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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
–
–
–
–
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:
– list1 = [1, 2, 3]
– list2 = [4, 5]
• Operations on lists
– Concatenate: list3 = list1 + list2
– Access nth element: list1[n]
Dictionaries
• Dictionaries are like arrays, except that
they are indexed by user defined keys
instead of non-negative integers.
• Example
– h={‘A’:’T’, ‘C’:’G’}
• Operations on dictionaries
– Lookup: h[key]
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
else:
max = y
Iteration
• for count in range(start,stop,step):
statements
• for item in collection:
do something with item
Exercises
• Write a Python program to
– Measure and print the average length of DNA
sequences in a file
– Determine the number of matches and
mismatches between two DNA sequences of
equal length
– Read a matrix from a text file into a two
dimensional array and determine the largest
number in the matrix
– Select random rows in a two dimensional
array and print them to a file