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The following is a function to write a recursive function that accepts an array and a callback. Function returns true if a single value in array returns true when passed to the callback otherwise it returns false.

I have trouble understanding the "if not" line written here(line 4), how does the cb interact with arr? Please help.

def isOdd(num):
    if num%2==0:
        return False
    else:
        return True
        
def someRecursive(arr, cb):
    if len arr == 0:
        return False
    if not(cb(arr[0])):     #Maybe till it is an empty array. DOUBT HERE
        return someRecursive(arr[1:],cb)
    return True
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    $\begingroup$ (While programming questions are off-topic here, the relevant part from the Python 3 reference reads not_test ::= comparison | "not" not_test […] The operator not yields True if its argument is false, False otherwise. The parentheses around cb(arr[0]) are dispensable. The statement in isOdd(num) might better read return num%2;, return num%2 != 0; or even return not (num%2 == 0);.) $\endgroup$
    – greybeard
    Jan 4, 2021 at 5:40
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    $\begingroup$ cb is a function, so cb(arr[0]) is simply calling cb with arr[0] as an argument. What don't you understand? $\endgroup$
    – xskxzr
    Feb 7, 2021 at 1:23

2 Answers 2

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The if not branches says that the program continues to recurse if the first element of the array does not satisfy cb (cb(arr[0]) returns false).

The function checks the condition cb on one (the first) element at every call, and give the rest to the next recursion. Since the first element is removed at every iteration, arr[0] is different in each function call.

Once not(cb(arr[0])) gives false, the function returns True, and the return value propagates back to previous iterations.

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  • $\begingroup$ Thank you for your immediate response, I think I understand this code better now. Have a good day my friend! $\endgroup$ Jan 4, 2021 at 18:47
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The function starts by checking for an empty array and returns false in that case.

Otherwise, it tests the first element of the array and returns true if the condition cb is met. Otherwise, it recurses with the array less the first element.

Said differently, the function recursively scans the array until some element fulfills the condition, and reports if there is one.

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  • $\begingroup$ Off topic here but the python expression any(cb(x) for x in arr) produces the same boolean more efficiently. $\endgroup$ Sep 23, 2023 at 4:31
  • $\begingroup$ @Gribouillis: doubly off-topic because the question was about recursion and the site is not about programming languages. $\endgroup$
    – user16034
    Sep 25, 2023 at 6:50

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