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For 2023 (and 2024 ...) - we are now fully retired from IT training.
We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk.

Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active!

I am also active in many other area and still look after a lot of web sites - you can find an index ((here))
Python - when to use the in operator

Python's in construct, when used with an if statement, lets you loop through all the members of a collection (such as a list or a tuple) and see if there's a member in the list that's equal to the pattern - thus

val = 17
if val in [1,4,5,7,12,14,17,20,34]: print "yes"


will print out "Yes". For looking for an element in a list, this is both quick coding and very efficient at run time, as the check is being done at a 'C level' - in other words, internally to Python without a constant series of reference back to the byte code. However - it IS still a loop.

Consider the following code which checks whether an integer entered by the user in between 0 and 30:

value = int(raw_input("Give me a number between 0 (inclusive) and 30 (exclusive): "))
if value in range(0,30): print "It is in range (1)"
if value < 30 and value >= 0: print "It is in range (2)"
if value in xrange(0,30): print "It is in range (3)"


All three work, and work well. BUT ... if the range we were checking was between 0 and 3 million, it would be a different story. The first test would create a list of 3 million integers internally and would then check our value against each of them - a real memory hog, and likely to break if the number was an order of magnitude higher that 3 million. The third test uses a generator function; not a memory hog, but never the less an internal loop looking at each number from 0 to 2,999,999 in turn and so really slow. The second test remains fast - just two checks to be made, no internal loop, no matter how high the top limit integer is!
(written 2006-08-16)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
Y103 - Python - Conditionals and Loops
  [299] What - no switch or case statement? - (2005-05-03)
  [353] Wimbledon Neck - (2005-06-20)
  [657] The ternary operator in Python - (2006-03-25)
  [668] Python - block insets help with documentation - (2006-04-04)
  [788] New - Conditional expressions in Python 2.5 - (2006-07-01)
  [909] Python is like a narrowboat - (2006-10-30)
  [1201] No switch in Python - (2007-05-23)
  [1477] Decisions - small ones, or big ones? - (2007-12-18)
  [1661] Equality, sameness and identity - Python - (2008-05-31)
  [1696] Saying NOT in Perl, PHP, Python, Lua ... - (2008-07-04)
  [2778] Learning to program in Python 2 ... and / or in Python 3 - (2010-05-24)
  [2899] Groupsave tickets - 3 or 4 train tickets for the price of 2 - (2010-08-02)
  [3083] Python - fresh examples from recent courses - (2010-12-11)
  [3200] How a for loop works Java, Perl and other languages - (2011-03-12)
  [3397] Does a for loop evaluate its end condition once, or on every iteration? - (2011-08-18)
  [3439] Python for loops - applying a temporary second name to the same object - (2011-09-14)
  [3558] Python or Lua - which should I use / learn? - (2011-12-21)
  [3620] Finding the total, average, minimum and maximum in a program - (2012-02-22)
  [3762] Learning to program - the if statement. Python. - (2012-06-12)
  [3895] Flowchart to program - learning to program with Well House - (2012-10-14)
  [4092] Identity in Python - (2013-05-17)
  [4210] If elif elif elif - multiway selection in Python - (2013-11-16)
  [4322] Learning to Program - the conditional statement (if) - (2014-11-21)
  [4323] Learning to program - Loop statements such as while - (2014-11-22)
  [4402] Finding sum, minimum, maximum and average in Python (and Ruby) - (2015-01-19)
  [4541] Setting up and tearing down with the Python with keyword - (2015-10-16)
  [4545] Method, Class, Module, Package - how to they relate in Python? - (2015-10-17)
  [4713] Equality (in Python) - (2016-10-30)
  [4723] Conditional operators in Python - (2016-11-05)


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Build on what you already have with OO
Python - when to use the in operator
Python makes University Challenge
Old Wardour Castle
Displaying data at 5 items per line on a web page
Comparison of Object Oriented Philosophy - Python, Java, C++, Perl
Butterflies in a Wiltshire garden
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This is a page archived from The Horse's Mouth at http://www.wellho.net/horse/ - the diary and writings of Graham Ellis. Every attempt was made to provide current information at the time the page was written, but things do move forward in our business - new software releases, price changes, new techniques. Please check back via our main site for current courses, prices, versions, etc - any mention of a price in "The Horse's Mouth" cannot be taken as an offer to supply at that price.

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