ITI1120-Lab 6 While Loops and Lists Solved

Task 1  
Go to the page below in your interactive textbook 

-read through

-click Forward until reaching the end on the Python Visualizing example (to understand it)

-and answer the two questions and the end:

https://runestone.academy/runestone/static/thinkcspy/MoreAboutIteration/ThewhileStatement.html  

Study of  while vs for … in     some elementary functions:     

Problem: sum of positive odd integers smaller than  n

A                                                                                    solution                                                                                          with                                                                                             for                                                                                           loop                                                                                                A                                                                                    solution                                                                                          with                                                                                                 a                                                                                         while                                                                                           loop

def sum_odd_while(n):

def sum_odd_for(n): odd_sum = 0 odd_sum = 0 i=1 for num in range(n):   while i < n: 

                        if num % 2 == 1:       if i % 2 == 1:

                                    odd_sum = odd_sum + num  odd_sum = odd_sum + i

            return odd_sum                     i=i+1 

return odd_sum

Problem: Sum of odd numbers in a given list a

A     solution with       for  loop       

def sum_list_for(a): ''' (list)-num ''' list_sum = 0 for i in range(len(a)):

if a[i] % 2 == 1:

list_sum = list_sum return list_sum

A  solution with       a     while     loop

def sum_list_while(a): ''' (list)-num list_sum = 0 i=0 while i < len(a):

if a[i] % 2 == 1:

list_sum = list_sum + a[i]

+a[i]        i=i+1 

return list_sum

Study    of  while vs    for …   in   another    important   elementary    functions: 

Problem: 

find a minimum element in a list (of length at least 1)
Two            solutions   with  for    loop  (for   loop  over          elements   and   over indices) def minimum_for_v1(a): ''' (list)-num ''' minimum = a[0] for element in a:

if element < minimum:

minimum = element

return minimum

def minimum_for_v2(a): ''' (list)-num ''' minimum = a[0] for i in range(len(a)):

if a[i] < minimum:

minimum = a[i] return minimum

A     solution with       a     while     loop

def minimum_while(a): ''' (list)-num''' minimum = a[0] i=0 while i < len(a):

if a[i] < minimum:

minimum = a[i]

i=i+1 return minimum

Task 2  and   Programming     Exercise    1

After studying the previous four functions

1.Open the file called seven_functions.py Copy/paste, one by one, the three solutions labeled with “# with while loop” into Python visualizer. Run through each example and understand how the solutions work and how the variables change in the loops. Play by changing what variable i is initialized to and how it is incremented   (do for example i=i+3) and see what happens. As always, you can find python visualizer here (make sure you choose Python 3) http://www.pythontutor.com/visualize.html#mode=edit

2.Programming exercise 1: Open file called prog-ex-1.py Complete the function sum_odd_while_v2 that takes an integer n as input parameter and computes the sum of all odd integers between 5 and n (inclusively). Use WHILE LOOP only and NO IF STATEMENTS.   

sum_odd_while_v2(10)

21

sum_odd_while_v2(-7)

0

Programming    Exercise    2 
Write a program using while loop that 

•asks the user for two integers (one by one or at the same time if you know how to handle it). Then it adds them and displays the sum. 

Then the user is asked to enter ‘yes’ if she wishes to perform the operation again, and otherwise if she enters anything other than ‘yes’ the program terminates. The operation 

(described in the bullet above) is repeated, as long as the user enters ‘yes’. 

Question      1.    What     does      the program       below    print      if     the input     user       entered was:

1         5          -3    0

Question      2.    Can you tell  what      the       program       does?    Write     one sentence       explaining    what      the function       does,       in    plain      English.

 

Question      1.               How many times does the program below print aha?

 

 Question      1.               What does the the code on the right print? 

ps. sometimes I say code, sometimes I say program.

I mean the same thing.

Task 6   
A programmer who wrote a function below thinks that the function tests if a given positive integer n=2 is a prime. 

However there is a logical mistake.

1.    Where is the mistake?

2.    For what number does the function return a wrong answer? 

Can you think of the smallest such number?

3.    Fix the mistake.

 

13

Programming    Exercise 3  
Write    a    function   first_neg that takes   a    (possibly   empty)       list of  numbers  as  input parameter,    finds   the first occurrence     of  a    negative number,   and     returns     the index  (i.e.     the position    in the list)      of  that     number.   If the list contains   no negative numbers  or  it    is   empty, the program   should return None. Use     while   loop    (and not for loop)   and     your    while loop    should stop    looping once    the first     negative number    is   found. 

