C language learning day_ 05

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• Learning reference B Stand on the video of teacher Hao bin , The source code in the article can be contacted by private letter if necessary .

function

• It's a tool , Not designed for a problem , It is designed for a large number of problems of the same type
• The number of functions processed is different , But the operation of data is the same
• What is a function
  • logically ： A separate block of code that performs a specific function
  • Physically ： Be able to process the received data , And return the value
    • You can also not accept data , No return value

example ：

/* Example of function */
# include <stdio.h>

int f(void)  //int Represents the return value type of the function ,void Indicates that the function cannot receive data 
{
    
    return 5;  // Returns... To the calling function 5
}

void g(void)  // Before the function name void Indicates that the function has no return value 
{
    
    return 3;  //error, no return value , Using this statement will report an error 
}

int main(void)
{
    
    int j = 10;

    j = f();  // No value is written in parentheses 
    printf("%d\n", j);  // Output f The return value of the function 5

    j = g();  //error, because g Function has no return value , So you can't assign values 

    return 0;
}

Why function is needed

• The operation on the data is the same , You need to operate on multiple data , You need to use functions to reduce the amount of code
• You can avoid writing a lot of repetitive code
• It is conducive to the modularization of the program

example ： existing a-f common 6 A variable , Each variable has a value , It is required to compare a、b,c、d,e、f Size between , And output a larger number .

/* existing a-f common 6 A variable , Each variable has a value , It is required to compare a、b,c、d,e、f Size between , And output a larger number */
# include <stdio.h>

int main(void)
{
    
    int a, b, c, d, e, f;
    a = 1, b = 2, c = 4, d = -3, e = 5, f = 10;  // Comma expression 
    /* Because the whole is not assigned to the variable , So it doesn't affect , You can also use semicolons to separate  a = 1; b = 2; ... */

    if (a > b)
        printf("max = %d\n", a);
    else
        printf("max = %d\n", b);

    if (c > d)
        printf("max = %d\n", c);
    else
        printf("max = %d\n", d);

    if (e > f)
        printf("max = %d\n", e);
    else
        printf("max = %d\n", f);

    return 0;
}

/* Running results */
max = 2
max = 4
max = 10
Press any key to continue

• Statements with the same function , Wrote many times , You can use functions , To call , Save code

Method 1 ：

/* existing a-f common 6 A variable , Each variable has a value , It is required to compare a、b,c、d,e、f Size between , And output a larger number */
# include <stdio.h>

void max(int i, int j)  // Defined function 
{
    
    if (i > j)
        printf("max = %d\n", i);
    else
        printf("max = %d\n", j);
}

int main(void)
{
    
    int a, b, c, d, e, f;
    a = 1, b = 2, c = 4, d = -3, e = 5, f = 10;
   
    max(a, b);  // Call function 
    max(c, d);
    max(e, f);

    return 0;
}

• void Indicates that the function has no return value ,max Is the name of the function ,i、j It's a formal parameter , Formal parameter
• Program execution , from main Start execution , Execute to max(a, b); after , Think this is a call to a function , Start checking if..., is defined max Function of
  • Call the max function , And then a Assign a value to i,b Assign a value to j, Execute function
  • After the function is executed , The address space allocated to the formal parameter will be freed

/* Running results */
max = 2
max = 4
max = 10
Press any key to continue

Method 2 ：

/* existing a-f common 6 A variable , Each variable has a value , It is required to compare a、b,c、d,e、f Size between , And output a larger number */
# include <stdio.h>

int max(int i, int j)  // Defined function 
{
    
    if (i > j)
        return i;  // take i Return to the calling function 
    else
        return j;
}

int main(void)
{
    
    int a, b, c, d, e, f;
    a = 1, b = 2, c = 4, d = -3, e = 5, f = 10;
   
    printf("max = %d\n", max(a, b));  // The main function will i Output 
    printf("max = %d\n", max(c, d));
    printf("max = %d\n", max(e, f));

    return 0;
}

• The output is the same as above
• The difference is that the called function in the first program directly outputs the result , And in the second program , The called function returns the value to the calling function , Processed by the main function

