The first C language program (starting from Hello World)

There is a conventional habit among programmers , When we learn any programming language , The first program written , Is to print a line of characters on the display “Hello World”.

Where does this habit come from , First review C The history of language , You can learn the origin of this habit .

1972 year ,C Language from Dennis Ritchie And colleagues at Bell Laboratories . Then ,C The designer of language Dennis Ritchie And famous computer scientists Brian W. Kernighan, Co authored an introduction C Language and its programming methods of the authoritative classic works ——《The C Programming Language》.

The first example program in the book , Is to output a string of characters on the screen “Hello World”.

so far , Many programming language textbooks also follow this habit . therefore , If you have seen Java,PHP,Go And other programming languages , You will find that their first example program is also “Hello World”.

It can be seen that everyone still recognizes this habit , And has continued this tradition in his own works .

Although no one can scientifically explain why Hello World So popular , however ,Hello World Programs have indeed become a milestone in the history of computer development .

Environmental preparation ：C Language development environment installation

Sample code ：

#include <stdio.h>

int main(){
     
   printf("Hello World");
   return 0;
}

The effect of the sample code ： The effect of this program is very simple , Is to output a line of characters on the screen "Hello World".

1. The main function

First , The main function main , A standard main function is as follows ：

int main() //  This is the main function  
{
     
    return 0; //  The return value of the main function  
}

main stay C The language represents a main function .

After the double slash is notes , Generally speaking , Comments are used to mark the purpose of this code or explain ideas . Because comments are not compiled as code , So no matter what comments are added , Will not have any impact on the actual operation of the code .

2. What is a function ？

In the field of Mathematics function , And... In programming languages function Completely different .

In programming languages , You can think of a function as a box , This box has the following features ：

1. At the beginning of execution , Function can be input with some values
2. In the process of execution , Functions can do some things
3. After execution , Function can return some values

Take the example code above as an example to explain ：

1. The main function has no input (main()： The input parameter is null )
2. The main function prints a line of words onto the screen （printf("Hello World");）
3. The main function returns 0（return 0;）

among , int Indicates the return type For integer type , int yes integer（ Integers ） Abbreviation . This is stipulated by the language standard , You can't write other words .

main yes Function name , main The brackets that follow () Inside is the input parameter , It is currently empty .
return Followed by a function Return value , by 0. and 0 It's an integer , And before the function name int Corresponding .

Sum up the formula of function :

 Function return value type   Function name ( Function input parameter value ) {
      
    Do something  
   return  Function return value ; 
}

What is enclosed by curly braces is called The body of the function , Note that the function body must be included by curly braces and cannot be omitted . Function name above curly braces 、 Function parameters and return values are called Function header .

3. Write your own function

demand ： According to the above formula of function writing , Write a function that adds two integers . This function needs to do ： Enter two integers , Return the result of their addition .

Since this function is used to calculate addition , We name the function add . Of course, the function name of the custom function can be written according to your preferences , Even if it's written as aaaaa It's OK . however , Have for function name semantic , It is convenient for people to read and understand , We usually use English as the function name .

//  This piece of code is called add Function definition of function  
int add(int a, int b) {
     
    return a + b; 
}

explain ： Define a return value as int type ( integer ) Function of , The function is called add, The input value is ： Integers a and Integers b,
Return value ： Returns the sum of two integers , Both addends are integers , So the result is also an integer .

This piece of code is called add function Function definition of .

4. The main function is the whole C The entrance to language programs

add function It can run directly ？ The answer is No good .

be-all C Language codes have a starting entry , And this entrance is The main function main . After entering the main function , To call other functions through the main function .

It also means that , Every C The language code , There can only be one main function .

Slightly modified code ：

#include <stdio.h>

int add(int a, int b) {
     
   return a + b;
}

int main() {
     
   int result;
   result = add(2, 3);
   printf("%d", result);
   return 0;
}

When the program is running , First of all The main function main . Then call what we just wrote add function 了 . We passed on 2 The values are integers 2 and 3 to add function .

The definition of the function specifies the need to pass a、b,2 Parameters , When we call , It must also be passed on 2 individual , And the type also needs to be consistent as far as possible , Otherwise, the compilation may report errors .（ If the type is different , The compiler will try automatic type conversion . If automatic type conversion fails, compilation fails .）

problem ： Who called the main function ？ The return value of the main function must be int Do you ？

answer ： The main function is called automatically at the beginning of the program , There is no need to actively call the main function in the program . The return value of the main function will be returned to the program calling this program .

C The language standard stipulates that the main function has a return value and must be int . If the program ends normally , Generally, the return value is set to 0.

5. To call a function , You must know the function first

The compiler starts with code , Read the code from top to bottom .

The compiler first sees the definition of a function , Describes a man called add Function of . next , stay main Required in add , Because the compiler already knows add The definition of , Therefore, the compiler can compile normally .(ps： First define , After use )

What happens if you reverse the definition of functions and the order of function calls ？

First , The compiler sees add The name , The compiler will be confused , add What is it? ？ The compiler cannot understand add What is it . therefore , The compiler will report an error , And stop compiling .

6. What is a variable ？

stay add function After the calculation , There needs to be something to accept add The returned value . therefore , We are add The previous statement states that int integer Of Variable .

What are variables ？ You can think of it as an empty box , It can contain any other value consistent with its type .

result It's just a name we gave it , Of course , You can call it anything . For example he, It's called xiangzi, Fine (ps： Naming should be semantic ).

