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lab38
and,or

Yesterday, we walked through examples of using and and or:

// Return the RU Username, except check for Jay Z who gets a special honorific.
//
def ruUserName( firstName, lastName ):
  if firstName == "Jay" and lastName = "Z":
    return "hisroyalflyness"
  else:
    firstInit = substring(firstname,0,1)
    return string.lower( firstInit + lastName )

// Charge $4.95 for shipping, unless the order is worth more than $100 total,
// or it has more than 50 items (in which case shipping is free).
// 
def shippingCharge( numItems, unitPrice ):
  if (numItems*unitPrice > 100  or  numItems > 50):
    return 0.00
  else:
    return 4.95

The precise semantics for and, or, not are what you'd expect; here they are in table form ¹:

    
      False and False  ==  False
      False and True   ==  False
      True  and False  ==  False
      True  and True   ==  True  
    
    
      False or False  ==  False
      False or True   ==  True  
      True  or False  ==  True  
      True  or True   ==  True  
    
    
      not False  ==  True  
      not True   ==  False

Note that you can actually pass/return the boolean values True and False to/from functions (just like you can pass/return particular numbers, or particular strings).

TASKS: we will check off the following two items:

Complete Codingbat.com > Python > Warmup > not_string.
Your solution must use our version of substring, where the programmer must ensure they aren't asking for characters that don't exist!

# substring : string, int, int → string
#   Given some text, a start-index, and a stop-index,
#   return the characters of `txt` from `start` up until (but not including) `stop`.
#   Indices are 0-based.
#   Examples:   substring( "radford", 3, 6) = "for"
#               substring( "radford", 0, 3) = "rad"
#
def substring(txt, start, stop):
  if (0 <= start  and  start <= stop  and  stop <= len(txt)):
    return txt[start:stop]
  else:
    raise IndexError("substring: bounds out of range: " + str(start) + "," + str(stop) + " for '" + txt + "'")

Complete Codingbat.com > Python > Logic-1 > sorta_sum.
Self-challenge (not to be checked off, at least not today):
in ruUserName, our code still doesn't quite use the actual university policy: if the last name is more than 8 characters, we'll only use the first 8.
Update ruUserName to correctly handle this situation (while preserving Jay Z's special case).
(Be sure to include substring as above, and also import string so that you can use string.find.)

Further self-challenge: Handle the case where somebody doesn't have a first name (in which case the empty-string will be passed in ²).

¹If you replace False,True with 0,1, what arithmetic function is similar to and? To or? …To not? ↩

² Technically, I'd split hairs over the distinction of "not having a first name" vs. "having a first name which is zero letters long". Granted, this isn't actually a distinction with people's names, but consider the situation where you look up a (last)name in a database: there is a difference between Sting not being in the database, vs. Sting being there but not having a last name.

A full programmer might say that “no last name” is represented by False, while a string (even the empty-string) represents a known last-name. Using False like this is called a “sentinel value”.

↩

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©2014, Ian Barland, Radford University
Last modified 2014.Apr.21 (Mon) Please mail any suggestions
(incl. typos, broken links)
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lab38 and,or

lab38
and,or