How to define a function

• The essence of function definition is to describe in detail the specific method why a function can realize a specific function
• return expression ; The meaning of
  • Terminate called function , Returns the value of the expression to the calling function
  • If the expression is empty , Then only terminate the function , No value returned
  • break Used to terminate loops and switch

example ：return and break The difference between

/*return and break The difference between */
# include <stdio.h>
void f(void)
{
    
    int i;

    for (i = 0; i < 5; ++i)
    {
    
        printf("Bad!\n");
        break;  // What ends is for loop 
    }

    return;  // What ends is f function 
    printf("Boy!\n");  // No output 
}

int main(void)
{
    
    f();

    return 0;
}

/* Running results */
Bad!
Press any key to continue

• The type of the return value of the function is also the type of the function
  • If the return value type before the function name is the same as that in the function execution body return expression Different types , Then the type of the final function return value is the return value type before the function name
• Format

 Type of function return value   Function name ( Formal parameter requirements of function )
{
    
     The body of the function 
}

example ： Type of function return value

/* Type of function return value */
# include <stdio.h>

int f()
{
    
    return 3.5;  // Because the return type of the function is int, So the final returned value is 3
}

int main(void)
{
    
    double x = 5.5;

    x = f();  // Call function , The value type is subject to the type before the function name 
    printf("%lf\n", x);

    return 0;
}

• function f The return value in is floating point , The type before the function name is an integer
• If the function value type is return The type returned shall prevail , Is returned 3.5. If the function value type is the type before the function name , Then the function value is 3

/* Running results */
3.000000
Press any key to continue

Classification of functions

• Parametric and nonparametric functions
• There are return values and no return values
• Library functions and custom functions
• Value transfer function and address transfer function
• Ordinary functions and main functions （main function ）
  • A program must have and only have one main function
  • The main function can call ordinary functions , Ordinary functions cannot call the main function
  • Ordinary functions can call each other
  • The main function is the entry and exit of the program

Program examples

example ： Judge whether a number is a prime number

/* Judge whether a number is a prime number */
# include <stdio.h>

int main(void)
{
    
    int val;
    int i;

    printf(" Please input the number to be judged ：");
    scanf("%d", &val);

    for (i = 2; i < val; ++i)
    {
    
        if (val % i == 0)
            break;  // End for loop 
    }
    
    if (i == val)
        printf("%d Prime number ！\n", val);
    else
        printf("%d Not primes ！\n", val);

    return 0;
}

/* Running results */
 Please input the number to be judged ：13
13 Prime number ！
---------------------
 Please input the number to be judged ：9
9 Not primes ！
Press any key to continue

Defined function

/* Judge whether a number is a prime number 【 Defined function 】*/
# include <stdio.h>

bool IsPrime(int val)  //bool Type data returns true or false 
{
    
    int i;
    for (i = 2; i < val; ++i)
    {
    
        if (val % i == 0)
            break;
    }

    if (i == val)
        return true;  //return Will terminate the function 
    else
        return false;
}

int main(void)
{
    
    int m;

    printf(" Please input the number to be judged ：");
    scanf("%d", &m);

    if (IsPrime(m))  // Judge whether the return value of the called function is true 
        printf("%d Prime number ！\n", m);
    else
        printf("%d Not primes ！\n", m);

    return 0;
}

• The output is the same as above

Function declaration

• The definition function should be written before calling the function , If it is written after calling the function , You need to write a function declaration
• Function pre declaration
  • Tell compiler , The coming letters represent a function
  • Tell compiler , Several letters represent the formal parameters and return values of the function
  • A function declaration is a statement , You must use a semicolon
  • The library function is declared through `# inculde < The file name of the file where the library function is located .h> To achieve

example ：

/* Function declaration */
# include <stdio.h>

void f(void);  // Function declaration 

int main(void)
{
    
    f();

    return 0;
}

void f(void)
{
    
    printf("Bad Boy!\n"); 
}

/* Running results */
Bad Boy!
Press any key to continue

Formal parameters and actual parameters

• The position and number of formal parameters and arguments must correspond to each other , Types must also be compatible

example ：

/* Formal parameters and actual parameters */
# include <stdio.h>

void f(int i, int j)  // Shape parameter 
{
    
    printf("%d %d\n", i, j);
}

int main(void)
{
    
    f(3, 5);  // Actual parameters 

    return 0;
}

/* Running results */
3 5
Press any key to continue

Reasonably design functions to solve practical problems

example ： The user enters a number , Output 0 All prime numbers between this number

