Date post: | 22-Dec-2015 |
Category: |
Documents |
View: | 216 times |
Download: | 3 times |
4/15/04 Introduction to Python 2
Overview Overview of program development Python Basics Python Types and Operators
Numbers and Arithmetic operators Strings Lists Dictionaries
Input & Output Example amino acid search program Programming Workshop #1
4/15/04 Introduction to Python 3
Programming Language and Development Software In this program, we’ll use Python
Interpretive Language Development software
IDLE python gui Pythonwin (recommended)
Do your work on either the hard disk or zip disk (not floppy disk, A: drive – too slow!)
4/15/04 Introduction to Python 4
Program DevelopmentProblem specification
Algorithm design
Test by hand
Code in target language
Test code / debug
Program
Problem solving
Implementation
4/15/04 Introduction to Python 5
Python Basics - Comments Python comments
# line comment Header comments
#Description of program#Written by:#Date created:#Last Modified:
4/15/04 Introduction to Python 6
Python Basics - Variables Python variables are not “declared”.
To assign a variable, just type: identifier=literal Identifiers
Have the following restrictions: Must start with a letter or underscore (_) Case sensitive Must consist of only letters, numbers or underscore Must not be a reserved word (LP pg 137)
Have the following conventions: All uppercase letters are used for constants Variable names are meaningful – thus, often multi-word
Convention 1: alignment_sequence Convention 2: AlignmentSequence
Python specific conventions: Avoid _X, __X__, __X, _, (LP pg 138)
4/15/04 Introduction to Python 7
Numbers Numbers
Normal Integers –represent whole numbers Ex: 3, -7, 123, 76
Long Integers – unlimited sizeEx: 9999999999999999999999L
Floating-point – represent numbers with decimal places
Ex: 1.2, 3.14159,3.14e-10 Octal and hexadecimal numbers
Ex: O177, 0x9ff, Oxff Complex numbers
Ex: 3+4j, 3.0+4.0j, 3J
4/15/04 Introduction to Python 8
Python Basics – arithmetic operations
+ add- subract* multiply/ divide% modulus/remainder
y=5; z=3x = y + z x = y – z x = y * z x = y / z x = y % z
x = 8x = 2x = 15x = 1x = 2
OperatorsExample
4/15/04 Introduction to Python 9
Python Basics – arithmetic operations
<< shift left
>> shift right** raise to power
y=5; z=3x = y << 1 x = y >> 2 x = y ** z
x = 10x = 1x = 125
OperatorsExample
4/15/04 Introduction to Python 10
Python Basics – Relational and Logical Operators
Relational operators== equal!=, <> not equal>greater than>= greater
than or equal
<less than<= less than or
equal
Logical operatorsand andor ornot not
4/15/04 Introduction to Python 11
Python Basics – Relational Operators Assume x = 1, y = 4, z = 14
Expression Value Interpretation
x < y + z 1 True
y == 2 * x + 3
0 False
z <= x + y 0 False
z > x 1 True
x != y 1 True
4/15/04 Introduction to Python 12
Python Basics – Logical Operators Assume x = 1, y = 4, z = 14
Expression Value Interpretation
x<=1 and y==3 0 False
x<= 1 or y==3 1 True
not (x > 1) 1 True
not x > 1 0 False
not (x<=1 or y==3)
0 False
4/15/04 Introduction to Python 13
Enclosed in single or double quotesEx: ‘Hello!’ , “Hello!”, “3.5”, “a”, ‘a’
Sequence of characters:mystring=“hello world!”
mystring[0] -> “h” mystring[1] -> “e”
mystring[2] -> “l” mystring[-1] -> “!”
Strings
-1 is last,
-2 next to last, etc…
4/15/04 Introduction to Python 14
String operations
mystring = “Hello World!”
Expression Value Purposelen(mystring) 12 number of characters in
mystring
“hello”+“world” “helloworld” Concatenate strings
“%s world”%“hello” “hello world” Format strings (like sprintf)
“world” == “hello”
“world” == “world”
0 or False
1 or True
Test for equality
“a” < “b”
“b” < “a”
1 or True
0 or False
Alphabetical ordering
4/15/04 Introduction to Python 15
Strings (2) substrings can be reassigned:mystring=“spoons”mystring[5]=“!”mystring -> “spoon!”
slicing:mystring[2:] -> “oon!”mystring[:3] -> “spo” #note last element is never included!
