1. Introduction to Python
Usman Roshan

2. Python
Interpreter language: the Python interpreter executes each statement one at a time.
No compilation or linking is required.
Python programs are called scripts which are simple text files. To execute a script we pass it through the interpreter.

3. Data types Basic data types Collections Integer Float String
Boolean (true or false)
Collections
Lists
Dictionaries (hash tables)
Sets
And more…

4. Expressions Numeric operators such as +, -, *, /, **
Logical operators such as and and or
String operators such as in, +, [], [:]
String and numeric functions such as len, abs, max, and min.

5. Assignment
name = value
Examples:
sequence = “ACCCATA”
x = 5
y = x + 2

6. Lists Ordered collection of basic types Examples: Operations on lists
Concatenate: list3 = list1 + list2
Access nth element: list1[n]

7. Lists Time to lookup, insert, and delete
We care about the worst case runtime for now
We will write runtimes as a function of the input size
The worst case runtime to lookup an item is the length of the list. But the time to lookup the ith item is actually constant. If we say print(l[i]) where l is a list then this takes constant time.
The worst case runtime to insert an item into the list is the length of the list
The worst case runtime to delete an item from the list is the length of the list

8. Dictionaries
Dictionaries are like arrays, except that they are indexed by user defined keys instead of non-negative integers.
They are also called hash-tables
Example
h={‘A’:’T’, ‘C’:’G’}
Operations on dictionaries
Lookup: h[key]

9. Hash-tables Time to lookup, insert, and delete
We will write runtimes as a function of the input size
The expected case runtime to lookup an item is constant
The expected case runtime to insert an item into the list is constant
The expected case runtime to delete an item from the list is constant

10. Input and Output
Input and output to files is done by creating a file object
open(path, mode)
Example:
f = open(“myfile.txt”,”r”)
f.readline()

11. Control structures Conditional statements if expression : statements1
else:
statements2
Example
max = 0
if(x > y):
max = x
max = y

12. Iteration for count in range(start,stop,step): for item in collection:
statements
for item in collection:
do something with item

13. Basic algorithmic techniques
In mainstream CS one is taught:
Dynamic programming
Greedy algorithms
Divide-and-conquer
In data science and machine learning we use coordinate and gradient descent extensively
What is coordinate and gradient descent?

14. Runtime analysis
Theoretical analysis: How long does it take for an algorithm to finish as a function of input size?
Real analysis: runtime in seconds depending upon machine