Python - Writing slice function

Question

I'm trying to write a function that will slice a list between two given values. If the stop value exceeds the list bounds, the stop value is the length of the list. If the start value exceeds the list bounds, I want to return an empty list. If the start value is less than the stop value, I want to return an empty list. I do not want to use any built-in functions or methods other than range() and list.append()

The code that is calling the function is:

str_list = ['s', 'h', 'r', 'u', 'g', 'g', 'e', 'd']

print("\nget_slice Test")
my_slice = list_function.get_slice(str_list, 2, 100)
print(list_function.to_string(str_list))
print(list_function.to_string(my_slice))

my_slice = list_function.get_slice(str_list, 6, 1)
print(list_function.to_string(my_slice))

my_slice = list_function.get_slice(str_list, 2, 5)
print(list_function.to_string(str_list))
print(list_function.to_string(my_slice))

The output I want to produce is:

get_slice Test
List is: s, h, r, u, g, g, e, d
List is: r, u, g, g, e, d
List is:
List is: s, h, r, u, g, g, e, d
List is: r, u, g

I'm defining the function as:

def get_slice(my_list, start, stop):

I've tried my_list = my_list[start:stop] but I get a 'NoneType' object is not iterable error. I also wanted to try using for k in range(0, length(my_list)):, as it was suggested that I use range(), however I can't think of a way that could be used for this function just yet.

Any assistance with this would be greatly appreciated.

Why are you doing that, when Python already supports slice notation? Is it homework? Learning? What have you tried so far? Show some code. — Michael Foukarakis, Nov 05 '13 at 14:36
I've tried `my_list = my_list[start:stop]` but I get a 'NoneType' object is not iterable error. I also wanted to try using `for k in range(0, length(my_list)):`, as it was suggested that I use `range()`, however I can't think of a way that could be used for this function just yet. — Dylsbo, Nov 05 '13 at 14:43
Edit your question to include your attempts, so it's clearer. — aIKid, Nov 05 '13 at 14:45

score 0 · Accepted Answer · edited May 23 '17 at 12:29

You just need to use the slice notation. That was covered here in this thread:

Explain Python's slice notation

Here is the solution printing the correct output:

def get_slice(my_list, start, stop):
        return my_list[start:stop][:]

def to_string(my_list):
        return 'List is: %s' % ''.join(my_list[:])

str_list = ['s', 'h', 'r', 'u', 'g', 'g', 'e', 'd']

print("\nget_slice Test")
my_slice = get_slice(str_list, 2, 100)
print(to_string(str_list))
print(to_string(my_slice))

my_slice = get_slice(str_list, 6, 1)
print(to_string(my_slice))

my_slice = get_slice(str_list, 2, 5)
print(to_string(str_list))
print(to_string(my_slice))

score 0 · Answer 2 · answered Nov 05 '13 at 15:21

I was not able to reproduce your problem, you should have posted the full code. Maybe a missing return and a fix on the variables? I don't know. But here's my solution:

def get_slice(my_list, start, stop):
    return my_list[start:stop]

def to_string(my_list):
    return "List is: " + ', '.join(my_list)

str_list = ['s', 'h', 'r', 'u', 'g', 'g', 'e', 'd']

print("\nget_slice Test")
my_slice = get_slice(str_list, 2, 100)
print(to_string(str_list))
print(to_string(my_slice))

my_slice = get_slice(str_list, 6, 1)
print(to_string(my_slice))

my_slice = get_slice(str_list, 2, 5)
print(to_string(str_list))
print(to_string(my_slice))

output:

get_slice Test
List is: s, h, r, u, g, g, e, d
List is: r, u, g, g, e, d
List is: 
List is: s, h, r, u, g, g, e, d
List is: r, u, g

Python - Writing slice function

2 Answers2