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"choose" in Python?

  #1  
May 9th, 2008
There's a function that repeatedly shows up in languages, usually with more or less the following form (Python syntax):
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Python Syntax (Toggle Plain Text) 
  1. var1 = raw_input("Input a number between one and three.")
  2.  
  3. choose var1:
  4.    case "1":
  5.       print "You inputted one."
  6.    case "2":
  7.       print "You inputted two."
  8.    case "3":
  9.       print "You inputted three."
Or something like that. I've also seen it called "switch." Does Python have anything like this?
Last edited by FreezeBlink : May 9th, 2008 at 7:59 pm. Reason: Added sentance on "switch"
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Re: "choose" in Python?

  #2  
May 9th, 2008
In Python you can set up a dictionary to mimic switch/case ...
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python Syntax (Toggle Plain Text) 
  1. def choose(s):
  2.     return {
  3.     '1' : lambda:"You inputted one.",
  4.     '2' : lambda:"You inputted two.",
  5.     '3' : lambda:"You inputted three."
  6.     }[s]()
  7.  
  8. var1 = raw_input("Input a number between one and three.")
  9. print choose(var1)
Even in C the switch/case statement is not as efficient as multiple if/elif statements, so Python did not implement it.
Last edited by vegaseat : May 9th, 2008 at 9:37 pm.
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Re: "choose" in Python?

  #3  
May 9th, 2008
That is pretty ingenious!
Does it work without using lambda? e.g
'1':"w/e", etc
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Re: "choose" in Python?

  #4  
May 9th, 2008
Actually, in this case you don't need lambda, since you are only returning a string. Please shorten the code to ...
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python Syntax (Toggle Plain Text) 
  1. def choose(s):
  2.     return {
  3.     '1' : "You inputted one.",
  4.     '2' : "You inputted two.",
  5.     '3' : "You inputted three."
  6.     }[s]
  7.  
  8. var1 = raw_input("Input a number between one and three.")
  9. print choose(var1)
Mildy more elegant ...
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python Syntax (Toggle Plain Text) 
  1. def choose(s):
  2.     return "You entered " + {
  3.     '1' : "one",
  4.     '2' : "two",
  5.     '3' : "three"
  6.     }[s]
  7.  
  8. var1 = raw_input("Input a number between one and three: ")
  9. print choose(var1)
Last edited by vegaseat : May 10th, 2008 at 12:03 am.
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Re: "choose" in Python?

  #5  
May 10th, 2008
And, um, while we're at it...what does Lambda do, anyway?
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Re: "choose" in Python?

  #6  
May 11th, 2008
Lambda creates an anonymous function, and is used to create a short inline function.

Lambda calculus is part of functional programming. It simply states that all computational constructs can be defined in terms of lambda. Interested? Read the article in:
http://linuxgazette.net/109/pramode.html
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Re: "choose" in Python?

  #7  
May 12th, 2008
I'm going to feel really stupid when I see the answer to my question, because I'm sure I've seen it before and I've just forgotten. Anyway, I was just fiddling with this example, and I can't remember how to add a default case. In the example given I want to return the text "Bad input" for any value other than 1, 2, or 3. What do I need to add to the dictionary?
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Re: "choose" in Python?

  #8  
May 12th, 2008
The standard try/except way would be:
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python Syntax (Toggle Plain Text) 
  1. def choose(s):
  2.     try:
  3.         return "You entered " + {
  4.         '1' : "one",
  5.         '2' : "two",
  6.         '3' : "three"
  7.         }[s]
  8.     except:
  9.         return "Bad input"
  10.  
  11. var1 = raw_input("Input a number between one and three: ")
  12. print choose(var1)
Or straight foreward with dictionary method get():
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python Syntax (Toggle Plain Text) 
  1. def choose2(s):
  2.     return "You entered " + {
  3.     '1' : "one",
  4.     '2' : "two",
  5.     '3' : "three"
  6.     }.get(s, "a bad number")
  7.  
  8. var1 = raw_input("Input a number between one and three: ")
  9. print choose2(var1)
This is closer to the switch/case default.
Last edited by ZZucker : May 12th, 2008 at 11:24 am.
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Re: "choose" in Python?

  #9  
May 12th, 2008
Thanks, I knew it was something simple that I had seen before.

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