Test your    function   with    at least    the examples below

first_neg([2, 3, -1, 4, -2])

2

first_neg([2, 3, 1, 4, 2])

first_neg([])

Programming    Exercise 4       
Write       a     function  sum_5_consecutive that takes     a  list  of    numbers      as    input     and  returns  True       if     there     are  5  consecutive numbers      in    the list  that        sum       to    zero.  Otherwise    it     returns  False.     The  function       should   also return  False      if     the list  has less than  5     elements      Solve     this in    two  ways:     

1.for loop       (over     indices  of    the list)

2.while    loop       (over     indices  of    the  list)

In      both      cases     you need      to    think  about    “stopping     condition”   in    order  to    avoid     

“IndexError: list index out of range”

3.      Test       your       function       with       at  least      the examples     below

sum_5_consecutive([2, 3, -3, 2, 4,-6])

True

sum_5_consecutive ([-10, 1, 1, 4, 2, 10, 13])

False

sum_5_consecutive([2, 1, -3, -3, -3, 2, 7, 4, -6])

True

sum_5_consecutive ([])

False

sum_5_consecutive ([1, -1,0])

False

Programming    Exercise 5  :

Different   ways to create useful   lists
Recall, for example, that 

a=[2] creates a list (refer to by variable a) with one element, a number 

2 in this case. b=[None] creates a list with one element. That element is an object of type None. That is sometimes useful when we are not ready yet to assign a value to an element of a list. c=[] creates an empty list, i.e. a list of length zero

Recall further that multiplying a list by an integer n, creates a new list that repeats the given list n times. Or that applying ‘+’ operator to two lists creates a new lists that concatenates the given two lists.

For example: 

[1,2]+[10,20,0] creates a list [1,2,10,20,0]

[7,2]*3 creates a list [7,2,7,2,7,2]

Finally, recall the slicing. For example, if a=[2,3,4,1], a[:] returns a new list that is a copy of list a.

Open          the    file    called         creating_various_lists.py. The   first   line   is       given          to      you.  It       asks  the user  to      enter          a        positive               even integer       n. For    each green         programing        exercise     below,       try to      find   at      least two   solutions   (e.g.  one   by     using a        loop  with  accumulator      pattern      and   another     by using          int*list).     

1.Create    a        list     a        (i.e    a        list     referred    to      by variable     a)      of      length        n        filled with  zeros

2.Create    a        list     b        of      length        n        filled with random     integers     between   1        and   100

3.Create    a                  variable     c        that  is       an     alias  of b

4.Set first   half   of      the    elements   of      c        to      zero, and   then print both b        and   c

5.Copy       list     b        into   a        list     d

6.Create    a        new  list     e        that  has    every         2nd element    of      b

Programming Exercise 6: Fibonacci    Numbers

Write a    function   called  fib(n) that   takes   as  input   an  integer     n    (greater    than    1)   and     creates     a    list containing      n     values such    that a[0]   =    1 a[1]     =    1 a[i]   =    a[i-1]   +    a[i-2] for     all  i in the range  1    <    i <  n and     it    prints  that     list once    it     creates     it

fib(7)

[1  1  2  3  5  8  13]

Once  you     are done,  trace   your     program   on paper  and     then     Python      visualizer  to  see what     the values in   the list will be  for     n=5.    

Programming   Exercise  7: 

Write a    function inner_product that   that     takes   as  input   two     lists     of  integers    (of same     length)      and     then     computes and returns the inner     

product of     the two     lists     of  integers.     The      inner   product     of  two     lists     x    =     [x1, x2,  ...,  xn] and     y     =    [y1, y2,  ...,  yn] is     the value   x1 y1 + x2 y2 + ... + xn yn .

inner_product([2,3,4], [1,0,2]) 10

More programming  exercises (outside  of  lab):

Do      everything     in   the first     13     exercises  here

https://runestone.academy/runestone/static/thinkcspy/Lists/Exercises.html  

******** and if you need more exercises ….

go to    the web       site below,   for  some      more     practice programming     questions      but be   aware    that        the web       site is      in    Python  2,    thus       if     you do   these      exercises      do   them     on          your      own       computer     AND       NOT       in      the little       browser       window of    the      web       site. http://codingbat.com/python