Definition: Calculate the length of the character before '\0' in the array.

Error-prone: Everyone must be familiar with strlen, but it should be noted that its return value is an unsigned integer (example).

Simulation implementation

1.1 YiShu (example)

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

int main()
{
	if (strlen("abcd") - strlen("abcdef") > 0)
	{
		printf(">\n");
	}
	else
	{
		printf("<=\n");
	}
	return 0;
}

According to the logic of this question, the subtraction of the two strlen functions =-3, the result should be <=. However, srtlen returns an unsigned integer, the subtraction of two unsigned integers is still an unsigned integer, then -3 will be a very large integer, and the program result is >. To modify, directly compare the two strlens, or perform mandatory conversion.

1.2 Simulation implementation

1.2.1 Counter form

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

int my_strlen(char *str)
{
	assert(str!=NULL);
	int count = 0;
	while (*str != '\0')
	{
		count++;
		str++;
	}
	return count;
}

int main()
{
	char arr[]="abcedf";
	printf("%d\n", my_strlen(arr));
	return 0;
}
//结果为6。

1.2.2 Pointer-pointer form

#include<stdio.h>

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

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

//结果为6。

1.2.3 Recursive form

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

int my_strlen(const char* str)
{
	if (*str != NULL)
	{
		return 1 + my_strlen(++str);
	}
	else
	{
		return 0;
	}
}

int main()
{
	char arr[] = "hello,world!";
	printf("%d\n", my_strlen(arr));
	return 0;
}
//结果是 12

2.strcpy

Definition: Copy the contents of the source array to the target array.

Error-prone: The source string must end with '\0', otherwise it will lead to out-of-bounds and program crash (for example).

pay attention! : strcpy will copy '\0' of the source string to the target space, the target space must be large enough, and the target space must be variable.

Simulation implementation

2.1 Easy to use (example)

#include<stdio.h>

int main()
{
	char arr1[20] = { 0 };
	char arr2[] = { 'a','b','c' };
	strcpy(arr1, arr2);
	return 0;
}

As can be seen above, the program crashes. It is because arr2 does not have a defined size, strcpy did not find '\0' after copying the content when copying, causing the program to crash.

2.2 Simulation implementation

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

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

int main()
{
	char arr1[20] = { 0 };
	char arr2[] = "abcdef";
	printf("%s\n",my_strcpy(arr1,arr2));
	return 0;
}
//结果是abcdef

3. broken

Definition: Appends the contents of the source string into the destination string space. Append from target space '\0'.

Error-prone: appending yourself will lead to an infinite loop.

pay attention! : The source and target strings must end with '\0', the target space must be large enough, and the target space must be modifiable.

Simulation implementation

3.1 Simulation implementation

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

char* my_strcat(char* dest, char* src)
{
	assert(dest && src);
	char* ret = dest;
	//让指针指向目标空间的'\0'处
	while (*dest)
	{
		dest++;
	}
	//在将源字符串内容拷贝到目标空间
	while (*dest++ = *src++)
	{
		;
	}
	return ret;
}

int main()
{
	char arr1[20] = "hello";
	char arr2[] = " world";
	printf("%s\n", my_strcat(arr1, arr2));
	return 0;
}
//结果是 hello world

4.strcmp

Definition: Compares the size of characters at corresponding positions in two strings until a difference or '\0' is found. When str1>str2, return the number >0; when str1<str2, return the number <0; when str1=str2, return 0.

Simulation implementation

4.1 Simulation implementation

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

int my_strcmp(const char* str1, const char* str2)
{
	assert(str1 && str2);
	while (*str1 == *str2)
	{
		if (*str1 == '\0')
		{
			return 0;
		}
		str1++;
		str2++;
	}
	if (*str1 > *str2)
	{
		return 1;
	}
	else
	{
		return -1;
	}
}

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

5. Length-limited string length

strncpy，strncat，strncmp

You can specify the length to perform corresponding operations, making the code relatively safe

6.strstr

Definition: Determine whether another string is a substring of the string in a string.

