Lists and Slicing in Python

[mac]

I need some beginner general programming help. I downloaded and was playing in the Python interpreter, reading the official Python tutorial docs. (V3.4)

I do get stumped when they start talking about manipulating strings and bring up subjects like slicing and indexing. I am already having issues processing the concepts.

[Icee]

A string can be interpreted as a sequence of characters. Indexing is then accessing an individual character within a string. For instance, the string "hello" is a sequence of 5 characters, 'h', 'e', 'l', 'l', and 'o'. "hello"[0] refers to character at index 0, which is 'h' (indexing starts with 0 and counts up to string length - 1). Slicing is accessing a substring between two indexes. "hello"[0:2] refers to the substring starting at index 0 (inclusive) and ending before index 2 (i.e., exclusive). This substring is then "he". Given a string S of length N, S[0:N] is the whole string. When slicing, either of the two indexes can be omitted. Omitting the first index (as in "hello"[:3]) is the same as specifying it to be 0, i.e. the slice starts at the very start of the whole string. Omitting the second index (as in "hello"[3:]) starts the slice at given index and slices up to the end of the string (thus "hello"[3:] yields "lo").

You can think of indexing as of looking at a string written on paper through a small window that allows you to see only one character at a time. The window can be moved. Slicing can be visualized as a similar concept, but the window can also be resized.

Hope that helps.

[mac]

But why isn't "Hello [0:2] seen as 'hel' instead of 'he' ?

Wait a minute, even though it counts from 0,they are sliced/indexed as if counting from 1 right?

[alexfru]

Are you still evaluating multiple languages or are you trying to learn several at once? I'd advise against the latter.

[iansjack]

It's just an arbitrary convention, the same as the fact that indexes start at 0 rather than 1 (although both conventions are chosen for good reasons). In a slice the first number is the first element of the slice, the second the first element beyond the slice.

It can also get a bit more complicated when you start considering negative indexes and assigning to slices. There's a discussion on Stack Exchange: http://stackoverflow.com/questions/5092 ... e-notation

[iansjack]

Are you still evaluating multiple languages or are you trying to learn several at once? I'd advise against the latter.

That has its pros and cons. But Python is a better choice for a first language than C.

[Muazzam]

Try http://www.codecademy.com/ tutorials on Python and JavaScript (You might find JavaScript easy). These tutorials are interactive, hence good and have community support. Official guides are often not beginners-friendly.
Note: I am not advertising anything, I just find it helpful.

[mac]

Are you still evaluating multiple languages or are you trying to learn several at once? I'd advise against the latter.

No, but I guess I need to "unlearn" everything else I read.

[Owen]

But why isn't "Hello [0:2] seen as 'hel' instead of 'he' ?

Wait a minute, even though it counts from 0,they are sliced/indexed as if counting from 1 right?

One needs to step back and consider the mathematical concept of intervals (or ranges).

Consider the interval with endpoints 0 and 10. Are the endpoints inside or outside the interval?

• If the interval contains both endpoints, then it is considered to be closed. It's commonly notated as [0, 10]
• If the interval excludes both endpoints, then it is considered to be open. It's commonly notated as (0, 10); though that's somewhat opaque, and I feel that the common alternate syntax ]0, 10[ is more clear.
• If the interval contains some mix, then it's half-open/half-closed, e.g. [0, 10[ (Please ignore these closing brackets; they are because PHPbb is confused: ]])

When dealing with the infinite precision of abstract maths, the distinction is very important: There isn't a way to rewrite an open end as a closed end or vise-versa in the real numbers, for example. When dealing with finite precision numbers, as most of the time in programming, the choice is somewhat arbitrary and stylistic, however half open ranges of the form [0, 10[ are preferred because they have some advantages: (PHPbb is confused again: ]])

• The starting end being closed just makes consistent sense. It would be weird if "hello"[0:2] returned "el", when hello[0] returns "h".
• The finishing end being open makes many things easier. For example, a slice from x of length y becomes list[x:x+y]. If the choice of a closed finishing end had been made, this would have been list[x:x+y-1]. For the converse, the length of slice [x:y], the sum is merely y-x. A closed end would have made that y-x+1. Essentially, having the second end open makes common mathematical operations simpler

You'll find very similar decisions in most common programming languages.

[KemyLand]

Summary:

In Python, the expression x[y:z] refers to a copy of the range of elements from y (inclusive) to z (exclusive) in the indexable object x.

[mac]

I'm beginning to find defining functions to be a very powerful feature and I've been messing around with it a few times.

Code: Select all

def print_greeting(s):
   print(s)
   print('your awesome')


print_greeting('welcome')

[KemyLand]

I'm beginning to find defining functions to be a very powerful feature and I've been messing around with it a few times.

Code: Select all

def print_greeting(s):
   print(s)
   print('your awesome')


print_greeting('welcome')

That's pretty good! Procedural programming (what you're doing with this) is the basis for C and thus eventually OSDev. Take in account that other programming paradigms can help you *a lot*, i.e: Object-Oriented Programming (this is like wine, it's good for you, as long as you don't abuse it!), Functional Programming (see Python's lambda expressions), and a long etc... Some languages (specially Python and C++, but most notabily Python) mix paradigms. That's a good model and has proved to be pretty self-sustainable. For example, you can have C-style functions in Python (not the parameters...), but if you need it, you can threat them as first-citizen objects (a property of Functional Programming) and use them as actual variables and arguments. You can also involve classes as deep as you want. Something that I like with Python, is that you can't abuse OOP, or you'll end up with un-maintainable code (i.e: def __init__(), with lots of self and references everywhere)!

[kzinti]

Functional Programming (see Python's lambda expressions)

you can threat them as first-citizen objects (a property of Functional Programming)

These things don't define functional programming. Just because you are using something called "lambda" in a given programming language doesn't make your program "functional". Nor is having functions as first-class objects make your language functional. The essence of functional programming is that you are dealing with immutable states. This can of course be accomplished in *any* language if you stay away from mutating anything. Of course, this isn't very efficient in Python where the philosophy is dramatically opposite: everything is mutable and can cause side-effects.

From http://en.wikipedia.org/wiki/Functional_programming:

In computer science, functional programming is a programming paradigm, a style of building the structure and elements of computer programs, that treats computation as the evaluation of mathematical functions and avoids changing-state and mutable data.

[KemyLand]

Although you're right that Python isn't the "essence" of Functional Programming, I did just provided some examples of it in presented in the language. BTW, I've never liked the idea of "what Wikipedia says is the truth", neither that "because the majority says so, it's so" . I don't like to relate Functional Programming with immutability. Although most functional-oriented programming languages do so, it limits your posibilities, as much as hard OOP does when lacking real procedures. Multi-paradigm languages are the future .

[kzinti]

BTW, I've never liked the idea of "what Wikipedia says is the truth"

I understand the sentiment, but in this case I quoted Wikipedia because they happen to have a good and proper definition of functional programming.

neither that "because the majority says so, it's so"

This isn't a popularity contest, "functional programming" has a specific meaning.

I don't like to relate Functional Programming with immutability.

But that is precisely what functional programming is! The word "functional" in "functional programming" means that you program with functions (in the mathematical sense). A mathematical function has no state and returns a result (output) from it's arguments (input). There is no global state or variables anywhere in sight. You can't say you don't like it and redefine what "functional programming" means and expect people to agree.

Although most functional-oriented programming languages do so,

Functional programming is a style of programming. The language can enfore it (i.e. Haskell) or not (i.e.: C, Python, ...)