How to teach beginners reversing a string in Python?

Question

I am teaching a course "Introduction to Computer Programming" to the first year math students. One has to assume that this is the first exposure of students to computer programming. Here are the main goals of my teaching:

• Students should learn and understand the basics of Python.
• Eventually they need to master sufficiently many Python tools so that they are able to select the right tool for a given problem.
• At the same time they have to learn basic skills of problem solving by computer programming.

My method of teaching is to give for each newly introduced concept a series of problems and teasers that motivate students. For instance, when introducing strings and lists a natural question is the task of string or list reversal. If I ask students to write a code that will check whether a string is a palindrome then I better tell them how to reverse it.

For lists, a natural solution myString.reverse() has at least two drawbacks:

1. It does not carry over to strings.
2. Students will see it a magic unless told about methods first.

The real question is: How should one introduce the problem of reversing a string in Python?

Answer 1

The two obvious ways are:

''.join(reversed(s))

and

s[::-1]

I think both are non-trivial for a programming newbie, but the concepts involved are not really that difficult.

The second way is easier to understand if you start by showing them what the results of s[::3], s[::2] and s[::1] are. Then s[::-1] will come naturally :)

Answer 2

Take a look at this discussion.

Answer 3

Introduce them to enough tools (array slicing and perhaps functional-style recursion, in particular) to accomplish the reversal. Then, let them struggle with trying to figure it out for a while. Take a few different answers and compare them, showing the pros and cons of each way.

Answer 4

You could teach them about stride notation (::) first and then slicing and apply both.

s = 'string'
s = s[::-1]
print s  # gnirts

References and more information:

• Extended Slices
• An Informal Introduction to Python
• Python string reversed explanation

In response to your comment, you can supply either arguments.

>>> s[len(s):None:-1]  
'gnirts'
>>> s[5:None:-1]  
'gnirts'
>>> s[::-1]  # and of course
'gnirts'

Answer 5

Absolute beginners guide to string reversal in python. ;)

# Tell them that,
# to reverse a string
# we read it backwards

s = 'string'  # input string
l = len(s)
rs = ''       # reversed string

for i in range(l-1,-1,-1): # range(start,end,step)
    rs += s[i]

print rs

But this is not considered pythonic and I am in favor of better methods everyone else have already posted.

Answer 6

Just ask them a riddle like this:

why

>>> 'dammitimmad'[::-1] == 'dammitimmad'
True

but

>>> 'dammit im mad'[::-1] == 'dammit im mad'
False

?