Lecture 4. while and for loops if else test Tuples Functions. Let us start Python Ssh (putty) to UNIX/Linux computer puccini.che.pitt.

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1 Lecture 4 while and for loops if else test Tuples Functions Let us start Python Ssh (putty) to UNIX/Linux computer puccini.che.pitt.edu Launching Python > python

2 Quick Reminder: while Loop Example >>> i = 0 >>> imax = 10 >>> dx = 0.5 >>> while i < imax : Hints: x = i * dx y = x ** 3 print x, y i += 1 Indentation (# of spaces) is important At the end, i = imax Output: Do not forget to initialize (line i = 0) and update (i += 1) counter

3 Quick Reminder: for Loop Example List A >>> A = range(0,15, 3) >>> print A >>> for i in A : Hints: print i, i** 2 For loop runs over elements of list A Formatting is again important No needs to update index i A[0] A[1] A[2]. A[n-1] Output:

4 Branching: if else test Syntax: if <comparison operator> : <do some smart work> else : # Optional! <do other smart work> if (condition) workflow else Example: Print tabulated function y(x)= exp(x) at y < y 0 >>> from math import * # import math lib >>> A = range (10) x1 >>> y0 = 1000 #cutoff x2 >>> for x in A : at y >= y0 x3 y = exp (x) if y < y0 : print x, y xn f(x1) f(x2) f(x3) f(xn)

5 Branching: if else test (Cont d) break operator Stop loop Example from previous slide >>> A = range (10) >>> y0 = 1000 >>> for x in A : y = exp (x) if else test if y < y0 : print x, y else : break Stop executing loop

6 Other Comparison Operators and, or, or not, and not, is Try this (will return True or False): >>> x = 5; y = 8; z = 17 # assign variables >>> x < y and y < z >>> x > y or y < z >>> not ( x > 0 and y > 0 ) Anther comparison. Example: >>> if 2 in A : else : print yes print no Hint: Any logic comparison, which returns True or False, can be used in if else test

7 Nested Lists Example >>> B = [[1,2], [3,4], [5,6], [7,8]] >>> print B[0] >>> [1,2] >>> print B[2][1] [1, 2] >>> 6 Elements of list B [3, 4] [5, 6] [7, 8] Add elements to nested list >>> print B >>> C = [10,11] B[0] B[1] List B B[2] B[3] >>> B.append(C) #new element appended at end of B >>> print B >>> B.append([20,21]) #this also works

8 Nested Lists: Useful Application Tabulated function as nested list >>> A = range (10); >>> F = [ ]; # creating list of indices, A # creating empty results list >>> for x in A : # loop over list A y = 5 * x + 1 F.append ([x, y]) # appending pair [x,y] to F print F # see how F is populating >>> print F >>> print F[1] >>> print F[1][0] >>> print F[1][1] # print all results # print only 2nd pair [x,y] # print x in 2nd pair # print y in 2nd pair

9 Nested Lists (Cont d) Simultaneous assignment of multiple variables Syntax >>> x0, x1, x2, x3,, xn = y0, y1, y2, y3,., yn Examples: >>> x, y = 1, 2 >>> print x, y Both work >>> x, y = [1,2] >>> B = [ [1,2], [3,4], [5,6] ] >>> a, b = B[0] >>> print a, b >>> x, y = 1, 2, 3 # Error: sizes of lhs & rhs must be same!

