C language explanation series - understanding of functions (4) declaration and definition of functions, simple exercises

Function declaration and definition

Declaration of functions

The declaration concept and function of function are as follows

1. The declaration of a function is to tell the compiler what a function is called , What are the parameters , What is the return value type . But does it exist , Function declarations do not solve
2. Function declarations usually appear before the use of functions . To satisfy the requirement of declaration before use .
3. The declaration of functions is usually in the header file .

We demonstrate the specific usage of function declaration in the code

int main()
{
    
	int num1 = 20;
	int num2 = 10;
	int max = get_max(num1, num2);
	printf("%d", max);
	return 0;
}

int get_max(int x, int y)
{
    
	return(x > y ? x : y);
}

Or the code that uses the function to solve the maximum value of two numbers , This time we put the custom function after the main function , Let's make the code run

Although we can also get our results, the compiler will have a warning

He said ours “get_max" Undefined , Here we need to use the function declaration to eliminate this warning .

When our custom function is after the main function , When we want to call a custom function , You need to use the declaration of the function , Tell the compiler that there is such a function before calling , This is it. ： Function declarations usually appear before the use of functions , To satisfy the requirement of declaration before use .

Definition of function

As the name suggests, the definition of function is The concrete realization of function , The function realization of the replacement function . When we understand the function declaration and definition , So how should we write the declaration and definition of functions in our daily work ？ In daily work, you must not declare functions like the above code , The declaration and definition of functions must be separate . for example

We can see that there is no definition in this source file get_max() This function , There is no function declaration , But we can still get the results we want by executing the code. Why ？ as a result of , We divide the declaration and definition of functions into two parts , Write in different places

The advantage of writing like this is , Our requirements can be divided into multiple modules for writing , Finally, we can achieve the desired effect by combining them , In this regard, we will share with you in detail in the future study , Today, you only need to know what is the declaration and definition of a function .

Simple exercises

After our sharing , I believe you also have a certain understanding of functions , Today we will lead you to practice several topics , Let's review our previous knowledge .

Write a function to determine whether a number is a prime

We also wrote such a topic in the previous exercise , So how to use function implementation ？

int prime_number(int x)
{
    
	int j = 0;
	for (j = 2; j < x; j++)
	{
    
		if (x % j == 0)
		{
    
			return 0;
		}
	}
	if (j == x)
	{
    
		return 1;
	}
}
int main()
{
    
	int num = 0;
	printf(" Enter a number ：");
	scanf("%d", &num);
	int i = prime_number(num);
	if (1 == i)
	{
    
		printf("%d Prime number ", num);
	}
	else
	{
    
		printf("%d Not primes ",num);
	}
}

Thought analysis ： First, we create a variable to receive the number of pending judgments we enter , Pass this number to the custom function , Judge if the return value is 1 So it's a prime number , The user-defined function implements the judgment part , If it is a prime, it returns 1, Not just back 0.

Write a function to realize binary search

// Use function to realize binary search of shaping ordered array 
int binary_search(int arr[])
{
    
	int left = 0;
	int sz = sizeof(arr) / sizeof(arr[0]);
	int right = sz - 1;
	int k = 0;
	printf(" Please enter the array element to query ：");
	scanf("%d", &k);

	while (left <= right)
	{
    
		int mid = (right - left) / 2 + left;

		if (k > arr[mid])
		{
    
			left = mid + 1;
		}
		else if (k < arr[mid])
		{
    
			right = mid - 1;
		}
		else
		{
    
			return mid;
		}
	}
	if (left > right)
	{
    
		return -1;
	}
}

int main()
{
    
	int arr[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	int num = binary_search(arr);

	if (num == -1)
	{
    
		printf(" Can not find ");
	}
	else
	{
    
		printf(" eureka , Subscript to be %d", num);
	}
	return 0;
}

Thought analysis ： Let's first create an integer ordered array , Pass the array to the custom function to find , If it can be found, the array subscript of the value to be checked is returned , If you can't get it back -1, Select the output through the return value , Give the corresponding prompt to the user . After writing, we execute our code

We found that , When looking for elements that exist in the array , Hint that we can't find , Where is the problem ？ We look for the answer in debugging

We found it was ours right There is an error in the value of , There is clearly 10 Elements ,right It should be equal to 9, Why become 1 What about it ？ The answer is , Array will not pass the whole array when passing parameters , Just pass the address of the first element of the array to find any element in the array , So when passing parameters in an array, you can not only use the above int binary_search(int arr[]) This way of writing , It can also be written as int binary_search(int *arr), The latter is more difficult to understand , But it reveals the essence , The array passing parameter is actually the passing of the first element of the array . So we will change our code ：

// Use function to realize binary search of shaping ordered array 
int binary_search(int arr[], int sz)
{
    
	int left = 0;
	int right = sz - 1;
	int k = 0;
	printf(" Please enter the array element to query ：");
	scanf("%d", &k);

	while (left <= right)
	{
    
		int mid = (right - left) / 2 + left;

		if (k > arr[mid])
		{
    
			left = mid + 1;
		}
		else if (k < arr[mid])
		{
    
			right = mid - 1;
		}
		else
		{
    
			return mid;
		}
	}
	if (left > right)
	{
    
		return -1;
	}
}

int main()
{
    
	int arr[10] = {
     1,2,3,4,5,6,7,8,9,10 };
	int sz = sizeof(arr) / sizeof(arr[0]);
	int num = binary_search(arr, sz);

	if (num == -1)
	{
    
		printf(" Can not find ");
	}
	else
	{
    
		printf(" eureka , Subscript to be %d", num);
	}
	return 0;
}

We will right() The value of this variable is found in the main function , Then pass it to the user-defined function to realize our requirements .

Write a function , Every time you call this function , Will be num The value of the increase 1.

Through the title, we know , You need to define a function to make the value of the number in the main function +1, In this way, a certain connection needs to be established inside and outside the function , We use Address call .

#include<stdio.h>
void add(int* pnum)
{
    
	*pnum = *pnum + 1;
}

int main()
{
    
	int num = 0;
	add(&num);
	printf("%d", num);

	return 0;
}