Oct 6
homework
First, a discussion the functions homework,
both some of my examples and looking at your submissions.
finish discussion of booleans
After finishing our class notes from last time
we'll continue with these.
the while loop
x = 10
while x > 1:
print "x is", x
x = x - 1
coding truth tables
One of the assignments for next week is like this.
Now let's write in class a program to loop through assignments
of (True, False) to (x,y) to see which (if any) of these are related.
(We talked about this in class on Tuesday - now let's write
some code that does that, rather than doing it by hand on the board.)
(not x) and (not y) # which of these are the same ?
not (x and y)
not (x or y)
(not x) or (not y)
Here's the code that I showed in class to do this :
formatline = " {:6} {:6} {:16} {:16} "
print formatline.format('a', 'b',
'not (a and b)',
'(not a) or (not b)')
print formatline.format('-'*6,
'-'*6,
'-'*16,
'-'*16)
for a in (True, False):
for b in (True, False):
print formatline.format(str(a), str(b),
str(not (a and b)),
str((not a) or (not b))
)
which outputs this :
a b not (a and b) (not a) or (not b)
------ ------ ---------------- ----------------
True True False False
True False True True
False True True True
False False True True
tidbits
The end of Tuesday's notes mentions a few more obscure
corners of conditionals (ternary one line forms,
and "shortcut" expressions - I'll point at them briefly
for those who are interested, and then let any of
you who are interested explore them yourselves.
try, catch
Discuss errors and how to handle them in Python :
try:
x = int(raw_input("What is x? "))
except ValueError:
print "You must enter a number."
This is like an (if: ... else: ...) conditional construct,
but here the branch happens when a python error takes place.
We say that the error is "caught" in the code, instead
being printed out and stopping the program.
To figure out what to put after the "except" part, do something
at the interactive prompt to make the error happen.
Sometimes this is called "throwing" and exception and "catching" it.
Class exercise : re-use the try/except code above
as the basis for a function get_integer() which asks the
user for an integer, and then keeps asking until
an integer is actually typed.
practice
Work some problems in class. While they're pretty math-geeky,
this site has many good programming problems :
A few good ones to start:
- 1 "Find the sum of all the multiples of 3 or 5 below 1000."
- 4 "Find the largest palindrome made from the product of two 3-digit numbers."
- 14 "Collatz numbers" (google xkcd collatz)
- 55 "An investigation of Lychrel numbers."
