Palindromes

hey guys i just have a question, if the user of these palindrome programs would put in a bunch of words, how would i make the program output just the palindromes, instead of it stating whether it is a palindrome or not?

also my professor mentioned to me that I should make two functions with the prototypes:

std::string reverse(const std::string& s);
bool is_palindrome(const std::string& s);

and that would make it easier to check whether a string is a palindrome...

basically i have to write out the functions...and then output the palindromes from a given input..im pretty sure i could figure out the bool function, but im not sure about the reverse.. also another problem is, im kind of a noob and im not sure exactly how to include the functions in the main function..can somebody just give me like a outline or a step by step or somethin just to show me the right way to do this? thanks a lot..

Moved from the previously resurrected thread Palindrome emordnilaP.

Quote:

---

Originally Posted by jewelz

im pretty sure i could figure out the bool function, but im not sure about the reverse

---

The underlying idea is the same. What do you have in mind for is_palindrome()?

^thanks..but can u explain that a little more..im kind of lost..

I very quickly edited my post. You might want to read my updated post.

Quote:

---

im not sure about the reverse..

---

O_o

The iterator based design of the C++ standard library containers is unparalleled.

You can, literally, write the 'std::string reverse(const std::string &)' function in a single line.

Look at your choice of references for the iterator based methods and constructors of 'std::string'.

Show us some diligence and you'll get some more help.

Soma

alright so..im required to have a header file containing the prototypes and a pal.cc file which defines the two functions ..and as for the bool function i think this would work..i had somebody look over it and he said it looks alright..wat do u guys think?

Code:

---

bool IsPalindrome(const std::string& s)
{
  int first = 0;
  int last = static_cast<int>(s.size()) - 1;

  while(first < last)
  {
    if(s[first] != s[last])
    return false;
   
    first++;
    last--;
  }

  return true;
}

---

so this code would go into the pal.cc right? im not exactly sure what to include in the header file besides the prototype (do i have to declare some things in the beginning?)
basically after i make the pal.cc i need to create another file.cc which would call the main function im assuming, and that should allow me to take in some input, and print out all the palindromes..

so now i need to figure out the reverse function..

anyone?

What progress have you made so far?

well in my previous post i tried out the bool function, im pretty sure it works, but i also i have some other questions on there regarding the organization of things.. (header file, pal.cc <---includes definitions of functions, and another (filename).cc <----includes the actual main function im assuming, which would allow for me to output all the palindromes..

so i have to work on the reverse function, and can u guys just help me figure out how to organize all this, such as what do i put in the header file, and how do i start off the pal.cc? thanks a lot..

What have you done regarding my statements?

What have you tried with the 'reverse' function?

Soma

would this work for the string reverse?

Code:

---

std::string reverse(const std::string& s);

{

for (i = s.length()-1;  i>=0; i--)
return  (s[i]);

}

---

Quote:

---

Originally Posted by jewelz

would this work for the string reverse?

---

No

s[i] is a char

so you return a last char instead of the string
your loop will not run at all

and if the length is 0 - your function will not have a return at all

no, that will return s[s.length()-1]

you are missing some basic concepts about functions and variables. you should read up on those a bit.

hmm..can somebody help me out with this? i'm really not sure what to do..

the algorithm in your last function has the right idea, but you're missing some more fundamental concepts like return statements and data types.

i should clarify - the for loop in your function has generally the right idea for reversing a string - start at the end and work your way to the beginning. however, that is not to say that is the most efficient or easiest way to go about solving your problem.

if you show me a function that displays how to use std::strings iterators, i'll help connect the dots for you on how to use them to solve your problem.


here, look at this:

http://www.cplusplus.com/reference/string/string/

ah darn it...using the STL reverse algorithm would be the easiest way, but i'm not allowed to use it..owned

the reversing algorithm is trivial. you have that part more-or-less correct, though it can be done more easily

std::string::iterator is a part of std::string; you appear to be allowed to use std::string.

using that site, i came up with this...i really gotta figure this out...sigh..

