I am currently practicing binary search and I'm having trouble understanding the difference in syntax between lo == 0, hi == len(cards) - 1 and lo, hi = 0, len(cards) - 1. The code works with the second method but not with the first one. Any help would be appreciated. Thank you.

The code is as follows:

def binary_search(cards, query):
    # lo == 0, hi == len(cards) - 1
    lo, hi = 0, len(cards) - 1
    while lo <= hi:
        mid = (lo hi) // 2
        result = locate_card(cards, query, mid)
        mid_card = cards[mid]
        print('lo: ', lo, 'hi: ', hi)
        print('mid: ', mid)
        print('mid_card: ', mid_card)
        print('result: ', result)
        print('\n')
        if result == 'found':
            return mid
        elif result == 'left':
            hi = mid - 1
        else:
            lo = mid   1
    return -1

def locate_card(cards, query, mid):
    if cards[mid] == query:
        if mid > 0 and cards[mid-1] == query:
            return 'left'
        else:
            return 'found'
    elif cards[mid] < query:
        return 'left'
    else:
        return 'right'
    
    return binary_search(cards, query)

if __name__ == '__main__':
    cards = [13, 11, 10, 7, 4, 3, 1, 0]
    query = 1
    print(binary_search(cards, query))

CodePudding user response：

A double equal-sign (==) is the operator to compare two object to see if they are equal. lo == 0, hi == len(cards) - 1 will compare lo to 0 and hi to len(cards)-1.

lo, hi = 0, len(cards) - 1 is the same as writeing

lo = 0
hi = len(cards) - 1

CodePudding user response：

The == operator is used when we want to use conditional expressions and want to compare two expressions that, if true, perform an operation, but the = operator is used to assign a value to a variable. for example:

if lo == 0 and hi == len(cards) - 1:
    print("Hello")

CodePudding user response：

I suggest playing around in the python interpreter, in order to better understand the three notations = (assignment), == (equality test), and , (used to build tuples).

Playing with == and ,: equality of tuples, or tuple of equalities

>>> 3 == 3, 4 == 4
(True, True)

>>> 3, 4 == 3, 4
(3, False, 4)

>>> (3, 4) == (3, 4)
True

>>> 3 == 3 and 4 == 4
True

Playing with = and ,: assignment with unpacking

>>> 0, 10
(0, 10)

>>> lo, hi = (0, 1)
>>> print(lo, hi)
0 1

>>> lo, hi = 1, 2
>>> print(lo, hi)
1 2

>>> lo = 3
>>> hi = 4
>>> print(lo, hi)
3 4

Tuple of assignment: that is not correct syntax

>>> lo = 5, hi = 6
SyntaxError: invalid syntax. Maybe you meant '==' or ':=' instead of '='?

Assignment expression with := (python >= 3.8)

>>> (lo := 7, hi := 8)
(7, 8)
>>> print(lo, hi)
7 8