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Intro to Python
Paul Martin
History
• Designed by Guido van Rossum
• Goal: “Combine remarkable power with very
clear syntax”
• Very popular in science labs
– "scientists often need to improvise when trying to
interpret results, so they are drawn to dynamic
languages which allow them to work very quickly
and see results almost immediately.“
• Guido van Rossum
Compiling and Comments
• Start Python by typing Python
– or Python myFileName
• # is symbol to comment line
– Can be placed on it’s own line or after code
Variable Assignment
• ‘=‘ is the assignment operator
• Can apply = operator to many vars at one time
– ex. x = y = z = 0
Variable Assignment Cont.
• No identifier required
– height = 5
• To declare a floating point variable
– width = 3.3
• Can declare a variable with an equation
– ex. area = width*height #width,height def above
Complex Numbers
• CmpNum = real + imagj
– ex. Cmpnum = 2.0 + 1j
• Access real numbers via Cmpnum.real
• Access imaginary numbers via Cmpnum.imag
• Magnitude can be found by abs(Cmpnum)
Strings
• Can be declare with single or double quote
– “Hello World” or ‘Hello World’
• Can use quote marker inside string when with
the escape character ‘\’
– ex. ‘This isn\’t the end’
• Can also use the other type of quote marker
– ex. “This isn’t the end”
String Concatenation and Length
• Word = ‘str’ + ‘ing’
• Word = ‘str’ ‘ing’
• Length = len(Word)
– For the above word, length will be equal to 6
String Indexing
• Start from index 0
• Word = ‘HelpA’
–
–
–
–
–
–
–
Word[4] = ‘A’ –letter at index 4
Word[0:2] = ‘He’ –letters index 0 <= x < 2
Word[0:100] = ‘HelpA’
Word[:2] = ‘He’ –letters index x < 2
Word[2:] = ‘lpA’ –letters index x > 2
Word[3:1] = ‘’
Accessing in reverse order via the negative sign
• Word[-2] = ‘p’
Lists
• List = [‘a’,’b’,2,3]
• Index Starts from 0
• Accessing Elements
– List[1] = ‘b’
– List[2:4] = [2, 3]
• Lists can be nested
– List2 = [1,List,2]
• List length accessed from len(List)
Mutating Lists
• Lists can be changed
– List[3] = List[3] + 40
• Elements can be removed/inserted
– Insert
• List[1:1] = [‘aa’,’ab’]
– Remove
• List[0:2] = []
If Statements
• If x == 0:
Do Something
elif x == 1:
Do Something Else
else:
Do Anything Else
• To end statement press enter on an empty line
While and Pass Statements
• while statement same function as C
• pass statement does nothing
– Used when program requires no action
• while x == 5:
pass
#this program will continue as long as x = 5 and
#user does not interrupt
Range Statements
• Useful for performing a loop x amount of
times
• range(10)
– creates list [0,1,2,3,4,5,6,7,8,9]
• range(5,10)
– creates list [5,6,7,8,9]
• range(0,20,5)
– Creates list [0,5,10,15]
For Statements
• Iterates over items of a sequence
• for x in a:
print x
• break and continue statements same as C
– break – exit nearest loop
– continue – continue to next iteration of loop
For Else Statements
• Executed if loop not terminated by break
• for n in range(2, 10):
if n == 11
print ’11 found’
break
else:
print ‘no 11 found’
Function Definitions
• def fun(m):
if m == 1:
print ‘m = 1’
• fun(1)
– prints ‘m = 1’
• fun(2)
– prints nothing
References
• http://python.org/
• http://www.cs.drexel.edu/~knowak/cs265_fall
_2010/cs_265_links.htm
• http://www.infoworld.com/d/developerworld/7-programming-languages-the-rise-620