Code:

---

string s;
string::reverse_iterator rit;
  for ( rit=s.rbegin() ; rit < s.rend(); rit++ )
    return rit;

---

jewelz, you're still plagued by a basic misconception about how functions work.

a function only returns at most ONE value each time it is called. a function with a return type specification of void never returns a value.

a return statement immediately stops execution of the function and returns the variable specified by the argument of the return statement.

so, in your pseudofunction above, the for loop accomplishes nothing, because the function will return rit on the first iteration, which is exactly equivalent to:

Code:

---

return s.rbegin();

---

the basic anatomy of a function is as follows:

Code:

---

// a free function (i.e. a function that is not a member function of a class)
returnType functionName(/*argument list here*/)
{
// function body
return instance_of_returnType; 
}

// a class-member function
returnType className::functionName(/*argument list here*/)
{
// function body
return instance_of_returnType;
}

---

you would use a function like this:

Code:

---

#include <vector>
std::vector<int> countTo(unsigned int number)
{
    std::vector<int>countList;
    for(unsigned int i=0;i<number;i++)
    {
          countList.push_back(i+1);
    }   
      return countList;
}

int main(int argc, char* argv[])
{
      std::vector<int> oneToTen = countTo(10);
}

---

note how the vector is populated over the course of the function execution and then ONE result is returned.

this is so frustrating...so basically i have to return the reversed string right? how would i do that..i cant just "return s;" that makes no sense i think..or should i create another string and then += that to the old string? im sorry im such a noob..but i really appreciate the help..

Quote:

---

Originally Posted by jewelz

or should i create another string and then += that to the old string?

---

SOOoooo close.

you wouldn't += onto the "old string", but you would += elements of the "old string" onto the new string. i think that's what you meant, but in programming these semantics are very important

you could modify the string passed into the function argument, except that int your suggested prototype it is passed by const reference. that means you are not allowed to modify it.

Quote:

---

Originally Posted by jewelz

im sorry im such a noob..but i really appreciate the help..

---

don't sweat it. you read a documentation page. that's a victory in and of itself imo. RTFM is job 1 :)

ok so creating the new string i would just declare:

string s2;

and then im not sure, how do i include this in the for loop?

final try for tonight...what do you guys think?

Code:

---

string s2 = "";
            for (int i = 1; i < s.length; i++)
            {
                s2 = s2 + s[s.length - i];
            }
            s2 = s2 + s[0];
            return s2;

---

If you are allowed to use std::string::reverse_iterator, then from original_string.rbegin() to before original_string.rend(), copy to the new string.

If you must use indices, then copy the original string into a new string, then reverse the old string by swapping the nth character with the nth last character, all the way from the character at position 0 to the middle of the string.

ok guys, so now i have to write a main function that will read words from input and print out all the palindromes, each being on a seperate line..

so first of all my professor said to use the bool palindrome function in terms of the reverse, so i came up with this:

Code:

---

bool Is_Palindrome(const std::string& s)
{

if (s==reverse(s))
return(true);
else
return(false);

}

---

and as for the main function should i do something like:

Code:

---

int main()
{
string s;
cout << "Input some words ";
getline (cin, s);
if (if (Is_Palindrome(s)==true)
cout<< s <<endl;

return 0;

}

---

i tried compiling it, but it gave me some really weird error..and its not even pointing at specific lines..

anyway let me give u guys the format i have for this program maybe it will help:

i have a header file called pal.h :

Code:

---

#ifndef GUARD_pal_h
#define GUARD_pal_h

#include <iostream>

std::string reverse(const std::string& s);
bool is_palindrome(const std::string& s);

#endif

---

a file called pal.cc which includes the definitions of the above functions:

Code:

---

#include "pal.h"
#include <string>

using std::string;

//function declaration
std::string reverse(const std::string& s)
{
    string s2 = "";
    for (int i = 1; i < s.length(); i++)
        {
            s2 = s2 + s[s.length() - i];
        }
    s2 = s2 + s[0];
    return s2;
}

bool Is_Palindrome(const std::string& s)
{

    if (s==reverse(s))
        return(true);
    else
        return(false);

}

---

and as for the main function i posted above i included "pal.h" and "pal.cc" and then the appropriate declarations and usings..what might be the problem?

