Formal parameters, arguments, main function parameters, arrays or pointers as function parameters of the knowledge in every corner of C language

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Definition and relation of formal parameter and argument

Formal parameters ：

The actual parameter ：

Relationship between formal parameters and arguments ：

Array as function parameter

A one-dimensional array is used as the shape of a function to participate in the arguments

A one-dimensional array is used as the shape of a function to participate in the arguments

Array as function parameter （ In the form of a pointer ）

Pointer as function parameter

main Function parameter problem

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Definition and relation of formal parameter and argument

Formal parameters ：

When defining a function , Specified parameters . A parameter that appears in a function definition can be seen as a placeholder , It has no data , You can only wait until the function is called to receive the data passed in , So called Formal parameters , abbreviation Shape parameter .

Formal parameter variables are only available in functions Memory is allocated only when called , After call , Free memory now , So formal parameter variables are only valid inside the function , Cannot use... Outside a function .

The actual parameter ：

When you call a function , Specified parameters . The parameters given when the function is called contain real data , Will be used by the code inside the function , So called The actual parameter , abbreviation Actual parameters .

Arguments can be constant 、 Variable 、 expression 、 Functions, etc , No matter what type of data the argument is , When you make a function call , Both of them There must be a definite value , In order to pass these values to the parameter , So we should use assignment in advance 、 Input and so on so that the argument gets a definite value .
 

Relationship between formal parameters and arguments ：

The function of formal parameters and arguments is to pass data , When a function call occurs , The value of the argument is passed to the formal parameter .

Array as function parameter

A one-dimensional array is used as the shape of a function to participate in the arguments

int a[10];
 
void function(int a[],int n)  // Formal parameters 
 
function(a,10)  // The actual parameter 

A one-dimensional array is used as the shape of a function to participate in the arguments

int a[5][10];
 
void function(int a[][10],int n)  // Formal parameters 
 
function(a,5)  // The actual parameter 

Array as function parameter （ In the form of a pointer ）

Some students will have questions , Didn't the above just say that arrays are used as function parameters , Why is there another one in the form of a pointer . Don't worry , We all know that an array is also a pointer （ That's the address ）, So arrays can also be represented in the form of pointers , I put this part here , It is just used as an array to buffer function parameters and a pointer to buffer function parameters

Let's directly use an example to compare the difference between the two forms when arrays are used as function parameters , To find the maximum value in a one-dimensional array

/*
find_max: Take the form of an array as a parameter to find the maximum value of a one-dimensional array 
    @a[]: One dimensional array that needs to be maximized 
    @n： Number of elements of one-dimensional array 
     Return value 
        max： The maximum value of one-dimensional array 
 */
int find_max(int a[],int n)
{
    int i;
	int max = a[0];
	for(i = 0; i < n; i++)
	{
		if(a[i] > max)
		{
			max = a[i];
		}
	}
	return max;
}

/*
find_max: Array takes the form of pointer as a parameter to find the maximum value of one-dimensional array 
    @a[]: One dimensional array that needs to be maximized 
    @n： Number of elements of one-dimensional array 
     Return value 
        max： The maximum value of one-dimensional array 
 */
int find_max(int *b,int n)
{
	int i;
	int max = *b; //*b = *&b[0]= b[0]
	for(i = 0; i < n; i++)
	{
		if(*(b+i) > max)
	    {
		max = *(b+i);
	    }
	}
	return max;
}

Add some difficulty , It is also to make everyone better understand , Let's input and output a two-dimensional array with an array as a pointer

void _3()
{
	int a[3][3];
	for(int i = 0;i<3;i++)
	{
		for(int j = 0;j < 3;j++)
		{
			scanf("%d",(*(a+i)+j));        // Input of two-dimensional array , Equivalent to scanf("%d",a[i][j]);
		}
	}
	for(int i = 0;i<3;i++)
	{
		for(int j = 0;j < 3;j++)
		{
			printf("%d  ",*(*(a+i)+j));    // Output of two-dimensional array , Equivalent to printf("%d ",a[i][j]); 
		}
		printf("\n");
	}
	printf("\n");
}

Pointer as function parameter

/*
_17: Use pointers as arguments , Exchange two values 
 */
void _17(int *p,int *q)
{
	int t;
	t = *p;
	*p = *q;
	*q = t;
 } 

int main()
{
    int a = 0,b = 1;
    _17(&a,&b);
    printf("a = %d, b = %d\n",a,b);
}

void func(int *p)
{
	*p = 250;
}

int main()
{
	int a;
	func(&a);//p = &a; 
    //typeof(p): int *
    //typeof(&a): typeof(a)* int *
	printf("%d\n",a);
}

main Function parameter problem

Talking about main function , We must all be able to play quickly “int main(){}" Such sentences , however main Function actually has three optional parameters , as follows

int main(int argc, char *argv[], char *envp[]);

argc: It's a int type , Represents the number of parameters on the command line
    argv: char*[], Is an array of string pointers .
        argv[0]: Save the address of the first parameter string of the program
        argv[1]: Save the address of the second parameter string of the program
        ....
        argv[argc] by NULL , Indicates the end of the parameter .

    char* envp[], It's also an array of strings , It is mainly to save the variable string in the user environment , With NULL end .
     according to C The rules of language ,argv[0] The value of is the name of the program that started the program .
        eq：./a.out    ./hello  
         So agrc The value of is at least one , if agrc The value of is 1, It indicates that there is no command line parameter after the program name .

Let's try to input the command line parameters on the terminal , Then print it out at the terminal . Also test the third parameter

int main(int argc,char *argv[])    // Print the command input by the terminal 
{
	int i = 0;
	while(argv[i++])
	{
		printf("%s ",argv[i]);
	}
	printf("\n");
	i = 0;
	while(envp[i++])
    {
        printf("%s\n", envp[i]);
    }
    return 0;
}

Excellent article in the past ：

C The data types of the knowledge in the corners of the language foundation _ Listen to music with bars _0 The blog of -CSDN Blog

C The pointer of knowledge in the corners of language _ Listen to music with bars _0 The blog of -CSDN Blog

C Arrays and functions of knowledge in every corner of the language _ Listen to music with bars _0 The blog of -CSDN Blog

C The pointer of knowledge in the corners of language _ Listen to music with bars _0 The blog of -CSDN Blog