I am trying to figure out how to solve this question. I know the answer, but I cannot figure out why it's correct.
primes = [2, 3, 5, 7, 11]
What does it contain after executing the following loop?
for i in range(2):
primes[4 - i] = primes[i]
answer: 2, 3, 5, 3, 2
I'm stuck in primes[4 - i]... part. Can somebody please explain this part? I know it is the index position.
range(n) gives you a sequence of numbers from 0 to n - 1.
If you replace the range function by its sequence in your code you basically get this:
for i in [0, 1]:
primes[4 - i] = primes[i]
Each time you loop, i will start from 0 and get bigger and bigger.
Which in turn means that 4 - i will start from 4 and get smaller.
1st loop: 4 -i = 4 - 0 = 4
2nd loop: 4 - i = 4 - 1 = 3
(and so on if your value in range was bigger than 2)
The line primes[4 - i] = primes[i] means that you're assigning to the index 4 - i the value stored at the index i.
In your example, your loop is actually doing this:
1st loop: primes[4] = primes[0]
2nd loop: primes[3] = primes[1]
Giving you the result you currently have.
This code starts by reversing the array, but it overwrites the end of the array so overall it just outputs a palindrome.
The code is just swapping the last with first,second last with second and so on. until the loop is over
The array is build with indices:
0,1,2,...,n
You can enter each index by selecting its position e.g. array[2] gets you the third value of the array.
Your code, like @script8man explained, selects the 4-ith index. in a range(2) (0,1), it will return the 4th and 3rd index, this index is overwritten with the i-th index (0,1).