/* The user enters a number , Output 0 All prime numbers between this number */
# include <stdio.h>

int main(void)
{
    
    int i, j;
    int val;

    printf(" Please enter a number ：");
    scanf("%d", &val);

    printf(" Prime numbers have ：");
    for (i = 2; i <= val; ++i)
    {
    
        for (j = 2; j < i; ++j)
        {
    
            if (0 == i % j)
                break;
        }
        if (i == j)
            printf("%d ", i);
    }

    printf("\n");

    return 0;
}

/* Running results */
 Please enter a number ：50
 Prime numbers have ：2 3 5 7 11 13 17 19 23 29 31 37 41 43 47
Press any key to continue

• The reusability of the above code is not high , Not easy to understand , There are limitations

Method 2 ： Defined function

/* Defined function , Outputs the prime number between the specified numbers */
# include <stdio.h>

bool IsPrime(int m)  // The definition function only determines whether a number is a prime number 
{
    
    int i;
    for (i = 2; i < m; ++i)
    {
    
        if (0 == m % i)
            break;
    }
    if (i == m)
        return true;
    else
        return false;
}

int main(void)
{
    
    int i, val;

    printf(" Please enter a number ：");
    scanf("%d", &val);

    printf(" Prime numbers have ：");
    for (i = 2; i <= val; ++i)
    {
    
        if (IsPrime(i))
            printf("%d ", i);
    }

    printf("\n");

    return 0;
}

Optimize ：

/* Defined function , Outputs the prime number between the specified numbers */
# include <stdio.h>

bool IsPrime(int m)  // The definition function only determines whether a number is a prime number , Yes, go back to true, Otherwise return to false
{
    
    int i;
    for (i = 2; i < m; ++i)
    {
    
        if (0 == m % i)
            break;
    }
    if (i == m)
        return true;
    else
        return false;
}

void TraverseVal(int n)  // Define the function to 1-n All prime outputs between 
{
    
    int i;

    printf(" Prime numbers have ：");
    for (i = 2; i <= n; ++i)
    {
    
        if (IsPrime(i))
            printf("%d ", i);
    }
    printf("\n");
}

int main(void)
{
    
    int val;

    printf(" Please enter a number ：");
    scanf("%d", &val);
    TraverseVal(val);  // Call function 

    return 0;
}

• The function is C The basic unit of language , Class is Java,C#,C++ The basic unit of

Common system functions

double sqrt(double x);  // seek x The square root of 
int abs(int x);  // Seek shaping x The absolute value of 
double fabs(double x);  // Finding floating point numbers x The absolute value of 

Scope and storage of variables

• Scope
  • Global variables
  • local variable
    • A variable or function parameter defined inside a function
    • Can only be used within this function
• How variables are stored
  • Static variables
  • Automatic variable
  • Register variables

example ：

/* Scope of variable */
# include <stdio.h>

void g()
{
    
    printf("k = %d\n", k);  //error, because k Variables are defined after the function 
}

k = 3;  // Global variables , After that, all functions can call 

void f(int i)  // local variable , Only this function can call 
{
    
    int j = 5;  // local variable 
    int i = 3;  //error, Because the variable name in the formal parameter conflicts with the variable name in the function 
}

• Inside a function , If the defined local variable name is the same as the global variable name , Local variables mask global variables

【 Article source code reference GitHub】

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 All of the above are original , If unknown or wrong , Please point out .

The author of this article ： Bad

Link to this article ： https://blog.csdn.net/qq_45668124/article/details/115186573

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