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If elif elif elif - multiway selection in Python

Most languages have "if / else if / else if / else " type constructs for selecting conditional code, although the format / keyword varies between languages, in the case if Python, the intermediate keyword is elif.

In all cases with this construct, you can think of it as like peeling an onion - taking a specific condition and dealing with it, and leaving other conditions in a subset to be considered later. In other words, once one option has been found that's true, the other options won't be checked nor will any further conditional code blocks be checked.

For example:

  if trainsPerDay == 8:
     a()
  elif trainsPerDay > 6 and trainsPerDay < 20:
     b()
  elif trainsPerDay > 6:
     c()
  etc


a() will be run in the value in trainsPerDay was 8.

If the value is between 6 and 20 BUT IS NOT 8, then b() will be run. b() isn't run if the tested value is 8, because that's already been dealt with in the earlier test.

c() says it will be run if the value is greater that 6. However, 8 was dealt with in the first test, and values between 6 and 20 in the second test, so in practice c() won't run until trainsPerDay is 20 or greater.

You could (rightly) suggest that my example could have been more clearly coded - but then it would have failed to make the point so clearly. Only one block will be performed, and you need to be very careful to put your blocks in the right order if you can hit a situation in which several would be true. And it's first matched, first run in that circumstance.

Source code examples [here] and [here].

I have just finished teaching learning to program in Python and Python Programming for 2013... next week I am teaching the more advanced Python Intermediate. Please get in touch is you want to make an extraordinarily late booking... or follow the links above which will give you a whole list of public course dates for each of the three courses for 2014.
(written 2013-11-16, updated 2013-11-19)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
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  [4723] Conditional operators in Python - (2016-11-05)
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  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4092] Identity in Python - (2013-05-17)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [3762] Learning to program - the if statement. Python. - (2012-06-12)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
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  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
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  [2899] Groupsave tickets - 3 or 4 train tickets for the price of 2 - (2010-08-02)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1201] No switch in Python - (2007-05-23)
  [909] Python is like a narrowboat - (2006-10-30)
  [835] Python - when to use the in operator - (2006-08-16)
  [788] New - Conditional expressions in Python 2.5 - (2006-07-01)
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  [657] The ternary operator in Python - (2006-03-25)
  [353] Wimbledon Neck - (2005-06-20)
  [299] What - no switch or case statement? - (2005-05-03)


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Handling JSON in Python (and a csv, marshall and pickle comparison)
Some other Articles
A busy week, and a long flight
Formatting options in Python
Python functions - an introduction to how they work
Handling JSON in Python (and a csv, marshall and pickle comparison)
If elif elif elif - multiway selection in Python
Lists in Tcl - fundamentals in a commented source code example
Tcl + Tk (Wish) - an introduction and revision example
Exception handling in Tcl
Writing the perfect program in Tcl?
Regular Expression Substitution - Tcl
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