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Dec 28th, 2004

Reversing a string?

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Hello ladies and gents,

I'm trying to reverse a string when one is entered with cin.getline, so when typing for example: testing

My output should become: gnitset

Ive written this piece of code and it works but, because the string has a certain length and not the whole length of the string is neccesarly being used the part wich is not used is also being shown in my output?

How do I get rid of this?

C++ Syntax (Toggle Plain Text)
int main()
{
	char tekst[20];
 
	cout<< "Put in a line of text! ";
 
	cin.getline(tekst, 20);
 
	cout<< "Reverse the string now!" <<endl;
 
	for (int i=0; i<sizeof tekst; i++)
		cout<< tekst [sizeof tekst-i-1];
 
	cout<<endl;
	return 0;
}
int main()
{
	char tekst[20];

	cout<< "Put in a line of text! ";

	cin.getline(tekst, 20);

	cout<< "Reverse the string now!" <<endl;

	for (int i=0; i<sizeof tekst; i++)
		cout<< tekst [sizeof tekst-i-1];
 
	cout<<endl;
	return 0;
}

Allready tried a search on this topic, came up with four possibilities and none really was sufficient for my problem, so thanks for the help in advance

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JoBe

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Dec 28th, 2004

Re: Reversing a string?

I copied this off of www.csharphelp.com

C++ Syntax (Toggle Plain Text)
int Reverse(char* str)
{
 
if (NULL==str)return -1; //no string
int l=strlen(str)-1; //get the string length
if (1==l)return 1;
 
for(int x=0;x < l;x++,l--)
{
str[x]^=str[l];  //triple XOR Trick
str[l]^=str[x];  //for not using a temp
str[x]^=str[l];
}
 
return 0;
 
}
int Reverse(char* str)
{

if (NULL==str)return -1; //no string
int l=strlen(str)-1; //get the string length
if (1==l)return 1;

for(int x=0;x < l;x++,l--)
{
str[x]^=str[l];  //triple XOR Trick
str[l]^=str[x];  //for not using a temp
str[x]^=str[l];
}

return 0;

}

Also, check out the following:

http://www.daniweb.com/code/snippet117.html

cscgal

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Dec 28th, 2004

Re: Reversing a string?

Don't use the physical size of the string as your limit, use the logical size. The physical size is the size of the array while the logical size is determined by where the terminating null character is:

C++ Syntax (Toggle Plain Text)
#include <algorithm>
#include <cstring>
#include <iostream>
 
using namespace std;
 
void sreverse(char *s)
{
  size_t begin = 0;
  size_t end = strlen(s) - 1;
 
  while (begin < end)
    swap(s[begin++], s[end--]);
}
 
int main()
{
  char s[20];
 
  cout<<"Enter a string: ";
  if (cin.getline(s, sizeof s)) {
    sreverse(s);
    cout<< s <<endl;
  }
}
#include <algorithm>
#include <cstring>
#include <iostream>

using namespace std;

void sreverse(char *s)
{
  size_t begin = 0;
  size_t end = strlen(s) - 1;

  while (begin < end)
    swap(s[begin++], s[end--]);
}

int main()
{
  char s[20];

  cout<<"Enter a string: ";
  if (cin.getline(s, sizeof s)) {
    sreverse(s);
    cout<< s <<endl;
  }
}

Or alternatively, straight printing as you were doing:

C++ Syntax (Toggle Plain Text)
#include <cstring>
#include <iostream>
 
using namespace std;
 
int main()
{
  char s[20];
 
  cout<<"Enter a string: ";
  if (cin.getline(s, sizeof s)) {
    for (size_t i = strlen(s) - 1; i != 0; i--)
      cout<< s[i];
    cout<< s[0] <<endl;
  }
}
#include <cstring>
#include <iostream>

using namespace std;

int main()
{
  char s[20];

  cout<<"Enter a string: ";
  if (cin.getline(s, sizeof s)) {
    for (size_t i = strlen(s) - 1; i != 0; i--)
      cout<< s[i];
    cout<< s[0] <<endl;
  }
}

Narue

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Dec 28th, 2004

Re: Reversing a string?

Narue, I see in your example you use #include <algorithm>
I never heard of this library? I did a google search for algorithm.h and can't seem to find a listing of functions?

cscgal

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Re: Reversing a string?

>I never heard of this library?
It's a handy header. Among the functions that it gives you is swap, which saves everyone from that XOR abomination that your code displayed. *shudder* At least it wasn't undefined behavior the way you wrote it. Most people try to palm off the one-liner version:

C++ Syntax (Toggle Plain Text)

a ^= b ^= a ^= b;

Narue

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Dec 28th, 2004

Re: Reversing a string?

Thank you very much ladies

Narue, ever tought of becoming a teacher, I'm certain you'd be great at it

JoBe

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Dec 28th, 2004

Re: Reversing a string?

I was actually thinking that it probably did cover the swap function() which is why I did a google search for it. When I couldn't come up with a list of functions it included, I figured I'd just ask you directly. I'm going to recede back into my PHP world just about now

cscgal

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Re: Reversing a string?

>Narue, ever tought of becoming a teacher
Yes, but if it's anything like the "tutoring" I do with the programmers working under me, I would be fired in an instant for verbal abuse.

It's not that I'm really that bad, it's just that people are too touchy these days.

Narue

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Re: Reversing a string?

ROFLMAO, oh those employees are so touchy these days :mrgreen:

Before I forget, thanks for the links Dani

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