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lab36
if-else statements, cont.

We'll start by discussion exam01.

Last time we talked about boolean expressions (and how operators like == (equal), >= (greater-or-equal), < (less-than), != (not-equal), etc. return a boolean (T/F) value).

Although JES treats booleans as 0 or 1 they're not — actual python uses False and True (note the upper-case) and these are different from ¹ 0 and 1.

For the Codingbat.com > Python > Warmup-1 > sum_double, we talked about four ways to compute the answer:

Checking the equal-inputs case first:

def sum_double(a, b):
  if (a == b):
    return 2*(a+b)
  else:
    return a+b

Checking the non-equal-inputs case first:

def sum_double(a, b):
  if (a != b):
    return a+b
  else:
    return 2*(a+b)

Instead of returning a value, assigning to a variable:

def sum_double(a, b):
  if (a!=b):
    answer = a+b
  else:
    answer = 2*(a+b)
  return answer

Use a variable to avoid repeated code:

def sum_double(a, b):
  sum = a+b
  if (a!=b):
    return sum
  else:
    return 2*sum

Note that none of these are the same as CodingBat's Show Solution button. That's because their solution violates two good practices, which we will use in this class:

No one-armed ifs: That is, every if must have an else².
Don't re-assign to a variable (with the exception of a loop's accumulator (so-far) variable or its index-variable). If you did this, it would mean (at the very least) that the variable's name is inaccurate at least half the time.

Task: We will work through the following codingbat problems:

CodingBat > Python > Warmup-1 > front3
For this one, we'll use an improved version of substring:

# 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 + "'")

CodingBat > Python > Warmup-1 > not_string
CodingBat > Python > Logic-1 > ~~sorta_sum~~ (We will check this one off in a future lab, after talking about and,or (and, nested-ifs).)

¹However, int(False) == 0 and int(True) == 1 ↩

²Even if it's … else: None, including the else helps avoid the common bug of forgetting to handle the other case. ↩

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©2014, Ian Barland, Radford University
Last modified 2014.Apr.10 (Thu) Please mail any suggestions
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lab36 if-else statements, cont.

lab36
if-else statements, cont.