CS1001.py

Extended Introduction to Computer Science with Python, Noshirvani University, winnter 2015

Python general comments

Course site at http://tau-cs1001-py.wikidot.com
Programming language -> Interpreter -> Machine language
IDLE (editor + interpreter), see site for installation instructions
Interactive mode vs. Script mode
Python version 3.2

print function - prints a textual representation to the console



In [9]:

    
from pprint import pprint
x=['z','a','w']
print(x)
x.sort()
print (x)









    



[1, 10, 5]
[1, 5, 10]



In [36]:

    
print("Hello", "world!")









    



Hello world!

Variables, types

int - integers: ..., -3, -2, -1, 0, 1, 2, 3, ...



In [37]:

    
x=5
y=-3
print(x, type(x))
print(y, type(y))









    



5 <class 'int'>
-3 <class 'int'>



In [38]:

    
x = 5.5
print(type(x))









    



<class 'float'>

float - floating point numbers, decimal point fractions: -3.2, 1.5, 1e-8, 3.2e5



In [39]:

    
x=5.0
y=-3.2
z=2.2e6
print(x, type(x))
print(z, type(z))









    



5.0 <class 'float'>
2200000.0 <class 'float'>

str - character strings, text: "intro2CS", 'python'



In [40]:

    
x = "CS1001.py"
y = 'I love python'
print(x, type(x))
print(y, type(y))









    



CS1001.py <class 'str'>
I love python <class 'str'>



In [41]:

    
print(type(4), type(4.0), type("4"))









    



<class 'int'> <class 'float'> <class 'str'>

bool - boolean values: True and False



In [42]:

    
i_love_python = True
python_loves_me = False
print(i_love_python, type(i_love_python))
print(python_loves_me, type(python_loves_me))









    



True <class 'bool'>
False <class 'bool'>

Operators

Mathematical operators

Addition:



In [45]:

    
4 + 5









    Out[45]:





9



In [46]:

    
x = 5
4 + x









    Out[46]:





9



In [47]:

    
x = 4.0 + 5
print(x, type(x))









    



9.0 <class 'float'>

Subtraction:



In [48]:

    
x - 3









    Out[48]:





6.0

Multiplication:



In [49]:

    
x * 3









    Out[49]:





27.0

Division - float and integral with / and //:



In [50]:

    
10 / 3, 10 // 3









    Out[50]:





(3.3333333333333335, 3)

Power:



In [52]:

    
2 ** 3, 2 ** 3.0, 3 ** 2









    Out[52]:





(8, 8.0, 9)

Modolu:



In [53]:

    
10 % 3









    Out[53]:





1

String operators

String concatenation using +:



In [55]:

    
"Hello" + " World"









    Out[55]:





'Hello World'

String duplication using *:



In [58]:

    
"Bye" * 2









    Out[58]:





'ByeBye'

Strings vs. numbers:



In [59]:

    
4 + 5









    Out[59]:





9



In [60]:

    
"4" + "5"









    Out[60]:





'45'



In [61]:

    
"4" + 5









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-61-f945f8c7e111> in <module>()
----> 1 "4" + 5

TypeError: Can't convert 'int' object to str implicitly



In [62]:

    
4 + "5"









    



---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-62-871c0c3bbca2> in <module>()
----> 1 4 + "5"

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Comparisons



In [63]:

    
5 < 4









    Out[63]:





False



In [64]:

    
5 > 4









    Out[64]:





True



In [65]:

    
5 >= 4









    Out[65]:





True



In [66]:

    
4 >= 4









    Out[66]:





True



In [73]:

    
4 <= 3









    Out[73]:





False



In [67]:

    
5 == 4









    Out[67]:





False



In [68]:

    
5 == 5.0









    Out[68]:





True



In [69]:

    
5 == "5"









    Out[69]:





False



In [70]:

    
5 != 4









    Out[70]:





True



In [71]:

    
2 + 2 == 4









    Out[71]:





True



In [72]:

    
2 => 3









    



  File "<ipython-input-72-76c8f045e4cf>", line 1
    2 => 3
       ^
SyntaxError: invalid syntax



In [77]:

    
x = 1 / 3 
print(x)
x == 0.3333333333333333









    



0.3333333333333333






    Out[77]:





