Palindrome testing program crashing.

[RobertL]

Working on a school assignment, essentialy, write a program that will receive a word (all lowercase, no spaces, terminated by a period), and determine if it is a palindrome. The program compiles fine, with no errors or warnings, but when run it crashes. I have compiled it with Microsoft Visual C++ 6.0 and Microsoft Visual C++ 2010 express and experienced the same problem.

EDIT: The exact error message that I got on my home computer is "Expression: string subscript out of range"

#include <iostream>
#include <string>
using namespace std;
void checkpal(int, string);
int main()
{ char ans;
do
{
string word1, worddelete;
int length, i;
cout << "This program checks palindromes\n.Enter your word, terminated by a period: ";
cin >> word1;
length = word1.length();
for(i = 0; i <= length - 1; i++)
{
worddelete[i] = word1[i];
}
length = length - 1;
cout << endl << "Your word is " << length << " characters long.\n";
checkpal(length, worddelete);
cout << "Would you like to check another word? 'y' for yes 'n' for no: ";
cin >> ans;
}
while(ans == 'y' || ans == 'Y');
return 0;
}





void checkpal(int length, string worddelete)
{
int i;
string fronthalf, backhalf;
for(i = 0; i < length / 2; i++)
{
worddelete[i] = fronthalf[i];
worddelete[length-i] = backhalf[i];
}
if(fronthalf.compare(backhalf) != 0)

cout << "This word is not a palindrome!";
else
cout << "This word is a palindrome!";
return;
}

[Zhuge]

worddelete, fronthald, backhalf, don't seem to ever be initialized...

[RobertL]

string word1, worddelete;

and

string fronthalf, backhalf;


Are sufficient, no?


EDIT: And I dont think that is the problem or else the compiler would be giving an error.

[Zhuge]

It is because you don't initialize them (thus making them empty strings), then try to access their elements with [], iterating over length, which is not their actual length (it's 0, because the are "").

[RobertL]

Alright, I see what you mean.

I made a mistake with

worddelete[i] = fronthalf[i];
worddelete[length-i] = backhalf[i];

It should be

fronthalf[i] = worddelete[i];
backhalf[i] = worddelete[length-i];

In order to fill the strings fronthalf and backhalf. But when i try it that way, it STILL wont run. Am I missing something else?
EDIT: I get the exact same error as previously.
EDIT2: Actualy, the prgram RUNS either way, but it crashes after I type in a word and hit enter.

[gcampton]

can you post your upgraded code in code tags. use the <> button over on the right----->

[dionisis]

When you declare worddelete or fronthalf or backhalf you don't assign a specific length to it so it is treated as null string (it has no space for a word to be assign to it)
and as a result you can't access individually each character of it because it simply doesn't have
For example:

fronthalf[i] = worddelete[i];
backhalf[i] = worddelete[length-i];

or

for(i = 0; i <= length - 1; i++)
{
worddelete[i] = word1[i];
}

Instead of that you should use the string assignment operator ( = ) to 'copy' a string to another
Try it and let me know about your progress

[RobertL]

I "think" I am following you. Since I am not declaring the size of each string, I cannot access individual characters.

That I understand, but if I'm following you correctly, how do I get rid of the '.' mandated by the assignment? Since that means

for(i = 0; i <= length - 1; i++)
{
worddelete[i] = word1[i];
}

Won't work at all.



So, what I think I need is some way of determining how long the word is. Probably by searching the string for '.'? In that case should I declare the strings as some arbitrarily high number? That seems like a crude way to go though.

#include <iostream>
#include <string>
using namespace std;
void checkpal(int, string);
int main()
{ char ans;
do
{
string word1, worddelete;
int length, i;
cout << "This program checks palindromes\n.Enter your word, terminated by a period: ";
cin >> word1;
length = word1.length();
for(i = 0; i <= length - 1; i++)
{
worddelete[i] = word1[i];
}
length = length - 1;
cout << endl << "Your word is " << length << " characters long.\n";
checkpal(length, worddelete);
cout << "Would you like to check another word? 'y' for yes 'n' for no: ";
cin >> ans;
}
while(ans == 'y' || ans == 'Y');
return 0;
}





void checkpal(int length, string worddelete)
{
int i;
string fronthalf, backhalf;
for(i = 0; i < length / 2; i++)
{
fronthalf[i] = worddelete[i];
backhalf[i] = worddelete[length-i];
}
if(fronthalf.compare(backhalf) != 0)

cout << "This word is not a palindrome!";
else
cout << "This word is a palindrome!";
return;
} 

[Zhuge]

If they are std::strings, just use .size() to get the size. Also, you ought to use getline() to get data into the string instead of just cin>>, which will stop at the first space.

