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This article introduces the relevant knowledge of "what are the C language functions and how to define and use them". In the operation of actual cases, many people will encounter such a dilemma. Next, let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

1. What is the function?

Definition of function in Wikipedia: subroutine

In computer science, a subroutine (English: Subroutine, procedure, function, routine, method,subprogram, callable unit) is a part of code in a large program that consists of one or more statement blocks. It is responsible for accomplishing a specific task and is relatively independent compared with other codes.

There are generally input parameters and return values that provide encapsulation of the process and hiding of details. This code is usually integrated into a software library.

2. Classification of functions in C language

Library function

Custom function

2.1 Library function

When learning C programming, you always want to print the result to the screen after a code has been written. A function is used to print the information to the screen (printf) in a certain format.

In the process of programming, we will frequently do some string copying work (strcpy)

In programming, we also calculate, always calculate the k power of n such operations (pow)

The above function can be called without having to write it yourself. In order to support portability and improve the efficiency of programs, a series of similar library functions are provided in the basic library of C language, which is convenient for programmers to develop software.

The library functions commonly used in C language are:

IO function

String operation function

Character operation function

Memory operation function

Time / date function

Mathematical function

Other library functions

Using library functions, you must include the header file corresponding to # include.

2.1.1 how to learn to use library functions

The official website of the recommended query tool:

MSDN (Microsoft Developer Network)

Www.cplusplus.com

2.1.2 Custom function

Custom functions, like library functions, have function names, return value types, and function parameters. These are all designed by ourselves, the composition of the function:

Ret_type fun_name (para1, *) {statement;// statement item} / / ret_type return type / / fun_name function name / / para1 function parameter / / example: writing a function can find the maximum value in two integers / / the design of the get_max function int get_max (int x, int y) {return (x > y)? (X): (y);} int main () {int num1 = 10 Int num2 = 20; int max = get_max (num1, num2); printf ("max =% d\ n", max); return 0;} 3, actual parameters of the function (arguments)

The actual parameters passed to the function are called arguments.

Arguments can be: constants, variables, expressions, functions, etc.

No matter what type of quantity the argument is, when making a function call, they must have certain values in order to pass these values to the parameter.

3.2 formal parameters (formal parameters)

Formal parameters refer to the variables in parentheses after the function name, because formal parameters are instantiated only when the function is called (allocating memory units), so they are called formal parameters.

Formal parameters are automatically destroyed when the function call is complete. Therefore, formal parameters are only valid in functions.

/ / example 2: writing a function can exchange the contents of two integer variables into a function, but cannot complete the task int exchange1 (int x, int y) {/ / when the parameter is passed to the parameter, the parameter is a temporary copy of the parameter, / / a modification of the parameter will not affect the argument int temp = x; x = y; y = temp } / / the correct version of int exchange2 (int* pa, int* pb) / / defines the pointer, receiving address {int temp = * pa; * pa = * pb; * pb = temp;} int main () {int a = 3; int b = 5; exchange1 (a, b); / / passing parameter is the value printf ("exchange1::a =% d b =% d\ n", a, b) / / exchange2 (& a, & b) before exchange; / / pass parameter is address printf ("exchange2::a =% d b =% d\ n", a, b); / / after exchange / / incoming address, custom parameter and argument are more closely related, and can change the value stored in the address / / at this time, the address of the parameter is the same return 0 as the address of the argument;}

In the above code, the parameter xquarter ypjpjpb in the exchange1 and exchange2 functions are formal arguments. A, b passed to exchange1 in the main function and & a, & b passed to the exchange2 function are actual parameters.

The running results are shown below. Exchange1 does not play the role of exchanging values as expected, and exchange2 can.

By monitoring variables, you find that:

Address where A5 value is 5&a0x113f8e4 value an address x5 parameter x value is 5&x0x113f800 parameter x address pa0x113f8e4 parameter pa value is an address

The variable an opens up space in memory at the address 0x113f8e4

The function exchange1 passes the argument a to the parameter x, and the value of the parameter x is also 5, but at this time, the parameter x opens up another space in memory, with the address 0x113f800, and the addresses of the parameter and the argument are different.

The value that the function exchange2 passes to the parameter pa,pa is the address of the variable a, 0x113f8e4, which holds the value of the variable an of 5.

Here you can see that when the exchange1 function is called, x, y has its own space and has exactly the same content as the argument. So we can simply think:

After the parameter is instantiated, it is actually equivalent to a temporary copy of the parameter. The values of parameters x and y are exchanged, but the values of parameters an and b are not affected. In general, we only use the numerical value, and use the shape parameter to transfer the value, which shows that the relationship between the shape parameter and the actual parameter is superficial and is limited to the copy of the surface value.

If you need to operate on the actual parameter value of the main function, such as exchange, the parameter needs to pass the address, which is more powerful. Passing the address shows that the relationship between the formal parameter and the actual parameter is further, and the value of the actual parameter can be modified directly through the address.

4. Function call 4.1 pass value call

The formal parameters and arguments of the function occupy different memory blocks, and the modification of the parameters will not affect the parameters.

4.2 address invocation

An addressing call is a way to call a function by passing the memory address of the created variable outside the function to the function parameter.

This way of passing parameters allows the function to establish a real relationship with the variables outside the function, that is, the variables outside the function can be manipulated directly inside the function.

4.3 exercise 4.3.1 determine whether a number is a prime

Write a function to determine whether a number is a prime or not

/ / return 1 means it is prime / / return-1 means it is not prime / / writing 1 2-n-1 trial division int is_sushu (int a) {/ / prime number is not divisible by other numbers except 1 and itself (int I = 2; I < Amur 1) 2-n-1 +) {/ / try dividing if (a%i==0) {/ / if there is divisible, directly return-1, jump out of the following cycle return-1 / / indicates that it is not a prime,}} / / break exit loop will be adjusted to this point / / when return returns, it will directly exit this function and return to the main function, with a higher level return 1;} int main () {int a = 0; scanf ("% d", & a); int num = is_sushu (a) If (num==1) {printf ("% d: is prime", a);} else {printf ("% d: not prime", a);} return 0;} / / write 2 2-sqrt (n) trial division, faster # includeint is_sushu (int num) {for (int I = 2; I)

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