True

Logical operators

not:



In [83]:

    
print(not True)
a = 2 == 5
print(not a)









    



False
True

and:



In [84]:

    
True and True









    Out[84]:





True



In [85]:

    
True and False









    Out[85]:





False



In [86]:

    
False and False









    Out[86]:





False



In [87]:

    
True or True









    Out[87]:





True



In [88]:

    
True or False









    Out[88]:





True

Conversions

Use the functions int(), float(), and str() to convert between types (we will talk about functions next time):



In [91]:

    
x = "6"
print(x, type(x))
x = int("6")
print(x, type(x))









    



6 <class 'str'>
6 <class 'int'>



In [92]:

    
float("1.25")









    Out[92]:





1.25



In [93]:

    
str(4)









    Out[93]:





'4'



In [94]:

    
int("a")









    



---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-94-91097a4105a2> in <module>()
----> 1 int("a")

ValueError: invalid literal for int() with base 10: 'a'



In [98]:

    
course = "intro" + str(2) + "cs"
print(course)
print("intro", 2, "cs", sep='')









    



intro2cs
intro2cs

Flow control

Conditional statements

The if condition formula - replace conditions and statements with meaningful code:

if *condition*:
    *statement*
    *statement*
    ...
elif *condition*: # 0 or more elif clauses
    *statement*
    *statement*
    ...    
else:             # optional
    *statement*
    *statement*

Example:



In [101]:

    
today = "Monday"
strike = "N"
my_recitation = "Monday"

if today == "Sunday":
    print("Shvizut Yom Alef")
    if strike == "Y":
        print("Stay home")
    else:
        print("Lecture in intro to CS!")
elif today == "Wednesday":
    print("Another lecture in intro to CS!")
elif today == my_recitation:
    print("Go to recitation!")
elif today == "Monday" or today == "Tuesday" or today == "Thursday" or \
                    today == "Friday" or today == "Saturday":
    print("no intro to CS")
else:
    print("Not a day")









    



Go to recitation!

Loops

While:

while *condition*:
    *statement*
    *statement*

Example - count how many times 0 appears in an integer number:



In [102]:

    
num = 2**100
print(num)









    



1267650600228229401496703205376



In [103]:

    
count = 0

while num > 0:   #what if we changed to >=0?
    if num % 10 == 0:
        count = count + 1
    num = num // 10

print(count)

For:

for *variable* in *iterable*:
    *statement*
    *statement*

Example - solve the same problem with a str type instead of int:



In [105]:

    
num = 2**100
count = 0
for digit in str(num):
    #print(digit, type(digit))
    if digit == "0":
        count = count + 1

print(count)

Builtin solution:



In [106]:

    
num = 2**100
count = str.count(str(num), "0")

print(count)

Efficiency

We can measure which solution is faster:



In [108]:

    
%%timeit
num = 2**100
count = 0
while num>0:   #what if we changed to >=0?
    if num % 10 == 0:
        count = count + 1
    num = num // 10









    



10000 loops, best of 3: 37.4 us per loop



In [109]:

    
%%timeit 
num = 2**100
count = 0
for digit in str(num):
    if digit == "0":
        count = count + 1









    



100000 loops, best of 3: 8.76 us per loop



In [110]:

    
%%timeit 
num = 2**100
count = str.count(str(num), "0")









    



100000 loops, best of 3: 2.82 us per loop

The builtin solution is 4 times faster than the for solution which is 3 times faster than the while solution.

Other notes

The while solution will not work for num <= 0
The while solution will not work for non-numerals (e.g, num = "Cola 0 is awesome!")
The builtin solution is implemented with C and that is why it is faster

Fin

This notebook is part of the Extended introduction to computer science course at Tel-Aviv University.

The notebook was written using Python 3.2 and IPython 0.13.1.

The code is available at https://raw.github.com/yoavram/CS1001.py/master/recitation1.ipynb.

The notebook can be viewed online at http://nbviewer.ipython.org/urls/raw.github.com/yoavram/CS1001.py/master/recitation1.ipynb.

The notebooks is also available as a PDF at https://github.com/yoavram/CS1001.py/blob/master/recitation1.pdf?raw=true.

This work is licensed under a Creative Commons Attribution-ShareAlike 3.0 Unported License.