[Tutor] Backwards message program

[Steven D'Aprano]

myles broomes wrote:
> Im trying to code a program where the user enters a message and it is returned backwards. Here is my code so far:
>  
>  
> message = input("Enter your message: ")
>  
> backw = ""
> counter = len(message)
> while message != 0:
>     backw += message[counter-1]
>     counter -= 1
> print(backw)
> input("\nPress enter to exit...")

When you want to do something with each item in a sequence, such as each 
character in a string, you can do it directly:

for char in message:
     print(char)


prints the characters one at a time.

Python has a built-in command to reverse strings. Actually, two ways: the hard 
way, and the easy way. The hard way is to pull the string apart into 
characters, reverse them, then assemble them back again into a string:


chars = reversed(message)  # Gives the characters of message in reverse order.
new_message = ''.join(chars)


Or written in one line:


new_message = ''.join(reversed(message))


Not very hard at all, is it? And that's the hard way! Here's the easy way: 
using string slicing.

new_message = message[::-1]


I know that's not exactly readable, but slicing is a very powerful tool in 
Python and once you learn it, you'll never go back. Slices take one, two or 
three integer arguments. Experiment with these and see if you can understand 
what slicing does and what the three numbers represent:


message = "NOBODY expects the Spanish Inquisition!"

message[0]
message[1]

message[39]
message[38]

message[-1]
message[-2]

message[0:6]
message[:6]

message[19:38]
message[19:-1]
message [19:-2]

message[::3]
message[:30:3]
message[5:30:3]


Hint: the two and three argument form of slices is similar to the two and 
three argument form of the range() function.


Python gives you many rich and powerful tools, there's no need to mess about 
with while loops and indexes into a string and nonsense like that if you don't 
need to. As the old saying goes, why bark yourself if you have a dog?



-- 
Steven