Character function and string function (I)

Preface

C The processing of characters and characters in language is very cumbersome , however C The language itself has no string type , Strings are usually placed in Constant string Medium or A character array in .
String constant For string functions that do not modify it

1、 Function introduction

1.1 strlen（ Find the string length ）

size_t strlen(const char * str);

• String to '\0' As an end sign ,strlen Function returns in a string '\0' The number of characters that appear before （ It doesn't contain '\0'）.
• The string that the parameter only wants must be '\0' end .
• Note that the return value of the function is size_t, It's unsigned （ Fallible ）
• strlen Simulation Implementation of function

notes ：（ Error model ）

#include <stdio.h>
int main()
{
    
	const char* str1 = "abcdef";
	const char* str2 = "bbb";
	if (strlen(str2) - strlen(str1) > 0)
    // because strlen The return type of is an unsigned integer , So you can't compare string sizes by subtraction 
    // Correct demonstration ：if(strlen(str2) > strlen(str1)
    // perhaps if((int)strlen(str2) - (int)strlen(str1) > 0)
	{
    
		printf("str2>str1\n");
	}
	else
	{
    
		printf("str1>str2\n");
	}
	return 0;
}

Simulation Implementation strlen function ：

#include <stdio.h>
#include <assert.h>
// Counter method 
size_t my_strlen(const char* str)
{
    
	size_t count = 0;
	assert(str);
	while (*str != '\0')
	{
    
		count++;
		str++;
	}
	return count;
}
int main()
{
    
	char arr[] = "abcdef";
	size_t n = my_strlen(arr);
	printf("%u\n", n);
	return 0;
}

Unlimited length string function ：
strcpy
strcat
strcmp

1.2 strcpy

char * strcpy ( char * destination, const char * source );

• Copies the C string pointed by source into the array pointed by destination, including the terminating null character (and stopping at that point).
• The source string must be in '\0' end .
• In the source string '\0' Copy to target space
• The target space has to be large enough , To ensure that the source string can be stored
• The target space has to be variable

// Example 1
#include <stdio.h>
int main()
{
    
	char name[20] = {
     0 };
	strcpy(name, "zhangsan");// Sure 
	//name = "zhangsan";//err,name The array name is the address , Address is a constant value , Cannot be assigned 
	char arr1[] = "word";
	strcpy(name, arr1);// Sure 
	char arr2[] = {
     'a','b','c' };
	strcpy(name, arr2);//err, The source string does not '\0' This end sign , Will cause cross-border visits 
	printf("%s\n", name);
	return 0;
}

// Example 2
int main()
{
    
	const char* p = "abcdef";
	char arr[] = "abc";
	strcpy(p, arr);//err; The target area cannot be modified 
	return 0;
}

Simulation Implementation strcpy function ：

// The first edition 
#include <assert.h>
char* strcpy(char* dest, const char* src)
{
    
	assert(dest);// Assertion , Judge whether the source function and the objective function are empty , Empty return 0, Don't go down 
	assert(src);
	char* ret = dest;
	while (*src != '\0')
	{
    
		*dest++ = *src++;
	}
	*dest = *src;// hold '\0' also copy To come over 
	return ret;
}

// The second edition （ Optimize First Edition ）
#include <assert.h>
char* strcpy(char* dest, const char* src)
{
    
	assert(dest&&src);// Assertion , Judge whether the source function and the objective function are empty , Empty return 0, Don't go down 
	char* ret = dest;
	while (*dest++ = *src++)// Judgment , Again copy'\0'
	{
    
		;
	}
	return ret;
}

1.3 strcat

char * strcat ( char * destination, const char * source );

• Appends a copy of the source string to the destination string. The terminating null character in destination is overwritten by the first character of source, and a null-character is included at the end of the new string formed by the concatenation of both in destination.
• The source string must be in '\0' end .
• The target space has to be large enough , It can hold the contents of the source string
• The target space must
• The string cannot be appended to itself （ Will be able to ’\0’ Cover ） Fall into a dead cycle

//strcat  String append function example 
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[20] = "hello ";
	strcat(arr, "world");
	printf("%s\n",arr);
	return 0;
}

Simulation Implementation strcat function ：

// Simulation Implementation strcat function 
#include <stdio.h>
#include <assert.h>
char* my_strcat(char* dest, const char* src)
{
    
	assert(dest && src);
	char* ret = dest;
	while (*dest != '\0')
	{
    
		dest++;
	}
	while (*dest++ = *src++)
	{
    
		;
	}
	return ret;
}
int main()
{
    
	char arr[20] = "hello ";
	my_strcat(arr, "world");
	printf("%s\n",arr);
	return 0;
}

1.4 strcmp

int strcmp ( const char * str1, const char * str2 );

• This function starts comparing the first character of each string. If they are equal to each other, it continues with the following pairs until the characters differ or until a terminating null-character is reached.

The standard stipulates ：

• The first string is larger than the second string , Return greater than 0 The number of
• The first string is equal to the second string , Then return to 0
• The first string is less than the second string , Then return less than 0 The number of

// The wrong sample 
#include <stdio.h>
int main()
{
    
	char arr1[20] = "zhangsan";
	char arr2[] = "zhangsanfeng";
	// Compare whether the two strings are equal 
	//arr1 and arr2 It's an array name , Array name is the address of the first element of the array 
	// This compares the address of the first element of two strings , Definitely not equal 
	// So this comparison is wrong 
	if (arr1 > arr2)
	{
    
		printf(">\n");
	}
	else if (arr1 == arr2)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

// The correct sample 
#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr1[20] = "zhangsan";
	char arr2[] = "zhangsanfeng";
	int ret = strcmp(arr1, arr2);
	if (ret > 0)
	{
    
		printf(">\n");
	}
	else if (ret == 0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

Simulation Implementation strcmp function ：

// The first edition 
#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[20] = "zhangsan";
	char arr2[] = "zhangsanfeng";
	int ret = my_strcmp(arr1, arr2);
	if (ret > 0)
	{
    
		printf(">\n");
	}
	else if (ret == 0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}

// The second edition （ Optimize First Edition ）
#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++;
	}
	return *str1 - *str2;// This modification 
}
int main()
{
    
	char arr1[20] = "zhangsan";
	char arr2[] = "zhangsanfeng";
	int ret = my_strcmp(arr1, arr2);
	if (ret > 0)
	{
    
		printf(">\n");
	}
	else if (ret == 0)
	{
    
		printf("==\n");
	}
	else
	{
    
		printf("<\n");
	}
	return 0;
}