[RobertL]

What is the difference between using .size() and .length()? But I see what you mean with the
getline(), I suspect that was the problem. (Though this computer does not have a compiler, so I can't check.)


And that should just be

cout << "This program checks palindromes\n.Enter your word, terminated by a period: ";
getline (cin,word1);

correct?

[Zhuge]

That's right.

http://www.cplusplus.com/reference/string/string/length/
(third or so line)

[RobertL]

#include <iostream>
#include <string>
using namespace std;
void checkpal(int length, char worddelete[]);
int main()

{ char ans;
do
{
string word1;
char worddelete[100];
int length, i;
cout << "This program checks palindromes\n.Enter your word, terminated by a period: ";
getline(cin, word1);
length = word1.length();
cout << endl << "Your word is " << (length - 1) << " characters long.\n";
length = length - 1;
for(i = 0; i < length; i++)
{
worddelete[i] = word1[i];
}

checkpal(length, worddelete);
cout << "Would you like to check another word? 'y' for yes 'n' for no: ";
cin >> ans;
}
while(ans == 'y' || ans == 'Y');
return 0;
}





void checkpal(int length, char worddelete[])
{
int i;
string fronthalf2, backhalf2;
char fronthalf[100], backhalf[100];
for(i = 0; i < length / 2; i++)
{
fronthalf[i] = worddelete[i];
backhalf[i] = worddelete[length-i];
}


if(fronthalf2.compare(backhalf2) != 0)

cout << "This word is not a palindrome!";
else
cout << "This word is a palindrome!";
} 

At the moment this is what I have.

Problem 1: Trying to figure out how to convert from type array to type string. Should figure this out on my own shortly(assuming it is possible).

Problem 2: Past the second loop it reports length as = -1. I have no idea why.

Problem 3: The program seems to have about a 50/50 chance of crashing instantly when typing in 'y' to initialize 'ans'. Not when you press enter, simply when you TYPE it. The window will just up and disappear.

[RobertL]

Diffrint iteration:

#include <iostream>
#include <string>
using namespace std;
void checkpal(int length, char worddelete[]);
bool is_equal ( char backhalf[], char fronthalf[], int length );
int main()

{ char ans;
do
{
string word1;
char worddelete[100];
int length, i;
cout << "This program checks palindromes.\nEnter your word, terminated by a period: ";
getline(cin, word1);
length = word1.length();

length = length - 1;
cout << endl << "Your word is " << length << " characters long.\n";
for(i = 0; i < length; i++)
{
worddelete[i] = word1[i];
}

checkpal(length, worddelete);
cout << "\nWould you like to check another word? 'y' for yes 'n' for no: ";
cin >> ans;
}
while(ans == 'y' || ans == 'Y');
return 0;
}


bool is_equal ( char backhalf[], char fronthalf[], int length )
{
length = length / 2;
for ( int i = 0; i < length; i++ ) {
if ( backhalf[i] != fronthalf[i] )
return false;
}

return true;
}


void checkpal(int length, char worddelete[])
{
bool a;
char fronthalf[100], backhalf[100];
for(int i = 0; i < length / 2; i++)
{
fronthalf[i] = worddelete[i];
backhalf[i] = worddelete[length];
cout << "front " << fronthalf[i] << "      back " << backhalf[i];
}

a = is_equal(backhalf, fronthalf, length);
if(a)
{
cout << "\nThis is a palindrome!";
}
else
{
cout << "\nThis is NOT a palindrome!";
}
}

For some bizarre reason the program returns false the first time and true the second time and won't run a third time. I have no idea why.