Lecture 8: ES 102 Functions, Searching and Sorting

Transcription

1 Lecture 8: ES 102 Functions, Searching and Sorting Introduction to Computing IIT Gandhinagar, India Oct 9th, 2013 Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

2 Revising Functions def f u n c t i o n n a m e ( p a r a m e t e r s ) : " function_docstring " f u n c t i o n s u i t e return [ e x p r e s s i o n ] Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

3 Function Example # F u n c t i o n d e f i n i t i o n i s h e r e def p r i n t m e ( s t r ) : " This prints a passed string into this function " print s t r ; return ; # Now you can c a l l p r i n t m e f u n c t i o n p r i n t m e ( "I m first call to user defined function!" ) ; p r i n t m e ( " Again second call to the same function " ) ; L8Programs/funexample.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

4 Add Multiply def addnum ( num1, num2 ) : " This function adds two numbers " sumnum=num1+num2 return sumnum def mulnum ( num1, num2 ) : " This function multiplies two numbers " prod=num1 num2 return prod a, b = 3,5 print addnum ( a, b ) print mulnum ( a, b ) L8Programs/AddMul.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

5 Add Multiply def addnum ( num1, num2 ) : " This function adds two numbers " sumnum=num1+num2 return sumnum def mulnum ( num1, num2 ) : " This function multiplies two numbers " prod=num1 num2 return prod a, b = 3,5 print addnum ( a, b ) print mulnum ( a, b ) L8Programs/AddMul.py def fun1 ( a, b ) : return a+b def fun2 ( a, b ) : return a b print fun1 ( 2, fun2 ( fun1 ( 1, 1 ), 4 ) ) L8Programs/midsemprob.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

6 Function Arguments: Required arguments : Arguments passed to a function in correct positional order Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

7 Function Arguments: Required arguments : Arguments passed to a function in correct positional order. Keyword arguments Caller identifies the arguments by the parameter name Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

8 Function Arguments: Required arguments : Arguments passed to a function in correct positional order. Keyword arguments Caller identifies the arguments by the parameter name. Default arguments An argument that assumes a default value if a value is not provided in the function call for that argument Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

9 Function Arguments: Required arguments : Arguments passed to a function in correct positional order. Keyword arguments Caller identifies the arguments by the parameter name. Default arguments An argument that assumes a default value if a value is not provided in the function call for that argument. Variable-length arguments : a function for more arguments than you specified while defining the function. will not be discussed Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

10 Keyword Arguments In a function call, the caller identifies the arguments by the parameter name. Allows you to skip arguments or place them out of order 3 def p r i n t i n f o ( name, age ) : " This prints a passed info into this function " print " Name : ", name ; print " Age ", age ; return ; # Now you can c a l l p r i n t i n f o f u n c t i o n p r i n t i n f o ( age =70, name=" Amitabh " ) ; #p r i n t i n f o ( Amitabh, age=70 ) ; L8Programs/KeywordArguments.py 3 Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

11 Default Arguments Assumes a default value if a value is not provided in the function call for that argument. 4 def p r i n t i n f o ( name, age = 35 ) : " This prints a passed info into this function " print " Name : ", name ; print " Age ", age ; return ; # Now you can c a l l p r i n t i n f o f u n c t i o n p r i n t i n f o ( age =70, name=" Amitabh " ) ; p r i n t i n f o ( name=" Amitabh " ) ; L8Programs/DefaultArguments.py 4 Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

12 Recursive Functions function defined by calls to itself! Loops and trees def f a c t o r i a l ( n ) : i f n == 0 : return 1 else : return n f a c t o r i a l ( n 1) print " 3!= ", f a c t o r i a l ( 3 ) print " 6!= ", f a c t o r i a l ( 6 ) L8Programs/Recursive fact.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

13 Fibonacci Seq def f i b ( n ) : i f n==0 : return 0 i f n==1 : return 1 return f i b ( n 1)+ f i b ( n 2) print f i b ( 1 ), f i b ( 2 ), f i b ( 3 ), f i b ( 4 ), "... ", f i b ( 1 0 ) #n=i n p u t ( p r o v i d e n ) #p r i n t n, th, F i b o n a c c i no i s, f i b ( n ) L8Programs/fibrecursive.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

14 Sequential Search When data itemize are unsorted, you search one by one in the list. Worst case : O(n) searches Best case: O(1) searches Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

15 Sequential Search: Python program def s e q s e a r c h ( l s t, item ) : pos=0 found=f a l s e while pos<l e n ( l s t ) : i f ( l s t [ pos]==item ) : print item, " found in position ", pos, " of list " found=true break else : pos=pos+1 i f ( not found ) : print item, " not found in list ", l s t return found t e s t l i s t = [ 1, 2, 32, 8, 17, 19, 42, 13, 0 ] s e q s e a r c h ( t e s t l i s t, 3 2 ) s e q s e a r c h ( t e s t l i s t, 2 5 ) L8Programs/SeqSearch.py Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

16 Binary Search Used when the data is already sorted (ascending or descending) Worst case : O(log 2 n) searches Best case: O(1) searches Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

17 Binary Search: Python program def b i n s e a r c h ( o r d l i s t, item ) : f i r s t =0 l a s t=l e n ( o r d l i s t ) 1 found=f a l s e while f i r s t <=l a s t : m i d p o i n t =( f i r s t +l a s t )/2 i f o r d l i s t [ m i d p o i n t]==item : print item, " found in position ", midpoint, found=true break e l i f item< o r d l i s t [ m i d p o i n t ] : l a s t=midpoint 1 else : f i r s t =m i d p o i n t+1 i f ( not found ) : print item, " not found in list ", o r d l i s t return found t e s t l i s t = [ 0, 1, 2, 8, 13, 17, 19, 32, 4 2 ] #b i n s e a r c h ( t e s t l i s t, 1 3 ) b i n s e a r c h ( t e s t l i s t, 9 ) Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

18 Sorting Place elements in some order Numerical: Ascending, Descending Strings: Alphabetical, Alphabetical reversed Alphanumerical.. Many algorithm: Bubble sort, Insertion sort, Merge sort, selection sort.. Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

19 Bubble sort Sorting by swapping and passing. Time: O(n 2 ) Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16

20 Practice problems Generate a sequence of numbers f [0] = 1, f [n] = f [n 1] + 2 n 1 using recursive functions To test whether 25 is present in the list lst=[ 1, 2, 32, 8, 17, 19, 42, 13, 0]. How many steps do you need? How about 0? What will be the steps if the list is already ordered? Write a recursive function to check whether a string is palindrome or not (ignore spaces). (check : Malayalam, Never odd or even, Some men interpret nine memos ) Write a program to sort a given sequence of numbers using bubble sort. Shivakumar,Bireswar (IIT Gandhinagar) Lecture 7 Oct 9th, / 16