def is_palindrome(dna):
n = len(dna)
for i in range(n):
if dna[i] != dna[:: -1]:
return False
return True
print(is_palindrome(input_1))
print(is_palindrome(input_2))
print(is_palindrome(input_3))
What I expected to happen:
I am expecting the input_1 to be True. But it is false.
What actually happened:
"Your function returned an incorrect answer. Input: ['ATTA'] Your answer: False Expected answer: True"
I am confused about this part of the answer, is there anyone can explain why the range is (n//2)?
what does “dna[n -i -1]” means? why i can not use “dna[::-1]” to pick the last one in the list?
for i in range(n // 2):
if dna[i] != dna[n - i - 1]:
dna[:: -1] #this reverse the string
dna[n - i - 1] #this subtract the length(n) by integer(i) minus 1
if dna[i] != dna[n - i - 1]: #this compare this 2 statements execute if is different (dna[i] slice the string)
sorry @info.victoromondi
I definitely get you confused about my question, what I am trying to understand is the following part in the standard answer offered by Dataquest:
for I in range(n//2):
if dna[i] = dna[n -i -1]:
Can you give me a more detailed explanation about why the range is (n//2) instead of n?
why we are using dna[n -i -1] instead of dna[-1]?
I stuck on this problem too. Now I get it by actually breaking down the logic on a piece of paper.
Why using dna[n -i -1] instead of dna[-1]? Because dna[-1] gets you only the last element while dna[n -i -1] picks for you any of the last elements…
Why the range is (n//2) instead of n? Well, it would still work with range(n). You get the result faster with fewer steps doing it halfway using range(n // 2).
def is_palindrome(dna):
for i in range(len(dna)):
if dna[i] == dna[-i-1]:
return True
return False
In the above case, the function returns True if the first and last character is the same. It won’t wait till the for loop checks all characters. Because of that, it will return True for all the inputs.
def is_palindrome(dna):
for i in range(len(dna)):
if dna[i] != dna[-i-1]:
return False
return True
On the other hand, the above code will only return False if the characters in comparison are not equal. If they are equal, it will patiently wait until all characters are checked and then return True.
“//” is used in python to get only the quotient part without the fraction or remainder from a division operation, I hope you’ve got it now.
As in, 7 // 2 will give you the result = 3.
@ drill_n_bass Drill N Bass
Why does the first program not “patiently wait until all characters are checked and then return True”? I thought “for loops” are used for the very purpose of going through each iteration before providing a result.
Furthermore, why does the second program “patiently wait until all characters are checked and then return True”. I’m not understanding the difference and the reasoning.
The second program has an if condition which is checking if the characters don’t match.
So, if they are equal, then the loop will keep iterating till they are not the same.
For example, take a random word lool.
In the first iteration, the if condition checks if l is not equal to l. Are they not equal? No. So, the loop continues.
In the second iteration, the if condition checks if o is not equal to o. Are they not equal? No. So, the loop continues.
Consider the example word lawl (I am literally making words up just to explain this, so ignore the randomness )
The first program will check if the first and last characters are equal on the first iteration. That is it will check if l and l are equal. They are. The if condition is True and the code returns a True. Even though we can see that a and w are not the same, the function still says the word is a palindrome.
The second program ensures that we if the word is actually a palindrome, we iterate through all the characters to confirm that. The first one stops as soon as any two characters will match and that is not desirable.
Thanks @the_doctor . It’s still confusing to me, but I think it’s one of those things where I will just move on and hopefully it will show up in another situation and turn a light bulb on in my head. I appreciate the explanation!
)