From C to capable -- use the pointer as a function parameter to find out whether the string is a palindrome character

Catalog

subject ：

Topic analysis ：

Their thinking ：

Code implementation

code annotation

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subject ：

Enter a string , Judge whether the string is palindrome string , It is required to use pointers as parameters .

Palindrome string refers to ： positive Ergodic and reverse Traverse the same string .

Example 1：

Input ：string = "ABBA"

Output ： Is a palindrome string or True

Example 2：

Input ：string = " Fog locks the mountain "

Output ： Is a palindrome string or True

Example 3：

Input ：string = “Hello word”

Output ： Not palindrome numbers or False

explain ： Positive traversal Hello word != drow olleH

Topic analysis ：

Palindrome string judgment is rarely used in practice , As an exercise, it is common .

Their thinking ：

According to the definition of palindrome number positive Ergodic and reverse Traverse the same string .

Then we can use pointers S Points to the first character of the string , We will point S Call it the leading pointer , Use the pointer E Points to the last character of the string , The pointer E Called tail pointer .

Then we compare whether the characters pointed to by the two pointers are the same , If not, the string is directly output instead of palindrome string , If the same, the first pointer S Move back one position so that it points to the next pointer , Repeat the above steps .

Finally, we can judge whether the string is a palindrome string according to the position of the pointer at the end of the loop , You can also judge whether the string is a palindrome string according to the end stage of the cycle .

 

Code implementation

# include "stdio.h"
# include "string.h"
# define SIZE 100

int main()
{
    void Justhw(char *p1, char *p2);
    char string[SIZE], *pStart, *pEnd;
    int n;

    printf("enter string(len<=%d):", SIZE);
    scanf("%s", string);
    n = strlen(string);

    pStart= &string[0];
    pEnd = &string[n-1];
    Justhw(pStart, pEnd);

    return 0;
}

void Justhw(char *p1, char *p2)
{
    while (p1 < p2) {
        if (*p1 != *p2) {
            printf(" This is not a palindrome string \n");// A
            return;
        }
        p1++;
        p2--;

    }
    // C

}

code annotation

# include "stdio.h"
# include "string.h"  //  Use strlen function , Import its header file 
# define SIZE 100

int main()
{
    void Justhw(char *p1, char *p2);  //  Initialization function 
    char string[SIZE], *pStart, *pEnd;  //  Initialization pointer 
    int n;  // n Represents string length 

    printf("enter string(len<=%d):", SIZE);
    scanf("%s", string);  //  Input string 
    n = strlen(string);   // strlen Function to get the length of the string 
//    if ((n&1) == 1) {
//        printf(" This is not a palindrome string \n");
//        return 0;
//    }

    pStart= &string[0];   //  Make the first pointer pStart Point to the first element of the string 
    pEnd = &string[n-1];  //  Make the tail pointer pEnd Point to the element at the end of the string 
    Justhw(pStart, pEnd);   //  Call the function to determine whether the palindrome 

    return 0;
}

void Justhw(char *p1, char *p2)
{
    while (p1 < p2) {  //  The cycle stop condition is ： Tail pointer  =>  Head pointer ,
        //  Because when the tail pointer > The first pointer indicates that the two pointers have traversed the string and moved one more step .
        //  The equal sign here is mainly when the palindrome string is an odd number of characters , When two pointers meet, the middle string 
        if (*p1 != *p2) {
            printf(" This is not a palindrome string \n");// A
            return;  //  there return No value returned , We use return Is the purpose of 
            //  In order to stop the program when it is judged that the string is not a palindrome string , Do not execute the following statement 
        }
        p1++;  //  The first pointer moves back 
        p2--;  //  The tail pointer moves forward 

    }
    // C
    printf(" This is the palindrome string \n"); // B

}

If you want to judge whether to palindrome according to the position of the pointer at the end of the loop , take A、B Position statement deletion , stay C Fill in

   if (p1-1 == p2 || p1 == p2)    
        printf(" This is the palindrome string \n");
   else
        printf(" This is not a palindrome string \n");

If palindrome , Then the string length parity is the same

When the string length is even , After the cycle ends, the two pointers move to meet and move one more step , The two pointers are still adjacent , that p1-1 = p2( Equate to p1 = p2+1)

 

When the string length is even , The two pointers just meet the middle character p1 = p2, After the cycle ends, move one more step

 

That's it today , See you next time .

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