Code:

---

bool Is_Palindrome(const std::string& s)
{
    return(s==reverse(s));
}

---

this is simpler

Well, your earlier method, the IsPalindrome function on the first page, is a lot better... Why even both reversing it all? I don't understand why your professor should say something like that... As a bad hint or as yet another silly must?

Quote:

---

Originally Posted by EVOEx

Well, your earlier method, the IsPalindrome function on the first page, is a lot better... Why even both reversing it all? I don't understand why your professor should say something like that... As a bad hint or as yet another silly must?

---

yeah i understand what you're saying, but the project description says we need both functions -_-

it's still not compiling properly..how should i fix up my main function?

this is the type of error im getting:

pal.o: In function `reverse(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
pal.cc: (.text+0x5c): multiple definition of `reverse(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
proj3a.o: proj3a.cc: (.text+0x5c): first defined here
pal.o: In function `Is_Palindrome(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
pal.cc: (.text+0x217): multiple definition of `Is_Palindrome(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)'
proj3a.o: proj3a.cc: (.text+0x217): first defined here

What is your current code? It looks like you are defining reverse twice, or there is a conflict with the standard library version.

everything in my last post is my current code...i didn't change anything from that..

Oh, I see the problem now:

Quote:

---

Originally Posted by jewelz

and as for the main function i posted above i included "pal.h" and "pal.cc" and then the appropriate declarations and usings..what might be the problem?

---

You should not be including "pal.cc". You should compile pal.cc separately and link it with the object file produced by compiling main.cc (or whatever source file the main function is in).

Instead of including <iostream> in pal.h, include <string> since you refer to std::string in pal.h.

Quote:

---

Originally Posted by laserlight

Oh, I see the problem now:

You should not be including "pal.cc". You should compile pal.cc separately and link it with the object file produced by compiling main.cc (or whatever source file the main function is in).

Instead of including <iostream> in pal.h, include <string> since you refer to std::string in pal.h.

---

so i dont have to include "pal.cc" anywhere?

Yes.

ok well this is what i came up with for printing the the palindromes:

Code:

---

int main()
{

    string s;
   
    while(cin >> s){
        if(isPalindrome(s)== true)
            cout << s << endl;

    }
return 0;

}

---

now for the last part, the program will take input and output the longest palindrome, along with its length..i came up with this:

Code:

---

int main()
{

    string s;
    vector<string> words;
   
    while(cin >> s) && (isPalindrome(s)== true)
                        {
                            words.push_back(s);
                        }
    string longest = words[0];

    for (int i = 1; i != words.size(); i++) {
        if(words[i].length() > longest.length()) {
            longest = words[i];
        }

        cout <<"Longest Palindrome is " << "\"" << longest << "\""
            << "whose length is " << longest.length() << endl;

   
        return 0;

    }

---

does this seem ok?

you do not need to store all the strings to find the longests

enogh to have 2 strings - longest_so_far and current

um..can u explain that a little more please? my method wouldn't output the longest palindrome?

It would, eventually. But you don't need to store all the words, just the longest (unless you need to keep all the palindromes for other reasons).

ok is this better??

Code:

---

int main()

{
typedef string:: size_type string _size;

string s;
string_size max = s.size();
string longest = s;
   
    while(cin >> s) && (isPalindrome(s)== true) {
        string_size size = s.size();
            if (size > max) {
              max = size;
              longest = s;
        }

}
 
  cout <<"Longest Palindrome is " << "\"" << longest << "\""
            << "whose length is " << max << endl;

   
        return 0;

}

---

you've got erroneous spaces in your typedef.

Code:

---

typedef string::size_type string_size;

---

is everything good now?? =)

that looks good

incidentally,i think it's very odd you weren't encouraged to do something like this:

Code:

---

bool isPalindrome(const std::string& s)
{
    std::string::const_iterator fit = s.begin();
    std::string::const_iterator rit = s.end();
    --rit;
    for(unsigned int i=0;i<s.size()/2;i++,fit++,rit--)
    {
        if(*fit==' ')
            ++fit;
        if(*rit==' ')
            --rit;
        if(*fit != *rit)
        return false;
    }
    return true;
}

---

hmm..getting a compilation error..it's saying:

error: expected identifier before '(' token

indicating the line with while (cin >> s)

Quote:

---

Originally Posted by jewelz

hmm..getting a compilation error..it's saying:

error: expected identifier before '(' token

indicating the line with while (cin >> s)

---

You need to surround the entire condition (both checks and the &&) inside one big set of parentheses.