We put add Back home 5, Put in result . therefore , result The value inside is 5 了 .

= Equal sign , stay C In language is Assignment operator , It has the function of loading the value on the right into the variable on the left . Like a function , The equal sign here is the same as that in Mathematics , It's also very different , It doesn't mean equal .

Assignment operator ： The value to the right of the symbol , An operator that puts variables on the left .

The example written above is add function Accept 2,3 For input , return 5, Assigned number = Assign a value to result The process of .

Can you write it like this ？ Get rid of int result; This article .

int main() {
     
    result = add(2, 3); 
    printf("%d", result); 
    return 0; 
}

The answer is No , Variables must be declared before using .

The compiler sees result The name , But never seen result The definition of , You will certainly wonder what kind of variable it is , It may even be a function instead of a variable . such , Unfortunately, the compiler can only give a hint of compilation errors , And end the compilation .

int result;

It must be like this , Declare a variable , Name is result , The type is int . Next , The editor can write down result For one int Variable of type . In the following code , You can use this happily result The variable .

7. identifier

In the previous code , Named by ourselves , The name used to refer to an entity , for example :add,result, The parameters of the function a,b It's all one identifier .

The identifier is a special identifier named by ourselves , Used to represent a variable 、 The name of a function or other entity .

for example ： Our custom function is named add It is to clearly indicate the intention of the function . And name the variable result, It is also to show that what is stored in the variable is The result returned by the function .

also , To enable the compiler to recognize identifiers , Must be declared or defined . for example ,add Is defined as a function ,result, a,b It is declared as an integer variable . If the compiler encounters an undefined or undeclared identifier , You will not be able to understand the specific name of this identifier and report an error .

stay C In language , The identifier can be named as you like , But the following rules must be followed ：

Identifiers can be in lowercase letters 、 Capital 、 Numbers and underscores to name . however , The first character of the identifier must be a letter or an underline , Not Numbers . also , Identifiers are case sensitive .

8. key word

problem ：int It's a identifier Do you ？
answer ： No . First int We didn't name it casually , secondly int Not the name of any entity .

int yes C One of the languages key word .

key word It is stipulated in the language standard , And it has special significance and use in the code . therefore , Keywords cannot be used as identifiers .

C The keywords in the language are shown in the following table .

auto	_Bool*	break	case
char	_Complex*	const	continue
default	restrict*	do	double
else	enum	extern	float
for	goto	if	_Imaginary*
inline*	int	long	register
return	short	signed	sizeof
static	struct	switch	typedef
union	unsigned	void	volatile
while

9. What is a literal constant ？

problem ： that , image 2,3, This value , Do you need to declare ？
answer ： Unwanted , They are Constant , Cannot be changed . And once it's written , We already know that they are integers int Constant of type .

alike , Literal constant of string It doesn't need to be declared , for example ："Hello World". Wrapped in double quotation marks , We think it is a string , To distinguish it from the numerical value .

Variables can be changed by assignment , Constant cannot be changed , So it cannot be assigned .

2 = 3; //  error  
"Hello" = "World"; //  error 

10. printf function

#include <stdio.h>

int main(){
    
    printf("Hello World");
    return 0;
}

and add equally , printf It's also a function , But this is not a custom function , It is a function of the system .
Let's take the string literal constant "Hello World" Passed on to printf function . When running code , We saw this line on the screen
operator . obviously , printf function The function of is to output the string to the console .

printf By word print（ Print ） And the words format（ Format ） The first letter of f form , Meaning for Format printing .

In the early , The output of the computer mainly depends on connecting the printer to print characters on paper . Today, , Most of the output of the computer is realized on the screen . however print This word has been retained . Sometimes we use the term printing , But in fact , We output characters in the console on the screen .

printf Function more usage

int main() {
     
    int result; 
    result = add(2, 3); 
    printf("%d", result); 
    return 0; 
}

Compare two pieces of code , We write Hello World Time passed only 1 A parameter for printf , But why add function Later, it can be transmitted 2 Give it one ？ Does the number and type of parameters defined by the function need to be consistent with that when the function is called ？

printf Is a very special function , It's a Argument function , Therefore, variable number and type of input parameters can be accepted . Here we don't need to care too much about how to write a variable parameter function , Just use it for the time being .

Through the following example , We can simply understand printf function More use of .

Used to print an integer ：printf(“%d”, integer int);

printf("%d", 12345);

Used to print two integers ：printf(“%d\n%d\n”, integer int1, integer int2);

// \n Means line break , Start from the next line and continue printing  
printf("A=%d\nB=%d\n", 123, 456);

To sum up printf Formula
printf(“XXX placeholder 1 XXX placeholder 2 XXX placeholder 3”, Replace 1, Replace 2, Replace 3);

printf The first parameter of must be character string , Here we pass in a string literal constant （ Wrapped in double quotation marks ）. among , To occupy a place % Conversion operation Express . for example ： integer int The placeholder for is %d . The following replacement parameters , The previous placeholders will be replaced in turn . printf Is a variable length parameter function , As long as the placeholder of the first string parameter is written correctly , Any number of replacement parameters can be added later .

11. #include command

printf function It's not the function we define , It's a function of the system . This function is written in the file stdio.h in , We're going to use printf You must first let the compiler understand printf .

Assume printf The function definition of is written in the file stdio.h in , use #include command , You can stdio.h Copy the code of into our code .

ps：stdio.h There is no definition printf function , But it has printf function Function declaration of . Beginners can understand it as stdio.h Inside , Yes printf Function definition of .