Dart “For 循环” “If Else” “布尔返回”

Question

To call my self a noob would make noobs look bad... lets have a look at the code first...

bool isPalindrome(String s){  
  for(int i = 0; i < s.length/2;i++){
    if(s[i] != s[(s.length-1) -i])
      return false;        
  }  
  return true;  
}

main() {
  print(isPalindrome("anna"));
  print(isPalindrome("cat"));
}

... Results true - false

I came across that in a list of Dart examples for "beginners" what is s.length even if i change the number of letters in "cat" to "cats" it's "false", so it's not counting the number of letters but "caac" is "true". Can someone please give an in depth explanation of what that code is doing.

Answer 1

Your misunderstanding isn't in the code, but in what a "palindrome" is. A palindrome is a word that is spelled the same forwards and reversed. For example, if you took the word "racecar" and reversed the order of the letters, the result would be "racecar", ie the exact same word.

The code returns true for "anna" because this is still true - "anna" backward is again "anna". It returns false for "cat" because spelled backward it is "tac", which is a completely different word thus demonstrating that "cat" is not a palindrome. (Likewise, "caac" backward is "caac" and thus a palindrome, and "cats" backward is "stac" and thus not a palindrome.)

The way the code works is it compares every letter in the word with the letter in the mirrored position. For example, the mirrored position of the first letter is the last letter, the second letter is the second-to-last letter, and so on. If the letters are the same, the loop continues on. If it reaches the end and every letter compared is the same, then the word is spelled the same forward and backward, meaning it's a palindrome.

V       
racecar  ==Same letter, continuing==
      ^ 

 V
racecar  ==Same letter, continuing==
     ^

  V
racecar  ==Same letter, continuing==
    ^

   V
racecar  ==Same letter, continuing==
   ^

==The end has been reached, the word is a palindrome, return true==

If however, it ever reaches a point where a letter and the letter in the opposite position are not the same, that means the word would not be spelled the same backward as it is forward, which means it is not a palindrome.

V
abcaa  ==Same letter, continuing==
    ^

 V
abcaa  ==Different letter, the word is not a palindrome, return false==
   ^

(The reason in the loop it only goes up to length/2 is because once the check passes the halfway mark, it will start comparing letters it already compared. ie in "abcde", comparing "b" to "d" is the same as comparing "d" to "b", so the loop cuts out halfway to avoid unnecessary comparisons.)