List of articles

• Function declaration and definition
• • Declaration of functions
  • Definition of function
• Simple exercises
• • Write a function to determine whether a number is a prime
  • Write a function to realize binary search
  • Write a function , Every time you call this function , Will be num The value of the increase 1.

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;
}