10 Nested Lists (Cont d) List comprehension: Creating table Reminder: >>> C = [ 2 * i for i in A ] >>> print C #produce list with elements C[i] = 2*A[i] Creating table A x A all combinations [A[i], A[j] ] >>> V = [ [ i, j ] for i in A for j in A ] >>> print V Reading lines from nested list: for loop >>> B = [ [1,2], [3,4], [5,6], [7,8] ] >>> for a, b in B: print a, b This reads nested elements of list

11 Extracting Part of List Slicing operator : Syntax: >>> A[n : m] # n & m are beginning and end of slice First and/or last boundary index can be omitted Examples: >>> A = range(10) >>> A [ : 3 ] >>> A [ 5 : ] >>> A [ 2 : 6 ] >>> A [ 1: -1 ] # all list except 1 st and last elements Assigning slice as new list >>> A1 = A[ 1 : -1 ] >>> print A1

12 Tuples Tuple is list that cannot be changed Example >>> t = (1, 2, 3, my name, , ) >>> type (t) <type 'tuple'> >>> mylist = [1,2] >>> type (mylist) <type 'list'> Traditionally used as storage of non-homogeneous data Advantages: Write-protected cannot be accidentally changed in loops Code with tuples works faster that that with lists Build-in functionality for variables assignment, printing, etc.

13 Tuples: Syntax Notes Simple syntax: brackets ( ) can be omitted >>> t = (1, 2, 3, my name, , ) >>> t1 = 4, 6, 8 # this also works Access to elements of tuple t [2] t [3] = 5 # ERROR: elements of tuple cannot be modified! Hint: you can re-assign tuple as whole, but cannot change single element Use of tuples Example: >>> a, b = 1, 2 >>> print %d, %d % (a, b) These are tuples

14 Operations with Tuples Nearly same functionality as lists Add two tuples >>> t = (1, 2, 3, my name, , ) >>> t1 = t + (10, 11) >>> print t1 How to create tuple with just one element? >>> t2 = a, # creates tuple with 1 element >>> type( t2 ) <type 'tuple'> >>> t3 = a <type 'str'> Trailing comma identifies t2 as tuple

15 Operations with Tuples How to add just a single element at end of tuple? >>> t1 = t + ( 10 ) # this gives ERROR! >>> t1 = t + ( 10, ) # correct syntax Loop over tuple >>> for elem in 1, 3, my string, 2.65 : print elem Tuple, parentheses omitted

16 Functions: Basics Function Collection of statements that can be called in Python code Functions in math library: sin(x), cos (x), exp(x), sqrt(x), pow(x,y), User-defined functions Function of one variable: Syntax: >>> def f(x) : <computations/operation> return <value>

17 Function of One Variable Example >>> def f(x): a = 1 b = 2 f_value = a * x + b return f_value >>> x = 1 >>> y = f (x) # function call >>> print x, y 1, 3 >>> f(21) # function call 23 Indentation again important function

18 Function of One Variable (Cont d) Input and output can be strings or other objects Example: Operation on string >>> def g ( s ) : s_ret = s +, sir return s_ret >>> g ( yes ) yes, sir >>> g ( no ) no, sir function

19 Global and Local Variables Rule 1: All variables defined in functions are LOCAL Example >>> def f1 ( x ) : coeff = 2 # def. of local variable return coeff * x >>> print f1 (2) >>> print coeff # give ERROR, main part does not # know about a Local variables Created inside function Destroyed when we leave function

20 Global and Local Variables Rule 2: Variables defined in main body are GLOBAL Example >>> myvar = 10 >>> def f2 ( x ) : coeff = 2 * myvar # use of global variable inside f2 return coeff * x >>> print f2 (2) 40

21 Global and Local Variables Global variables can be modified locally (passed by value) Example Init value Temporary redefined value Init value >>> myvar = 10 >>> print myvar 10 >>> def f3 ( x ) : myvar = 2 # variable inside f3 print myvar in f3 =, myvar # print local value return myvar * x >>> f3 (2) myvar in f3 = 2 # output of f3(x) 4 # output of f3(x) >>> print myvar # value returns back to global one 10

22 Global and Local Variables How to change global variable in function? (pass by reference) Declare it as global within function! Example: >>> myvar = 10 >>> def f4 (x) : global myvar # use global keyword myvar = 5 #change value globally return myvar * x >>> f4(1) 5 >>> print myvar #modified by f4(x) 5