This is the code for python 2. There are some lines that do not work with python 3 because of the grammar changes.
adapted from ‘data_containers.ipynb’ by Eric Firing at Univ. of Hawaii
Let’s start with numbers.
Python’s system of numbers is quite simple: there are
True or False) typesThe types are determined dynamically, so operations involving integers and floating point numbers yield floating point numbers.
print 1 + 2
print 1.0 + 2
print(1 + 2)
3
3.0
3
Division can be tricky because traditional computer languages use integer division on integers. This was the case by default with Python 2.
print 4/3
print 4.0/3
1
1.33333333333
There are some built-in functions that operate on numbers, e.g.:
print int(4/3)
print round(4/3)
print abs(4.2 - 5) # Note binary floating point inability
# to represent a decimal number exactly.
print pow(2, 3)
1
1.0
0.8
8
The pow() function can be replaced by
print 2**3
8
For more math functions, one can import the math module from the standard library:
import math
print(math.sin(1))
print(math.sqrt(2))
0.841470984808
1.41421356237
We will rarely use the math module, however, because numpy provides the same functionality and much more.
Boolean values are either True or False, and result from conditional expressions like this:
print("1 > 2 is", 1 > 2, "but 1 < 2 is", 1 < 2)
('1 > 2 is', False, 'but 1 < 2 is', True)
Here is a more complex conditional expression:
print(1 > 2 or 3 < 4)
True
Python strings can be created with the str() function, and as literals using single quotes, double quotes, triple single quotes, or triple double quotes. The triples can enclose blocks of text spanning multiple lines. The following are all valid strings, each assigned to a variable name:
a = 'Single quotes'
b = "Double quotes"
c = "a string that has 'single quotes' inside it"
d = """This is a multiline
sentence string, ending with a linefeed.
"""
e = '''
This is also valid. It starts and ends with a linefeed.
'''
for example in (a, b, c, d, 3):
print(example)
Single quotes
Double quotes
a string that has 'single quotes' inside it
This is a multiline
sentence string, ending with a linefeed.
3
Strings can be added (concatenated):
print(d + e)
This is a multiline
sentence string, ending with a linefeed.
This is also valid. It starts and ends with a linefeed.
Tuples and lists are very general sequences–that is, they are containers that preserve order, and they can contain any kind of object at all. There is one big practical difference: tuples are immutable, lists are mutable.
To create a tuple from scratch, use round parentheses and commas.
t1 = (1, 2)
t2 = (3, (4, 5), 7, 8, "some string")
print t1
print t2
print t1+t2
print t1*2
(1, 2)
(3, (4, 5), 7, 8, 'some string')
(1, 2, 3, (4, 5), 7, 8, 'some string')
(1, 2, 1, 2)
Lists have many methods, and support addition and multiplication:
a = ["list1", 1, 2]
b = ["list2", 3, 4]
print a + b
print a * 2
c = a.extend(b) # 'extend' adds elements to the end (assigning to c only to suppress printing)
print a # This is identical to the sum, a + b, above.
c = a.append(b) # 'append' addes the argument as a whole
print a
['list1', 1, 2, 'list2', 3, 4]
['list1', 1, 2, 'list1', 1, 2]
['list1', 1, 2, 'list2', 3, 4]
['list1', 1, 2, 'list2', 3, 4, ['list2', 3, 4]]
Lists, tuples, and strings all support the same indexing syntax.
# Using the built-in "range" function,
# make a list on which we can practice indexing:
x = list(range(0, 100, 10))
print(x)
print range(0,100,10)
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]
[0, 10, 20, 30, 40, 50, 60, 70, 80, 90]
Take the first 5 values, then the last 5 values:
print(x[:5])
print(x[-5:])
[0, 10, 20, 30, 40]
[50, 60, 70, 80, 90]
See how easy that was? Now take every second value starting from the second (index 1):
print x[1::2]
[10, 30, 50, 70, 90]
[40, 30, 20, 10, 0]
In the above examples we are indexing a list with a slice, so we are getting a list back, even if it has only one element in it, or if it is empty. If we want to get an element from the list, then we index with a single integer:
print(x[0])
print(x[-1])
0
90