Transcript CSC598BIL675-2016

Basic operators - strings
• Python supports concatenating strings using the addition operator:
#!/usr/bin/python
helloworld = "hello" + " " + "world”
print helloworld
• Python also supports multiplying strings to form a string with a repeating
sequence:
lotsofhellos = "hello" * 10
print lotsofhellos
Basic operators - lists
• List can be joined by addition operator:
#!/usr/bin/python
even_numbers = [2,4,6,8]
odd_numbers = [1,3,5,7]
all_numbers = odd_numbers + even_numbers
print all_numbers
• Python also supports multiplying strings to form a string with a repeating
sequence:
print [1,2,3] * 3
String formatting
• Python uses C-style string formatting to create new, formatted strings.
#!/usr/bin/python
name = "John"
print "Hello, %s!" % name
% is the operator to format a set of variable in a tuple
ame = "John"
age = 23
print "%s is %d years old." % (name, age)
mylist = [1,2,3]
print "A list: %s" % mylist
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%s … string
%d … integer
%f … float
%.<N>f … <N> is precision after comma
tuple
String operations
#!/usr/bin/python
astring = “Hello world!”
print astring
print len(astring)
returns length of astring
print astring.index("o") returns the index of o in astring
print astring.count("l")
returns how often l occurs in astring
print astring.split(”o")
splits astring at o’s into lists of strings
print astring[3:7]
returns the 3rd to the 6th letter of astring
print astring.upper(), astring.lower()
returns all letters of astring as upper/lower case
print astring.startswith("Hello")
returns true/false if astring starts with “Hello”
print astring.endswith("Hello")
returns true/false if astring ends with “Hello”
Conditions
• Python uses boolean variables to evaluate conditions. The boolean values
True and False are returned when an expression is compared or evaluated:
x=2
print x == 2 # prints out True
print x == 3 # prints out False
print x < 3 # prints out True
• The "and" and "or" boolean operators allow building complex boolean
expressions:
#!/usr/bin/python
name = "John"
age = 23
if name == "John" and age == 23:
print "Your name is John, and you are also 23 years old."
if name == "John" or name == "Rick":
print "Your name is either John or Rick."
in, if, is, not
• The "in" operator could be used to check if a specified object exists within
an iterable object container, such as a list.
#!/usr/bin/python
name = “John”
if name in ["John", "Rick"]:
print "Your name is either John or Rick.”
• if statement
if <statement is true>:
<do something>
....
....
elif <another statement is true>: # else if
<do something else>
....
....
else:
<do another thing>
in, if, is, not
#!/usr/bin/python
x=2
if x == 2:
print “x equals 2!”
else:
print “x does not equal 2!”
• Unlike the double equals operator "==", the "is" operator does not
match the values of the variables, but the instances themselves.
#!/usr/bin/python
x = [1,2,3]
y = [1,2,3]
print x == y prints out True
print x is y prints out False
• not inverts a boolean expression
#!/usr/bin/python
print not False
prints out True
print (not False) == (False) prints out False
loops
• for loops iterate over a given sequence.
#!/usr/bin/python
primes = [2, 3, 5, 7]
for prime in primes:
print prime
• for loops can iterate over a sequence of numbers using the range and
xrange functions #!/usr/bin/python
for x in xrange(5): # or range(5)
print x
# Prints out 3,4,5
for x in xrange(3, 6): # or range(3, 6)
print x
# Prints out 3,5,7
for x in xrange(3, 8, 2): # or range(3, 8, 2) prints in range 3-8 in steps of 2
print x
primes = [2, 3, 5, 7]
for prime in primes:
print prime