Ok now on to fix the problem that I have been having since completing the first part of the assignment (output just palindromes)...the compiler is telling me:

pal.h:4: error: 'string' in namespace 'std' does not name a type
pal.h:5: error: 'string' is not a member of 'std'
pal.h:5: error: 's' was not declared in this scope

and its saying that i cant use the isPalindrome function:

error: 'isPalindrome' cannot be used as a function

if u guys need to look at my code for header file and implementation file for two functions, it's on the previous pages..

Quote:

---

Originally Posted by jewelz

Ok now on to fix the problem that I have been having since completing the first part of the assignment (output just palindromes)...the compiler is telling me:

pal.h:4: error: 'string' in namespace 'std' does not name a type
pal.h:5: error: 'string' is not a member of 'std'
pal.h:5: error: 's' was not declared in this scope

and its saying that i cant use the isPalindrome function:

error: 'isPalindrome' cannot be used as a function

if u guys need to look at my code for header file and implementation file for two functions, it's on the previous pages..

---

That tells me you don't have #include <string> in your pal.h file (if that file needs string, you have to include <string> in that file).

yeah but one of my classmates said he didn't need to include that in his pal.h and it ran fine..plus i thought header files are generally not supposed to have them anyway?

Quote:

---

Originally Posted by jewelz

yeah but one of my classmates said he didn't need to include that in his pal.h and it ran fine..plus i thought header files are generally not supposed to have them anyway?

---

Then either (a) he is not using strings (surely he's not using char[] gasp horror) or (b) he's including some other file that in its turn includes string.h.

Header files are most certainly supposed to include all the header files needed for its own content to make sense.

well i included the <string> and compiled it again, this time the only problem is that it's saying that there is an undefined reference to 'isPalindrome(std.....)

i dunno what to make of that error.. pal.h is included but idk whats going on..

And is isPalindrome prototyped in pal.h? More importantly, is it prototyped the way it is defined and the way it is called?

here is my code for the entire project:

pal.h

Code:

---

#ifndef GUARD_PAL_H_
#define GUARD_PAL_H_

#include<string>

std::string reverse(const std::string& s);
bool isPalindrome(const std::string& s);

#endif

---

implementation file - pal.cc

Code:

---

#include "pal.h"
#include <cstdlib>

//function declaration
std::string reverse(const std::string& s)
{
    string s2 = "";
    for (int i = 1; i < s.length(); i++)
        {
            s2 = s2 + s[s.length() - i];
        }
    s2 = s2 + s[0];
    return s2;
}

bool isPalindrome(const std::string& s)
{
    return(s==reverse(s));

}

---

main function:

Code:

---

#include "pal.h"
#include <string>
#include <cstdlib>
#include <iostream>

using std::endl;
using std::string;
using std::cout;
using std::cin;


int main()

{
    typedef string::size_type string_size;
   
    string s;
    string_size max = s.size();
    string longest = s;
   
    while((cin >> s) && (isPalindrome(s)== true)) {
        string_size size = s.size();
        if (size > max) {
            max = size;
            longest = s;
        }
    }

    cout << "Longest Palindrome is " << "\"" << longest << "\""
        << "whose length is " << max << endl;
   
    return 0;
}

---

there is never a reason to compare a bool to a bool.

Code:

---

isPalindrome(s)== true

---

is identical to

Code:

---

isPalindrome(s)

---

Oh, so linker error. Make sure pal.cc is in your project, or included in your command-line compilation.

Thanks guys i got it to compile, but i have a problem when running the program.. basically the program is supposed to read from the input and output the longest palindromes, along with it's length..however what it does is..it takes the first word inputted for example "abba" and then says that it is the longest palindrome right away and then waits for more input..i basically need the program to check through the list AFTER the user is done inputting words..also another problem is that the program exits if the user inputs a non-palindrome word.

i believe either you changed your code from what's posted above or you have misspoken.

yeah..i made a silly mistake =) anyway thanks for all the help guys..really appreciate it