Python代码说明需求

Question

def array_front9(nums):
    end = len(nums)
    if end > 4:
        end = 4

    for i in range(end):
        if nums[i]==9:
            return True
    return False

I need to understand the above python code and why two return statement in 'for loop'. This is seriously confusing me.

Answer 1

This could be rewritten much simpler (that is, "more pythonic") as this:

def array_front9(nums):
   return 9 in nums[:4]

The first half of the code is setting the loop limit to the first 4 elements, or less if the array nums is shorter. nums[:4] does essentially the same thing by creating a copy that only contains up to the first 4 elements.

The loop is checking to see if the element 9 is found in the loop. If found, it returns immediately with True . If it's never found, the loop will end and False is returned instead. This is a longhand form of the in operator, a built-in part of the language.

Answer 2

Let me explain:

def array_front9(nums):   # Define the function "array_front9"
    end = len(nums)       # Get the length of "nums" and put it in the variable "end"
    if end > 4:           # If "end" is greater than 4...
        end = 4           # ...reset "end" to 4

    for i in range(end):  # This iterates through each number contained in the range of "end", placing it in the variable "i"
        if nums[i]==9:    # If the "i" index of "nums" is 9...
            return True   # ...return True because we found what we were looking for
    return False          # If we have got here, return False because we didn't find what we were looking for

There are two return-statements in case the loop falls through (finishes) without returning True .

Answer 3

The second return isn't in the for loop . It provides a return value of False if the loop "falls through", when none of nums[i] equal 9 in that range.

At least, that's how you've indented it.

Answer 4

You could rewrite this to be more clear using list slicing:

def array_front9(nums):
    sublist = nums[:4]

    if 9 in sublist:
        return True
    else:
        return False