C程序如何反转一个数字需要解释

Question

While searching for a C program on how to reverse a string, i came across the below program. I am already familiar with a program where we take the length of the string and then minus each character and find the reverse. But this a different program. So can someone please tell me how this code works? Help will be gratefully accepted. compiler used is Borland Turbo c.

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

void main() {
    char str[50];
    char rev[50];
    int i, j, n;
    clrscr();
    printf("enter the string to be reversed:");
    scanf("%s", &str);
    for (i = 0; str[i] != 0; i++) {
        n = i - 1;
    }
    for (j = 0; j <= i - 1; j++) {
        rev[j] = str[i];
        n--;
    }
    printf("the reverse of the string is:%s", rev);
    getch();
}

Answer 1

Your code doesn't work as it is supposed to.Have you tried it ?

Consider this approach to reverse a string :

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

int reverse(char *Str);
void swap(char *x, char *y);

int main(int argc, char *argv[]) {
    char Str[255];
    printf("enter the string to be reversed : ");
    scanf("%s", Str);
    reverse(Str);
    printf("the reverse of the string is : %s\n", Str);
}

int reverse(char *Str) {
    size_t len = strlen(Str);
    size_t n = len / 2;
    char *begin = Str;
    char *end = (Str + len) - 1;

    while (n > 0) {
        swap(begin, end);
        begin++;
        end--;
        n--;
    }
    return 0;
}

void swap(char *x, char *y) {
    char tmp;
    tmp = *x;
    *x = *y;
    *y = tmp;
}

Answer 2

In C, strings are NUL-terminated meaning that it has '\\0' at the end signifying the end of the string. The code you've posted currently has two issues:

1. This:

    scanf("%s", &str);

   should be

   scanf("%s", str);

   as %s expects a char* , not a char(*)[50] .

2. This:

    rev[j] = str[i];

   should be

   rev[j] = str[n];

3. rev should be NUL-terminated before printing. Add

   rev[j] = '\\0';

   just before

   printf("the reverse of the string is:%s", rev);

   to avoid Undefined Behavior.