当前位置：网站首页>Operators are also important if you want to learn the C language well

For a better demonstration, the following code will be in Linux Demonstration under the platform （ \ Is a continuation character ）： This code , Which line is problematic ？

There may be a little friend here who has questions , Why is this the only line that has gone wrong ？ We know that annotations are recognized by the compiler in the preprocessing phase , Here we can use Linux An order of ：gcc -E test.c -o test.i That's right test.c Program translation , Finally, the preprocessing results are preserved , Then you can use vim Open our preprocessing result file .

Finally, we can come to the conclusion that ： A comment that is replaced is essentially replaced with a space ！

Here we want to make another simple point ,C Language annotations cannot be nested ,/* Always with the nearest */ matching , Interested partners can go to test ：

int main()
{
    /*

    /*printf("hello world");
    printf("hello world");*/

    */
    return 0;
}

Here's another interesting code , If you use C Language notes , If only one appears /* The following... Could not be found */ Words , Then he will default that all the following code is annotated ！

int main()
{
    int x = 10;
    int y = 5;
    int ret = 0;
    int* p = &y;
    ret = y/*p;
    return 0;
}

So how to solve the above situation ？
stay / And * Add a space in ： ret = y / *p;
hold *p Enclosed in brackets ： ret = y / (*p);
Recommend the second , Because it looks more intuitive

1.2 How to write good notes ？

[ Suggest 1 ] Notes should be accurate and easy to understand , There must be no controversy , Wrong comments are harmful .

[ Suggest 2 ] Make sure that the comments are consistent with the code , Useless comments should be deleted in time .

[ Suggest 3 ] For global data ( Global variables 、 Constant definition ) It must be annotated .

[ Suggest 4 ] A clear statement can be left uncommented , such as ：i++; // i Self increasing 1.

[ Suggest 5 ] The position of the comment should be adjacent to the code being described , Can be on the same line as the statement , Not below .

[ Suggest 6 ] When the code is long , Especially when there are multiple sets of them , There should be a note at the end of some paragraphs .

[ Suggest 7 ] The indentation of comments should be consistent with that of code .

[ Suggest 8 ] Annotation code should focus on ,“ Why do ” , instead of “ How do you do it? ”.

[ Suggest 9 ] The unit of value must be annotated .

[ Suggest 10 ] In complex functions , In branch statement 、 Appropriate comments are required after the loop statement ends , It is convenient to distinguish each branch or loop .

2、 Continuation and transition characters

2.1 Continuation function

C Use backslashes in language ( \ ) It means the flight is cut off . The compiler will remove the backslash , The character following the backslash is automatically continued to the previous line . But notice ： No space after backslash , Interested partners can come down and test ：

int main()
{
    int a = 1;
    int b = 2;
    int c = 3;
    if (1 == a && \
        2 == b && \
        3 == c)
    {
        printf("You can see me!\n");
    }
    else
    {
        printf("You can not see me!\n");
    }
    return 0;
}

Maybe some friends will be curious , Obviously I don't have to \ You can also change lines without reporting errors , Then why use \ Means line feed ？ As a programmer , Others may also read our code , If not \ Straight line breaks can be strange , You can also tell the compiler that this place is a continuation line , More semantic ！

2.2 Escape character

C In language , There are some characters , It's his literal meaning , such as 'n','b','t'. There are also some characters , It has a special meaning , such as ：" , ', \ The essential meaning of escape is ： Literally to special , Or special to literal .

\ There are two uses ：
When the continuation character uses
Escape characters use (a. Literally to special b. Special to literal )

Let's focus on \n and \r Maybe all my friends have heard of it Line break and enter Well ！ So what they essentially mean is different ！ But now many compilers make Line break ( \n ) It also contains enter ( \r ) The function of ！ What is the difference between them ？

Line break ： Move the cursor to the next line
enter ： The cursor returns to the beginning of the current line

Here you can try it C Language to write a simple countdown function , You can clearly distinguish between line feed and carriage return ！

Here we can also take a look at a pen test ：

// What does the following program output ？
#include <string.h>
#include <stdio.h>
int main()
{
    printf("%d\n", strlen("abcdef"));
    // \62 Parsed into an escape character 
    printf("%d\n", strlen("c:\test\628\test.c"));
    return 0;
}

Here is the first print function , If we know ,strlen() The function is to find the string \0 The number of characters before , So the first one to print is 6 , The second print function , First of all \t Parsed into an escape character , \62 It is also parsed as an escape character (8 Base number ) So the second print should be 14 ！

3、 Single and double quotes

3.1 Basic concepts

about C Everyone who knows the language knows it , Single quotation marks are all character constants , The double quotation marks are all string constants , But it is still easy for beginners to make mistakes , such as 'a', and "a" Is a completely different concept , In the string , With \0 For the end , It doesn't belong to the element content of string in essence , It can only be said that it is a string end identifier , But it takes up space ！ That is to say 'a' Will take up one byte and "a", It will occupy two bytes .

With the above simple concept, let's take a look at an example ：

There may be a little friend who has questions , The first two and the last one printf I can still understand the printed results , But why does the third one occupy four bytes ？ This is obviously not a character constant ？（C++ The third size in is 1 Bytes , Because this is about C Language so we don't talk about other languages ）

Actually in C99 Provisions of the standard ,'1' It's called an integer character constant (integer character constant), To be seen as int type .

Just use “ The standard stipulates ” To prove infeasible , We will use another example to prove ：

Don't be surprised to see this result , I just said that , The standard stipulates , Integer character constants are treated as int type , So he will have four bytes of space , And my current computer is a small end storage , So the low byte order is placed at the low address , Those in charge , Will a 4 Bytes of data into char Data truncation occurs in variables of type , A byte of the low address will be put into char In variables of type , So in the end c There is only one in the variable 'd' character ！

But I don't recommend the above way ！！！ If only we knew the truth .

Then some of the little friends here started to experiment again , So I wrote this code ：

Because an integer character constant has only 4 Byte space , How can he let go 5 Bytes or more ？

3.2 A special case

Obvious , Null integer character constants are not allowed , But an empty string can appear , Because the string end identifier \0 Although it can not count the contents of string elements , But it also takes up a lot of space .

4、 Logical operators

4.1 && ( Logic and )

Concept ： Cascade two （ Multiple ） Logical expression , It has to be true at the same time , The result is true .

Example ：

For the short circuit of logic and , Because our compiler scans from left to right , So if the left side of the expression is false , It will not execute the logic and the expression on the right , This is what we call short circuit phenomenon ：

4.2 || ( Logic or )

Concept ： Cascade two （ Multiple ） Logical expression , At least one must be true , The result is true .

Example ：

The above example also contains the short circuit phenomenon of logical or , In logic or , Because the compiler scans the code from left to right , So as long as the left side of the expression is true , You don't need to make the right judgment .

4.3 A written test of logic and logic or

Because for the front ++ And post ++ I haven't said yet , So the students who have the foundation can have a look first , If you don't have a foundation, you can wait until I issue the symbol in the next issue. The second issue will come back to see this problem .

The first logic and expression ,a The initial value of the variable is 0 , And it's post ++, Use the value of the expression first , In the process of self increasing , Logic and must satisfy that the values on both sides of the expression are true , If one is false, the following expression will not be executed , So only a The variable has changed .
The second logical or expression , Logic or as long as one of the two sides of the expression is true ,a++ To judge as false , Will continue to judge ++b, In front of ++ It's self increasing first , therefore ++b Expression is true , You won't execute the following expression , in other words , Only a and b The value of the variable has changed .