C++ 回文函数

Question

 #include <iostream>
 #include <string>


using namespace std;


bool isPalindrome(string str){



    for(int i = 0; i <= str.length()-1; i++){
        if(str[i] != str[str.length()-1-i]){
            return false;
        }else {
            return true;
        }
    }
}

main(){

    string text;

    do{
        cout << "Enter some Text: " << endl;
        cin >> text;
        if(isPalindrome(text)){
            cout << "The text is a palindrome" << endl;
        }
        else{
            cout << "The text is not a palindrome" << endl;
        }

    }while(text != "Q");

    return 0;
}

Can someone explain to me what is wrong with my code? If I input "otto" as text, the code produces the correct answer. If In input "ottop" as text, the code is working as well, but if I input "ottopo" i get as output that "ottopo" is a palindrome, which it obviously is not.

I seem to be missing something or have maybe overlooked something. I know i could use c++ standard library funtions for this, but i really want to know why it's not working the way i want it to.

I have implemented palindrome functions in java, python, javascript, ruby ect...I just can't find my mistake within this c++ code! I know it's really simpel, but it's sooo frustrating!

Answer 1

You are returning true on the first iteration of the loop. And you only need to iterate on half the string.

Corrected:

    size_t len = str.length();

    for(int i = 0; i < len/2; i++){
        if(str[i] != str[len-1-i]){
            return false;
        }
    }
    return true;

Answer 2

You don't have to have a return true after the else statement there. Put that outside the for loop and your code should work.

As an extra nitpick, you don't have to check until strlen(s) - 1, but strlen(s) / 2 is actually enough. I'll leave figuring that out to you, as it's quite straightforward and not too different from what you are doing, at all.

Answer 3

As it has been said, the return true; should not be in the else , but after the end of the for loop. This can also be accomplished in just a few lines. A string is a palindrome if it is the same reversed as it is normally. All you need is

reversedText = reverse(text.begin(), text.end());
if (text == reversedText)
    //indicate that it is a palindrome
else
    //indicate that it is not a palindrome`

Just be sure to include algorithm. I can understand if you don't want to use a library for some reason, but I just want to make sure that you know that this can be accomplished very easily.

Answer 4

You should set a flag to 1 if it is not a palindrome, then break after:

for (int i = 0; i < len/2; i++) {
    if (str[i] != str[len-1-i]) {
        // here .......
    }