Summary of the second lesson

1. Constant

There are four kinds of constants , They are literal constants ,const Defined constant ,define Defined identity constants and enumeration constants .

Literal constants

Literal constants are literal constants , Such as 1,2,3,4,"ahsbb"'a', These are literal constants .

const Defined constant

Such as const int a＝3, This defines a constant variable , But here's the thing const Just make it unchangeable , And its essence is variable . Such as （const int a＝3;char arr[a]＝{1,2,3｝） Theoretically, this code can run successfully , But it still doesn't work , because a The essence of is a variable , And the brackets in the array should be constants , So it can't run .

define Defined identity constant

       

  Like this, you define an identity constant , And here you can successfully run the instructions define Definition Of Constant. .

Enumeration constants

     

Enumeration constants like this , And the result of the program output is 0, Because I'm not right male Assign initial value to , So it's from 0 At the beginning . So the keyword of enumerating constants is enum.

2. character string

What is the difference between outputting strings one by one and outputting strings directly

Like the first one （arr1） String output , The result is consistent with the expectation , But the second kind has deviation . Why? ？

First of all, let me explain %s The output rules of ： Keep outputting until you encounter '\0' Output ends .

And the first string is input into the character array （arr1） In the middle of the day , stay 'd' A... Is automatically entered after '\0', So normal output .

The second one is （arr2） Direct input 'a','b','c','d' No input '\0' So there's a mess .

Empathy , Library function strlen Also encountered '\0' end , But here's the thing strlen Not put '\0' Count into the length , And if you want to use without warning, you need to add a header file in front #include <string.h>. Last ,strlen Only string length can be calculated , and sizeof It's different .

3. Escape character

The picture is from (7 Bar message ) C Language commonly used escape character table _a746742897 The blog of -CSDN Blog _c Language commonly used escape character table https://blog.csdn.net/a746742897/article/details/77543029

The last two are relatively important and relatively difficult , For example, what is the answer of the following code

The answer is 14.

because '\t' Represents a character （ Horizontal tabs ）,'\62' It represents a two digit octal number ascii（ Corresponding 49）, because \628 Already exceeded ascii The maximum of , So I just took \62, That is to say 1. So the running result is 14.

Explain the last two escape characters above ：\ddd It doesn't have to have three octal numbers , An octal number can also .

Empathy ,\xdd The same goes for , Similarly, three hexadecimal numbers are not necessary , One is also possible .

4. Select statement , Loop statement , Knowledge of functions and arrays

Is to simply understand , No deep research , So I won't elaborate too much here .

5. Monocular operator and binocular operator

Generally speaking, monocular operator means that there is only one variable in the left and right operations , Such as "++","--" And so on. . Binocular operators are two variables that operate on the left and right , Such as "+","-" wait , Also, there is no deep study , So here is just a simple explanation .