Analysis and Simulation of strlen function

️ Function analysis and simulation implementation

Function analysis

The function prototype ：
size_t strlen (const char* str)

The header file ： <string.h>

effect （function）：Get string length

Translation translation ： Get the string at \0 The number of characters before .

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Parameters ：const char* str

Translation translation ： Receive a character pointer .

give an example ：

int main()
{
     
    char str1[] = "abcdef";  // Store the string in the character array 
    char* str = "123456";    //str Is a pointer variable , The address of the first element of the string 
    int len1 = strlen(str1); //  The address of the first element of the array name , Pass it on to strlen
    int len2 = strlen(str);  // take str Pass to strlen
    return 0;
}

Return type ：size_t（ Unsigned integer ）

Translation translation ： Returns a greater than or equal to 0 The positive integer .

Three methods are simulated strlen

1、 Iterative method implementation strlen

#include<assert.h>
size_t my_strlen(const char* str)
{
     
    assert(str);  // Because it needs to be right str To dereference , prevent str by NULL Cause an error 
    int count = 0;
    while(*str++)  //'\0' Of ASCLL The code value is 0, Can be used as a loop end condition 
    {
    
        count++;
    }
    return count;
}

2、 The pointer — The pointer

The pointer — The pointer ： The absolute value of subtracting two pointers is the number of elements between pointers , Premise is ： Pointers to the same space can be subtracted

#include<assert.h>
size_t my_strlen(const char* str)
{
    
    assert(str);
    char* start = str;
    while(*str != '\0')   
    {
    
        str++; // Make str Found in the \0 Previous address 
    }
    return str - start; // Pointers to the same space are subtracted , Return the number of elements between them 
}

3、 Recursive method

#include<assert.h>
size_t my_strlen(const char* str)
{
    
    assert(str);
    if(*str != '\0')
    {
    
        return 1 + my_strlen(++str); // When str The content pointed to is not '\0' When the , Recursion ,str Point to the next content 
    }
    else
    {
    
        return 0; 
    }
}

matters needing attention

1、 Understanding of return parameters

give an example ：

Analyze the following code ：

answer ：

strlen(arr1) The result must be 3,strlen（arr2） The result must be 4, How can it print out "arr1 > arr2" As a result ？

because strlen The return type of the function is size_t The unsigned integer of , Then the result of subtracting two unsigned integers must also be unsigned integers , That is, the result must be greater than or equal to 0.

So it's using strlen When the result returned by the function compares the length of two strings , Try to use Relational operator Compare .

2、strlen Pay attention to

①：strlen The function calculates \0 The number of characters before , If there is \0, The result of the calculation will end in advance

②：strlen The parameter required by the function is an address , Cannot be constant , It can't be a wild pointer .