When array is used as a function parameter, it is better to use the array size as a function parameter

1. A good habit ： When an array is used as a function parameter , It is better to take the array size as an argument of the function .

2. If the array size is uncertain , Plus maybe the code is not written carefully , May cause array overflow ( An array ), However, the compiler does not report errors , Here's an example , This explains why it is best to specify the array length as a parameter .

This is a function that sums the array elements and calculates the average value

#include<stdio.h>
int getAvgScore(int score[])
{
	int *p;
	int num = 0;
	int sum = 0;
	p = score;
	//while ((*p) != -1)
	for (int i = 0; i < 5; i++)// There is a problem with the code written here , It has been retrieved sscore[4], But in fact, the array has only three elements , There is an array overflow problem , But the compiler does not report errors ！
		{
		sum += *p;
		num++;
		p++;
	}
	return sum / num;
}

int main()
{
	int score[] = { 10, 10, 10};// here -1 As the end of the array 
	int a = 0;
	a = getAvgScore(score);
	printf("a=[%d]\n", a);

	return 0;
}

This result is obviously incorrect , The reason is that the size of the input parameter group is not determined , There is an array overflow problem , although score[3] The following value is no longer an array element , But it is still calculated into the program . When this kind of mistake occurs , The program can still run normally without error , But the result is wrong , This may take a lot of time to find mistakes .

therefore , To avoid this , You can use the following two methods ：

1. Pass the length of the array along with the size of the array as a parameter to the function , Make a clear statement . So , Make the following changes to the program （ Take the length of the array as one of the parameters ）：

#include<stdio.h>
int getAvgScore(int score[],int size)// Be careful 
{
	int *p;
	int num = 0;
	int sum = 0;
	p = score;
	//while ((*p) != -1)
	for (int i = 0; i < size; i++)// Be careful 
	{
		sum += *p;
		num++;
		p++;
	}
	return sum / num;
}

int main()
{
	int score[3] = { 10, 10, 10 };// here -1 As the end of the array 
	int a = 0;
	int size = sizeof(score) / sizeof(int);
	a = getAvgScore(score,size);
	printf("a=[%d]\n", a);

	return 0;
}

Results the correct , by 10.

2. If it is inconvenient to add the length of the array in the function parameter list , You can set the flag of the end of the array in the array element .（ For character arrays, it is '\0',int Type array common -1 As an end sign ）

#include<stdio.h>
int getAvgScore(int score[])// Be careful 
{
	int *p;
	int num = 0;
	int sum = 0;
	p = score;
	while (*p!=-1)// Cycle termination conditions 
	
	{
		sum += *p;
		num++;
		p++;
	}
	return sum / num;
}

int main()
{
	int score[] = { 10, 10, 10,-1 };// here -1 As the end of the array 
	int a = 0;
//	int size = sizeof(score) / sizeof(int);
	a = getAvgScore(score);
	printf("a=[%d]\n", a);

	return 0;
}

  The results are still correct .

in general , The purpose is to keep a good habit , Try to write code according to the specification , This is not to say that these are inevitable , But according to the specification , The procedure can be more rigorous .

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