Python: What does for x in A[1:] mean?

Question

I was trying to understand Kadane's algorithm from Wikipedia, when I found this:

def max_subarray(A):
    max_ending_here = max_so_far = A[0]
    for x in A[1:]:
        max_ending_here = max(x, max_ending_here + x)
        max_so_far = max(max_so_far, max_ending_here)
    return max_so_far

I'm not familiar with Python. I tried to google what this syntax does but I couldn't find the right answer because I didn't know what's it called. But, I figured A[1:] is the equivalent of omitting A[0], so I thought for x in A[1:]: is equivalent to for(int i = 1; i < A.length; i++) in Java

But, after changing for x in A[1:]: to for x in range(1,len(A)), I got the wrong result

Sorry if this is a stupid question, but I don't know where else to find the answer. Can somebody tell me what this syntax does and what is it called? Also, could you give me the equivalent of for x in A[1:]: in Java?

Answer 1

Here are some of the example that I have tried

>>> a=[1,5,9,11,2,66]

>>> a[1:]
[5, 9, 11, 2, 66]

>>> a[:1]
[1]

>>> a[-1:]
[66]

>>> a[:-1]
[1, 5, 9, 11, 2]

>>> a[3]
11

>>> a[3:]
[11, 2, 66]

>>> a[:3]
[1, 5, 9]

>>> a[-3:]
[11, 2, 66]

>>> a[:-3]
[1, 5, 9]

>>> a[::1]
[1, 5, 9, 11, 2, 66]

>>> a[::-1]
[66, 2, 11, 9, 5, 1]

>>> a[1::]
[5, 9, 11, 2, 66]

>>> a[::-1]
[66, 2, 11, 9, 5, 1]

>>> a[::-2]
[66, 11, 5]

>>> a[2::]
[9, 11, 2, 66]

I think you can understand more by this examples.

Answer 2

This is array slice syntax. See this SO question: Explain Python's slice notation .

For a list my_list of objects e.g. [1, 2, "foo", "bar"], my_list[1:] is equivalent to a shallow copied list of all elements starting from the 0-indexed 1: [2, "foo", "bar"]. So your for statement iterates over these objects:

for-iteration 0: x == 2 
for-iteration 1: x == "foo" 
for-iteration 2: x == "bar" 

range(..) returns a list/generator of indices (integers), so your for statement would iterate over integers [1, 2, ..., len(my_list)]

for-iteration 0: x == 1 
for-iteration 1: x == 2
for-iteration 2: x == 3

So in this latter version you could use x as an index into the list: iter_obj = my_list[x].

Alternatively, a slightly more pythonic version if you still need the iteration index (e.g. for the "count" of the current object), you could use enumerate:

for (i, x) in enumerate(my_list[1:]):
    # i is the 0-based index into the truncated list [0, 1, 2]
    # x is the current object from the truncated list [2, "foo", "bar"]

This version is a bit more future proof if you decide to change the type of my_list to something else, in that it does not rely on implementation detail of 0-based indexing, and is therefore more likely to work with other iterable types that support slice syntax.

Answer 3

Unlike other languages, iterating over a sequence in Python yields the elements within the sequence itself. This means that iterating over [1, 2, 4] yields 1, 2, and 4 in turn, and not 0, 1, and 2.

Answer 4

What does "for x in A[1:] mean?

There are two parts to this. First, let's consider A[1:], which is slice notation on a list or array. Consider the following list:

A = [3,4,5,6]

In this example A[0] is 3 and A[1] is 4 and so on. When you use the : that can mean take everything or everything from a certain index. So A[1:] is the slice notation that says, "Take elements 1 to the end" of the list. So in my simple example A[1:] gives you a slice of the list that is [4,5,6]

The second have of the question is about the for x... part. In this case this is the Python notation that says for each element in the list. So:

A = [3,4,5,6]
for x in A[1:]:  #remember A[1:] is [4,5,6]
    print(x)

This will print 4 on the first loop, 5 on the second loop, and 6 on the third loop.

Answer 5

A = [1, 2, 3]

A[1:] == [2, 3]

This is used to truncate your list from the first element.

And note that lists are mutable, if you find something like A[:] that means, they want to create a double of this list, without altering the original list, and use A[::-1] instead of reversed(A) to reverse the list.

Answer 6

Simply A[1:] means access elements from 1st position (don't consider A[0]). That's the only logic we need to consider.

A=[23,45,6,77,456,3]
print(A[1:])

output:

45,6,77,456,3