Recursive array

Question

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

Answer 1

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.

Answer 2

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.