Member Avatar for Lensva

Code works, however i'm having problems with reverse string- i cant figure out how to index the elements. for cycle returns errors, what else could i use?
The end goal would be to compare string line with string reverse. If adequate elements match its a palindrome.

#include <iostream>
#include <string>
using namespace std;

int main()
{
    string line;
    cout<< "sentence: \n";
    getline (cin, line);

    cout<< "in reverse: \n";
    string::reverse_iterator reverse;
    for (reverse=line.rbegin(); reverse<line.rend(); reverse++)
    cout<< *reverse;

return 0;
}
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  • Latest Post Latest Post by rohitvipin

Dani AI

Generated

Nice start, . Two quick iterator gotchas tripping you up: (1) use != to stop a reverse-iterator loop (it != line.rend()), not <. (2) Do not step to rend()+1; that is past the reverse range and is undefined to dereference. If you really want random access on the reversed view, you can index from the back via *(line.rbegin() + i) or store a reverse iterator and use rit[i]. Also prefer std::string::const_reverse_iterator when you are only reading. Finally, avoid naming a variable reverse; it shadows the standard algorithm name and makes code harder to read.

is right that you do not need to build a separate reversed string. A two-pointer pass is faster and simpler, and you can easily add common palindrome rules (ignore case and punctuation). Here is a drop-in helper that does exactly that:

#include <string>
#include <cctype>

bool is_palindrome(const std::string& s) {
    std::size_t i = 0, j = s.size();
    if (j == 0) return true;
    --j;
    while (i < j) {
        while (i < j && !std::isalnum(static_cast<unsigned char>(s[i]))) ++i;
        while (i < j && !std::isalnum(static_cast<unsigned char>(s[j]))) --j;
        if (i < j) {
            char a = static_cast<char>(std::tolower(static_cast<unsigned char>(s[i])));
            char b = static_cast<char>(std::tolower(static_cast<unsigned char>(s[j])));
            if (a != b) return false;
            ++i; --j;
        }
    }
    return true;
}

If you still want a one-liner using iterators, ’s idea of comparing the forward range to the reverse range is fine for exact, case-sensitive checks. And while mentioned std::reverse, keep in mind that actually mutates the string; use it on a copy if you go that route. Also note for completeness: C’s strrev mentioned earlier is non-standard and not portable in C++.

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All 7 Replies

Member Avatar for zhelih

Try line.rend()+1;

Member Avatar for Lensva

could you illustrate that on my example? :)

Member Avatar for hammerhead

There is a strrev() function in string.h for char array. I dont know enough to help you with your code but I suggest you to follow a different method for checking palindrome in a string.

Instead of checking a string with a reverse string, make two pointers one at the starting of the string and other at the end. For a string to be a palindrome the starting should match the end, if so then increment the starting pointer and decrease the end pointer. Again both should be equal for the string to be a palindrome. Keep doing so until start equals end.

Member Avatar for dougy83

To check if an iterator has passed over all elements compare it to the end (for iterators) or rend (for reverse iterators).

string::reverse_iterator rit = st.rbegin();
   string::iterator it = st.begin();
   for(; rit != st.rend() && it != st.end(); rit++, it++){
// do comparisons here
commented: thanks for the help +1
Member Avatar for Arpy Giri

for c++ there is an inbuilt function to reverse a string just give
std::reverse(string_name.begin(),string_name.end())
you will get it

commented: was exaclt what im looking +1
Member Avatar for Lensva

Thanks people, specially Arpy Giri for suggesting such an easy way out

Member Avatar for rohitvipin 
#include<iostream>
#include<string>
using namespace std;
void main()
{
	string str;
	cout<<"Enter the string: ";
	cin>>str;
	string::reverse_iterator ri=str.rbegin();	//reverse iterator used to read the string from back to front
	if(equal(str.begin(),str.end(),ri))			//equal compares 2 strings in the provided order
		cout<<str<<" is a Pallindrome!"<<endl;
	else
		cout<<str<<" is not a Pallindrome!"<<endl;
}
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