Simulation implementation

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

char* my_strstr(const char* str1, const char* str2)
{
	assert(str1 && str2);
	const char* s1 = str1;
	const char* s2 = str2;
	char* cur = str1;
	while (*cur)
	{
		s1 = cur;
		s2 = str2;
		while (*s1&&*s2&&(*s1 == *s2))
		{
			s1++;
			s2++;
		}
		if (*s2 == '\0')
		{
			return cur;
		}
		cur++;
	}
	return NULL;
}

int main()
{
	char arr1[] = "abcdefgh";
	char arr2[] = "cdef";
	char* ret = my_strstr(arr1, arr2);
	if (ret == NULL)
	{
		printf("找不到子串\n");
	}
	else
	{
		printf("%s\n",ret);
	}
	return 0;
}
//结果是cdefgh

7.strtok

Definition: String cutting, given a character set, cutting the content of the source string.

pay attention! : strtok finds a mark in the string, ends with '\0', and returns a pointer to this mark; strtok will change the manipulated string, it is recommended to copy the content to a new string to ensure the source Strings are not modified. ; When passing a non-NULL pointer, only find the first mark; when passing NULL, search from the saved position backward.

Use of strtok

7.1 Use of strtok

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

int main()
{
	char arr[] = "hello#world*first";
	char sep[] = "#*";
	char buf[20];//创建新的字符串
	char* str = NULL;
	strcpy(buf, arr);

	for (str = strtok(buf, sep); str != NULL; str = strtok(NULL, sep))
	{
		printf("%s\n",str);
	}
	//上面for循环相当于：
	//printf("%s\n",str = strtok(buf, sep));
	//printf("%s\n",str = strtok(NULL, sep));
	//printf("%s\n",str = strtok(NULL, sep));
	//但是这三条语句只对本次arr数组有效，如果arr字符串更长，分割字符集越多就不适用。

	return 0;
}
//结果是：
//hello
//world
//first

8th street error

Definition: Return the error message corresponding to the error code.

pay attention! : Use this function to include the header file #include<errno.h>; introduce the global variable errno (error code).

Use of strerror

8.1 Use of strerror

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

int main()
{
	int* p = (int*)malloc(INT_MAX);//INT_MAX是一个非常大的数。
	if (p == NULL)
	{
		printf("%s\n",strerror(errno));

		//与strerror相同功能，只是不需要调用printf打印，并且可以自定义前缀。
        perror("malloc"); 

		return 1;
	}
	return 0;
}
//结果是
//Not enough space
//malloc: Not enough space

9. Other string functions

Returns true if the argument meets the following conditions

iscntrl : any control character

isspace : blank characters: space ' ' , form feed '\f' , line feed '\n' , carriage return '\r' , tab '\t' or vertical tab '\v'

isdigit : decimal digits 0~9

isxdigit : hexadecimal digits, including all decimal digits, lowercase letters a f , uppercase letters A F

islower : lowercase letters a~z

isupper : uppercase letters A~Z

isalpha : the letter a z or A Z

isalnum : letters or numbers, a z, A Z, 0~9

ispunct : punctuation marks, any graphic characters that are not numbers or letters (printable)

isgraph : any graphic character

isprint : any printable character, including graphic characters and whitespace characters

toupper : Convert characters in a string to uppercase

tolower : convert the characters in the string to lowercase

C language - string function (required collection)

1.strlen

1.1 YiShu (example)

1.2 Simulation implementation

1.2.1 Counter form

1.2.2 Pointer-pointer form

1.2.3 Recursive form

2.strcpy

2.1 Easy to use (example)

2.2 Simulation implementation

3. broken

3.1 Simulation implementation

4.strcmp

4.1 Simulation implementation

5. Length-limited string length

6.strstr

7.strtok

7.1 Use of strtok

8th street error

8.1 Use of strerror

9. Other string functions

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