reverse a string using pointers - c++

Question

I have to admit, i have no idea how to use pointers, but I tried non the less. the problem with my program is that it shows the string in reverse, except for what was the first letter being missing and the entire string is moved one space forward with the first element being blank.
for example it show " olle" when typing "hello".
#include <iostream>
#include <string>
using namespace std;
string reverse(string word);
int main()
{
char Cstring[50];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(string word)
{
char *front;
char *rear;
for (int i=0;i< (word.length()/2);i++)
{
front[0]=word[i];
rear[0]=word[word.length()-i];
word[i]=*rear;
word[word.length()-i]=*front;
}
return word;
}
The new code works perfectly. changed the strings to cstrings. the question technicaly asked for cstrings but i find strings easier so i work with strings then make the necesary changes to make it c string. figured out ho to initialize the rear and front as well.
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}

Answer 1

char *front;
char *rear;
then later
front[0]=word[i];
rear[0]=word[strlen(word)-1-i];
Not good. Dereferencing uninitialized pointers invokes undefined behavior.
Apart from that, your code is overly complicated, it calls strlen() during each iteration (and even multiple times), which is superfluous, and the swap logic is also unnecessarily complex. Try using two pointers instead and your code will be much cleaner:
void rev_string(char *str)
{
char *p = str, *s = str + strlen(str) - 1;
while (p < s) {
char tmp = *p;
*p++ = *s;
*s-- = tmp;
}
}
The thing is, however, that in C++ there's rarely a good reason for using raw pointers. How about using std::reverse() instead?
string s = "foobar";
std::reverse(s.begin(), s.end());

Answer 2

inline void swap(char* a, char* b)
{
char tmp = *a;
*a = *b;
*b = tmp;
}
inline void reverse_string(char* pstart, char* pend)
{
while(pstart < pend)
{
swap(pstart++, pend--);
}
}
int main()
{
char pstring[] = "asfasd Lucy Beverman";
auto pstart = std::begin(pstring);
auto pend = std::end(pstring);
pend -= 2; // end points 1 past the null character, so have to go back 2
std::cout << pstring << std::endl;
reverse_string(pstart, pend);
std::cout << pstring << std::endl;
return 0;
}

Answer 3

you can also do it like this:
#include <iostream>
#include <cstring>
using namespace std;
string reverse(char word[20]);
int main()
{
char Cstring[20];
cout<<"enter a word: ";
cin>>Cstring;
string results = reverse(Cstring);
cout <<results;
}
string reverse(char word[20])
{
char a='a';
char b='b';
char *front=&a;
char *rear=&b;
for (int i=0;i< (strlen(word)/2);i++)
{
*front=word[i];
*rear=word[strlen(word)-1-i];
word[i]=*rear;
word[strlen(word)-1-i]=*front;
}
return word;
}
it successfully works on my system ,i.e. on emacs+gcc on windows 7

Answer 4

Taken from C How To Program Deitel & Deitel 8th edition:
void reverse(const char * const sPtr)
{
if (sPtr[0] == '\0')
return;
else
reverse(&sPtr[1]);
putchar(sPtr[0]);
}