Introduction to Python Usman Roshan

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.

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

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.

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

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

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

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]

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

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()

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

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

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?

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