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I was working on this Leetcode problem. I managed to solve it and later looked upon the solution to find this :-

class Solution(object):

## A is the Given Array
def isMonotonic(self, A): 
    return (all(A[i] <= A[i+1] for i in range(len(A) - 1)) or
            all(A[i] >= A[i+1] for i in range(len(A) - 1)))

My doubt is regarding the usage of all() in this manner - all(A[i] <= A[i+1] for i in range(len(A) - 1)), clearly all() is being called with a boolean parameter and it works. But when I executed all(True) in python console I got TypeError: 'bool' object is not iterable. So basically how did the usage of all() in this manner work in the code but not in the console.

My another doubt is in A[i] <= A[i+1] for i in range(len(A) - 1). When i reaches the last index value of the list shouldn't A[i] <= A[i+1] raise an IndexError: list index out of range.

hsiraaH
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1 Answers1

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This is an example of list comprehension.1 In this instance, the input to all() is a list of booleans, which could be rewritten as a for loop like so:

bools = []
for i in range(len(A) - 1):
    bools.append(A[i] <= A[i + 1])
all(bools)

For your second doubt, we can look to the definition of the range() function (emphasis mine):

For a positive step, the contents of a range r are determined by the formula r[i] = start + step*i where i >= 0 and r[i] < stop.

Essentially, the range method is exclusive on the maximum, meaning the generator range(len(A)-1) will only go up to len(A) - 2.

1: Technically this isn't a list but a generator. The difference is that a generator only creates new values as needed, rather than all of them at the same time. See this answer on the definition of yield for a more in-depth look.

bbnumber2
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