C language: deep understanding of character functions and string functions (1)

References in this article are from cplusplus Website ：cplusplus.com - The C++ Resources Networkhttp://www.cplusplus.com/

Catalog

One 、strlen Find the string length

Two 、strcpy Copy string

3、 ... and 、strcat Append string

Four 、strcmp Compare strings

5、 ... and 、strncpy Copy string

6、 ... and 、strncat Append string

7、 ... and 、strncmp Compare strings

---

One 、strlen Find the string length

First, I will briefly introduce the information I have found strlen：

Next , Use one strlen To introduce the simulation implementation strlen：

// Calculate string length 
#include <stdio.h>
#include <string.h>
int main()
{
	char arr[] = "abcdef";
	int ret = strlen(arr);
	printf("%d\n", ret);
	return 0;
}

Simulation Implementation strlen The way to do this is 3 Kind of ：

1. Calculator method

#include <stdio.h>
#include <string.h>
#include <assert.h>

int my_strlen(const char* str)
{
	assert(str);
	int count = 0;
	while (*str)
	{
		count++;
		str++;
	}
	return count;
}

int main()
{
	char str[] = "abcdef";
	int ret = my_strlen(str);
	printf("%d\n", ret);
	return 0;
}

2. Cannot create timers for temporary variables （ Recursive method ）

#include <stdio.h>
#include <string.h>
#include <assert.h>

int my_strlen(const char* str)
{
	assert(str);
	if (*str == '\0')
	{
		return 0;
	}
	else
	{
		return 1 + my_strlen(str + 1);
	}
}

int main()
{
	char str[] = "abcdef";
	int ret = my_strlen(str);
	printf("%d\n", ret);
	return 0;
}

3. The pointer - Pointer method （ Subtracting two pointers calculates the number of elements between two pointers ）

#include <stdio.h>
#include <string.h>
#include <assert.h>

int my_strlen(const char* str)
{
	assert(str);
	char* ptr = str;
	while (*ptr != '\0')
	{
		ptr++;
	}
	return ptr - str;
}

int main()
{
	char str[] = "abcdef";
	int ret = my_strlen(str);
	printf("%d\n", ret);
	return 0;
}

Two 、strcpy Copy string

About strcpy Information ：

Be careful ：1. The source string must be in '\0' end

           2. Will put... In the string '\0' Copy to target space

           3. The target space has to be large enough , To ensure that the source string can be stored

           4. The target space has to be variable

Next , Use one strcpy To introduce the simulation implementation strcpy：

//strcpy Copy string 
#include <stdio.h>
#include <string.h>
int main()
{
	char arr1[] = "abcdef";
	char arr2[] = "xxxxxxx";
	char* ret = strcpy(arr2, arr1);
	printf("%s\n", ret);
	return 0;
}

Simulation Implementation strcpy：

#include <stdio.h>
#include <string.h>
#include <assert.h>

char* my_strcpy(char* dest, const char* src)
{
	char* ret = dest;
	assert(dest && src);
	while (*dest++ = *src++)
	{
		;
	}
	return ret;
}

int main()
{
	char arr1[] = "abcdef";
	char arr2[20] = "xxxxxxxxx";
	char* ret = my_strcpy(arr2, arr1);
	printf("%s", ret);
	return 0;
}

The specific simulation implementation method is introduced in detail in the previous article .

3、 ... and 、strcat Append string

About strcat Information ：

Be careful ：1. The source string must be in '\0' end

           2. The target space has to be large enough , It can hold the contents of the source string

           3. The target space must be modifiable

Next , Use one strcat To introduce the simulation implementation strcat： 

//strcat Append string 
#include <stdio.h>
#include <string.h>
int main()
{
	char arr1[10] = "abcd";
	char arr2[] = "ef";
	printf("%s\n", strcat(arr1, arr2));
	return 0;
}

Simulation Implementation strcat：

#include <stdio.h>
#include <string.h>
#include <assert.h>

char* my_strcat(char* dest, const char* src)
{
	char* ret = dest;
	assert(dest && src);
	while (*dest)
	{
		dest++;
	}
	while (*dest++ = *src++)
	{
		;
	}
	return ret;
}

int main()
{
	char arr1[10] = "abcd";
	char arr2[] = "ef";
	printf("%s", my_strcat(arr1, arr2));
	return 0;
}

Simply speaking , Simulation Implementation strcat Is to find the target string '\0' The address of , Then add elements from this address .

