Download CS2304: Functions and Control Structures

CS2304: Python for Java Programmers
CS2304:
Functions and Control
Structures
Monti 2014
CS2304: Python for Java Programmers
Functions
•  We’ve already seen the basic hello world
function:
def main():
print(“Hello World”)
Monti 2014
CS2304: Python for Java Programmers
General Function Format
•  Remember indentation is very important, indentation
indicates a block of code.
•  You can’t “overload” functions but Python is flexible
about the number of parameters you actually need to
specify.
def function(parameters):
statement1
statement2
etc…
Monti 2014
CS2304: Python for Java Programmers
Aside: The Pass Statement
•  Lets say you wanted to create an empty block.
•  There aren’t brackets like in Java/C++.
•  This doesn’t work:
def test_function(parameters):
# nothing here
•  However, this works:
def test_function(parameters):
pass
•  pass is a place holder in a function, control
structure, or class.
Monti 2014
CS2304: Python for Java Programmers
The Membership Operator: “in”
•  For lists, tuples, and strings we can test if
something is “in” the collection.
#
l
#
t
#
s
this is a list
= [1, 2, 3, 4]
here’s a tuple
= (1, 2, 3)
and of course a string
= “Hello World!”
Monti 2014
CS2304: Python for Java Programmers
The Membership Operator: “in”
•  For lists, tuples, and strings we can test if
something is “in” the collection.
>>> 1 in l
True
>>> 5 in l
False
>>> 5 not in l
True
>>> ‘H’ in s
True
>>> ‘z’ in s
False
Monti 2014
CS2304: Python for Java Programmers
Range Function
# creates a sequence from 0 to num - 1
range(num)
# creates a sequence from start to end – 1
range(start, end)
# like previous, but skips “steps” numbers
range(start, end, steps)
•  Conceptually you can think of range() as
returning a list of numbers.
# taking a range and creating a list is easy
list(range(num))
•  Aside: in Python 2.X it range actually returned a
list. Now it’s list “like”, but more efficient.
Monti 2014
CS2304: Python for Java Programmers
And Now A For Loop…
•  General form:
for var in collection:
statement(s)
•  A first for loop:
for x in range(10):
print(x)
Monti 2014
CS2304: Python for Java Programmers
And Now A For Loop…
•  Range is fairly flexible, here we print every third
number.
for x in range(0, 10, 3):
print(x)
•  Range also allows for more interesting start/end
points:
for x in range(50, 100):
print(x)
Monti 2014
CS2304: Python for Java Programmers
And Now A For Loop…
•  Using the “in” operator we can also iterate
through lists:
for x in l:
print(x)
•  And tuples:
for x in t:
print(x)
Monti 2014
CS2304: Python for Java Programmers
And Now A For Loop…
•  What about strings?
For x in s:
print(x)
•  This actually works too, iterating through the
characters:
H
e
l
…
Monti 2014
CS2304: Python for Java Programmers
Comparison Operators
•  Python has the standard set of comparison
operators you’ve come to expect:
>>> 1
True
>>> 1
False
>>> 1
False
>>> 1
== 1
!= 1
< 1
> 1
•  You also have => and <= though they aren’t
shown.
Monti 2014
CS2304: Python for Java Programmers
Comparison Operations
•  Conveniently, comparison operators can be
chained:
>>> a = 9
>>> 0 <= a <= 10
True
Monti 2014
CS2304: Python for Java Programmers
Logical Operators
•  Python also has the standard set of logical
operators you’ve come to expect:
>>> True and True
True
>>> True and False
False
>>> False or True
True
>>> 1 and 1
1
•  Instead of symbols the logical operators are
English words.
Monti 2014
CS2304: Python for Java Programmers
After All Of That… If Statements
•  These work as you would expect.
•  General form:
if condition:
statement(s)
•  A first if statement:
x = 1
if x in l:
print(x)
Monti 2014
CS2304: Python for Java Programmers
After All Of That… If Statements
•  If statements using comparison operators…
if x > 10:
x += 1
•  Multiple comparison operators can be combined
using logical operators:
if x > 10 and y < 5:
x -= 1
y += 1
Monti 2014
CS2304: Python for Java Programmers
After All Of That… If Statements
•  There are also the larger multi-way if statements:
if condition:
statement(s)
else:
statement(s)
if condition:
statement(s)
elif condition2:
statement(s)
else:
statements(s)
Monti 2014
CS2304: Python for Java Programmers
The While Loop
•  General form:
while condition remains True:
statement(s)
•  A first while loop:
while x < 10:
print(x)
Monti 2014
CS2304: Python for Java Programmers
The While Loop
•  No do…while loop, so code like this is common:
while(True):
# do something
# when we want to stop the loop
if x > 10:
break
•  There is also a continue statement.
•  Both break and continue can be used in for /
while statements.
Monti 2014
CS2304: Python for Java Programmers
Else Statements With Loops
•  There are else clauses for while and for loops:
while x < 10:
# assume we have some code here
else:
# executed when the condition
# is false or the loop ends
•  The else clause is not executed in the event of an
exception or break statement.
•  These exist for both for / while statements
Monti 2014
CS2304: Python for Java Programmers
FYI: Regarding Conditions
•  An empty string, list, tuple, None (null), and the
number 0 all evaluate to False
# empty list
x = []
if not x:
x += [1]
# None is like null or NULL
if x is None:
x = SomeFunction()
Monti 2014
CS2304: Python for Java Programmers
Aside: Object References
•  Every variable, list, tuple, etc. in python is really
an object reference.
•  Or a “pointer” to an object
•  Some types are immutable:
•  Strings, integers, tuples
•  Whereas, some are mutable:
•  Lists, dictionaries
•  If you change an immutable object, a new object
is created, and the variable name (x) is “pointed”
to the new object.
•  Since every variable is a reference, any variable
can be set to None (null).
Monti 2014
CS2304: Python for Java Programmers
The Identity Operator
•  If you want to test whether two object references
are equal, NOT their values, you may use the “is”
operator:
>>> a = [1, 1, 1]
>>> b = [1, 1, 1]
>>> a is b
False
>>> a = b
>>> a is b
True
•  Testing whether a variable is None is common.
•  Don’t use values with is, weird stuff can happen.
Monti 2014
CS2304: Python for Java Programmers
Exceptions
•  General Form:
try:
# code that can break
statement(s)
except ExceptionType1 as var1:
# handle ExceptionType1
statement(s)
except ExceptionType2 as var2
# handle ExceptionType2
Monti 2014
CS2304: Python for Java Programmers
A More Complex Example
while(True):
try:
l = input(“Give me an int”)
t = int(l)
break
except ValueError as err:
print(“Enter a valid integer.”)
Monti 2014