mystring[1:3]-> “po”
4/15/04 Introduction to Python 16
Strings (3) “%” operator:
sort of “fill in the blanks” operation:mystring=“%s has %n marbles” % (“John”,35)
mystring -> “John has 35 marbles”
%s replace with string %n,%i replace with integer %f replace with float
Values to put in blanks
“blanks”
4/15/04 Introduction to Python 17
Lists
mylist=[“a”,”b”,3.58,”d”,4,0]mylist[0]mylist[2]
a3.58
Indexing
mylist[-1]mylist[-2]
04
Negative indexing (counts from end)
mylist[1:4] [“b”,3.58,”d”] Slicing (like strings)
“b” in mylist“e” not in mylist
1 or True1 or True
mylist.append(8) [“a”,”b”,3.58,”d”,4,0,8]
Add to end of list
4/15/04 Introduction to Python 18
Tuples Tuples – sequence of values
like lists, but cannot be changed after it is createdmytuple=(1,2,3,4)mytuple=(1,”a”,”bc”,3,87.2)mytuple[1]=“3”
Used when you want to pass several variables around at once
Error!
4/15/04 Introduction to Python 19
Dictionaries Dictionaries – map ‘keys’ to ‘values’
like lists, but indices can be of any type Also, keys are in no particular order Eg:mydict={‘b’:3, ’a’:4, 75:2.85}mydict[‘b’] -> 3mydict[75] -> 2.85mydict[‘a’] -> 4
4/15/04 Introduction to Python 20
Dictionaries
mydict={“r”:1,”g”:2,”y”:3.5,8.5:8,9:”nine”}mydict.keys() ['y', 8.5, 'r', 'g', 9] List of the keys
mydict.values() [3.5, 8, 1, 2, 'nine'] List of the values
mydict[“y”] 3.5 Value lookup
mydict.has_key(“r”) True or 1 Check for keys
mydict.update({“a”:75})
{8.5: 8, 'a': 75, 'r': 1, 'g': 2, 'y': 3.5, 9: 'nine'}
Add pairs to dictionary
4/15/04 Introduction to Python 21
Dictionaries – other considerations Slicing not allowed Referencing invalid key is an error:>>> mydict={8.5: 8, 'a': 75, 'r': 1, 'g': 2, 'y':
3.5, 9: 'nine'}>>> mydict["red"]Traceback (most recent call last):
File "<interactive input>", line 1, in ?KeyError: 'red‘
Use mydict.get(“red”) instead, it returns None if key is not found
4/15/04 Introduction to Python 22
Input/Output Function raw_input() designed to read a line of
input from the user 1 optional argument: string to prompt user If int or float desired, simply convert string:
int(mystring)->convert to int (if possible)
float(mystring)->convert to float (if possible)
>>> mystr=raw_input("Enter a string:")Enter a string:Hello World!>>> mystr'Hello World!'
4/15/04 Introduction to Python 23
Output Function print
Prints each argument, followed by space
After all arguments, prints newline
Put comma after last arg to prevent newline
“add” strings to avoid spaces
print “a”,”b”,”c”a b c
print “a”,”b”,”c”,a b c
print “a”+”b”+”c”abc
Newline!
No Newline!
No spaces!
4/15/04 Introduction to Python 24
Output Example>>> print "hello","world";print "hello","again"
hello world
hello again
>>> print "hello","world",;print "hello","again"
hello world hello again
>>> print "hello %s world" % "cold and cruel"
hello cold and cruel world
>>> print "hello","cold"+ " " + "and","cruel","world"
hello cold and cruel world
4/15/04 Introduction to Python 25
Creating a Python Program Enter your program in the editor
Notice that the editor has a color coding Comments Key words Etc…
Also notice that it automatically indents Don’t override!! – this is how python tells when
block statements end! If doesn’t indent to proper location – indicates bug
4/15/04 Introduction to Python 26
Running your Program To build your program
Under File->Run… Select No Debugging in the drop-down
window Fix any errors, then run again
4/15/04 Introduction to Python 27
Programming Workshop #1
Write a Python program to compute the hydrophobicity of an amino acid
Amino Acid Hydrop. VALUEA 1.8C 2.5D -3.5E -3.5F 2.8G -0.4H -3.2I 4.5K -3.9L 3.8M 1.9N -3.5P -1.6Q -3.5R -4.5S -0.8T -0.7V 4.2W -0.9Y -1.3
Program will prompt the user for an amino acid and will display the hydrophobicity