Four 、strcmp Compare strings

of strcmp Information ：

Next , Use one strcmp To introduce the simulation implementation strcmp：

//strcmp Compare strings 
#include <stdio.h>
#include <string.h>
int main()
{
	char arr1[] = "abc";
	char arr2[] = "abc";
	int ret = strcmp(arr1, arr2);
	if (ret < 0)
	{
		printf("arr1<arr2");
	}
	else if (ret > 0)
	{
		printf("arr1>arr2");
	}
	else
	{
		printf("arr1=arr2");
	}
	return 0;
}

Simulation Implementation strcmp：

#include <stdio.h>
#include <string.h>
#include <assert.h>

int my_strcmp(const char* arr1, const char* arr2)
{
	assert(arr1 && arr2);
	while (*arr1 == *arr2)
	{
		if (*arr1 == '\0')
		{
			return 0;
		}
		arr1++;
		arr2++;
	}
	return *arr1 - *arr2;
}

int main()
{
	char arr1[] = "abc";
	char arr2[] = "abc";
	int ret = my_strcmp(arr1, arr2);
	if (ret > 0)
	{
		printf("arr1>arr2");
	}
	else if (ret < 0)
	{
		printf("arr1<arr2");
	}
	else
	{
		printf("arr1=arr2");
	}
	return 0;
}

  Simulation Implementation strcmp The idea is ： Compare each character in the string one by one , If equal, compare the next character , Until the comparison '\0' Also equal , Then judge that the two strings are equal ; If unequal characters are found in them , The result of subtracting two characters will be returned , Because subtracting two characters is actually their ASCII Code value subtraction , The return value is just int type .

thus , The above strcpy、strcat、strcmp All three string functions are called string functions with unlimited length , That is, the length of the operation string cannot be specified 、 Scope, etc ; But the next step is to introduce three string functions with limited length ——strncpy、strncat、strncmp, When they operate, they can specify the length of the operation string . The basic situation of these three string functions is similar to the above three , Just one more parameter , This parameter is used to specify the scope , So the following three string functions are only simulated , No more emphatic explanations .

5、 ... and 、strncpy Copy string

Simulation Implementation strncpy：

#include <stdio.h>
#include <string.h>
#include <assert.h>

char* my_strncpy(char* dest, const char* src, size_t num)
{
	assert(dest && src);
	char* ret = dest;
	while (num--)
	{
		*dest = *src;
		if (*dest == '\0')
		{
			return ret;
		}
		dest++;
		src++;
	}
	return ret;
}

int main()
{
	char arr1[10] = { 0 };
	char arr2[] = "abcdef";
	char* ret = strncpy(arr1, arr2, 4);
	printf("%s\n", ret);
	return 0;
}

6、 ... and 、strncat Append string

Simulation Implementation strncat：

#include <stdio.h>
#include <string.h>
#include <assert.h>

char* my_strncat(char* dest, const char* src, size_t num)
{
	char* ret = dest;
	assert(dest && src);
	while (*dest)
	{
		dest++;
	}
	while (num--)
	{
		*dest = *src;
		if (*dest == '\0')
		{
			return ret;
		}
		dest++;
		src++;
	}
	*dest = '\0';
	return ret;
}

int main()
{
	char arr1[10] = { 'a', 'b', 'c', 'd', '\0' };
	char arr2[] = "efgh";
	printf("%s\n", my_strncat(arr1, arr2, 2));
	return 0;
}

7、 ... and 、strncmp Compare strings

Simulation Implementation strncmp：

#include <stdio.h>
#include <string.h>
#include <assert.h>

int my_strncmp(const char* arr1, const char* arr2, size_t num)
{
	assert(arr1 && arr2);
	while (num--)
	{
		if (*arr1 == *arr2)
		{
			if (*arr1 == '\0')
			{
				return 0;
			}
			arr1++;
			arr2++;
		}
		else
		{
			return *arr1 - *arr2;
		}
	}
}

int main()
{
	char arr1[] = "abcee";
	char arr2[] = "abcef";
	int ret = my_strncmp(arr1, arr2, 4);
	if (ret > 0)
	{
		printf("arr1>arr2");
	}
	else if (ret < 0)
	{
		printf("arr1<arr2");
	}
	else
	{
		printf("arr1=arr2");
	}
	return 0;
}

What we should pay attention to here is , In addition to the original parameters, the three string functions with limited length , A parameter will also be added to specify the range , The type of this parameter is size_t type , So it can only be a positive integer , And in bytes .