running error. The system will not prompt .

This shows how important a good programming style is to programmers , Develop good programming habits , From you and me .

原网站

版权声明
本文为[Desperate Azi]所创，转载请带上原文链接，感谢
https://yzsam.com/2022/181/202206301156417923.html