Download Homework #7 - SOLUTIONS

CSCI-UA.0002-010: Introduction to Computer Programming
Fall 2013
Homework #7 - SOLUTIONS
The following programs are due at the beginning of class on Tuesday, November 5.
You can submit your programs online via NYU Classes. Please submit a separate .py file
for each program, and put your name and the problem/assignment number in a comment
at the top of the program.
1. Modify your guessing game program from homework #5 (problem #3) so that it randomly picks a whole number between 1 and 100 and then asks the user to guess this number
(with hints like ”Higher!” and ”Lower!”)
# SOLUTIONS : Homework 7 − Problem 1
import random
s e c r e t n u m b e r = random . r a n d i n t ( 1 , 1 0 0 )
count = 1
g u e s s = i n t ( i n p u t ( ” I ’m t h i n k i n g o f a number between 1 and 1 0 0 .
Can you g u e s s what i t i s ? ” ) )
w h i l e g u e s s != s e c r e t n u m b e r :
count += 1
i f guess < secret number :
p r i n t ( ” Higher ! ” )
g u e s s = i n t ( i n p u t ( ”What number am I t h i n k i n g o f ? ” ) )
e l i f guess > secret number :
p r i n t ( ” Lower ! ” )
g u e s s = i n t ( i n p u t ( ”What number am I t h i n k i n g o f ? ” ) )
i f count > 1 :
p r i n t ( ” You g u e s s e d i t ! And i t o n l y took you ” , count , ” t r i e s
.”)
else :
p r i n t ( ” You g u e s s e d i t ! And i t o n l y took you ” , count , ” t r y . ” )
2. The average value in the following data set is 5.
3, 6, 7, 5, 4
When statisticians want to measure how much variation there is from the average, they use
a statistic called the standard deviation. To calculate the standard deviation, first compute
the difference of each data point from the average and square each result.
(3 − 5)2
(6 − 5)2
(7 − 5)2
(5 − 5)2
(4 − 5)2
= (−2)2 = 4
= (−1)2 = 1
= 22 = 4
= 02 = 0
= (−1)2 = 1
Next, add up each of these numbers, divide by the total number of data points minus one,
and take the square root of the result:
r
r
4+1+4+0+1
10
=
≈ 1.58
5−1
4
The standard deviation is this number, 1.58. This is a tedious calculation even when you
only have five data points! Write a function calculate standard deviation that takes a list
of numbers as an argument and returns the standard deviation. Use your function to write
a program that calculates the standard deviation for the data set
3, 6, 7, 5, 4, 2, 2, 1, 10, 9, 4, 6, 4, 5, 2, 7, 8, 3, 9, 3
# SOLUTIONS : Homework 7 − Problem 2
from math import s q r t
def calculate mean ( list of numbers ) :
total = 0
count = 0
f o r num i n l i s t o f n u m b e r s :
t o t a l += num
count += 1
r e t u r n t o t a l / count
def calculate standard deviation ( list of numbers ) :
total = 0
count = 0
mean = c a l c u l a t e m e a n ( l i s t o f n u m b e r s )
f o r num i n l i s t o f n u m b e r s :
t o t a l += (num − mean ) ∗∗2
count += 1
v a r i a n c e = t o t a l / ( count − 1 )
return sqrt ( variance )
sample = [ 3 , 6 , 7 , 5 , 4 , 2 , 2 , 1 , 1 0 , 9 , 4 , 6 , 4 , 5 , 2 , 7 , 8 , 3 ,
9 , 3]
s t d e v = c a l c u l a t e s t a n d a r d d e v i a t i o n ( sample )
s t d e v = format ( stdev , ’ , . 4 f ’ )
p r i n t ( ” The s t a n d a r d d e v i a t i o n i s ” , stdev , ” . ” , sep = ’ ’)
# ALTERNATE SOLUTION : Homework 7 − Problem 2
# This s o l u t i o n i s more p o l i s h e d ( but more c o m p l i c a t e d . )
# I t g e t s t h e l i s t o f numbers from t h e u s e r , and a v o i d s
# t h e s p e c i a l c a s e where you would a c c i d e n t a l l y d i v i d e by 0 .
from math import s q r t
def get sample () :
list of numbers = [ ]
num = ’ ’
p r i n t ( ” This program c a l c u l a t e s t h e s t a n d a r d d e v i a t i o n o f a
sample . ” )
p r i n t ( ” P l e a s e e n t e r t h e data i n your sample , one number a t a
time , and e n t e r ’ done ’ when you ’ r e done . ” )
w h i l e num != ’ done ’ :
num = i n p u t ( ” The next number i s ? ” )
i f num != ’ done ’ :
num = i n t (num)
l i s t o f n u m b e r s += [ num ]
return list of numbers
def calculate mean ( list of numbers ) :
total = 0
count = 0
f o r num i n l i s t o f n u m b e r s :
t o t a l += num
count += 1
i f count != 0 : #This i f s t a t e m e n t i s t o a v o i d d i v i d i n g by 0 .
r e t u r n t o t a l / count
def calculate standard deviation ( list of numbers ) :
total = 0
count = 0
mean = c a l c u l a t e m e a n ( l i s t o f n u m b e r s )
f o r num i n l i s t o f n u m b e r s :
t o t a l += (num − mean ) ∗∗2
count += 1
i f count != 1 : #This i f s t a t e m e n t i s t o a v o i d d i v i d i n g by 0 .
v a r i a n c e = t o t a l / ( count − 1 )
return sqrt ( variance )
else :
return 0
sample = g e t s a m p l e ( )
s t d e v = c a l c u l a t e s t a n d a r d d e v i a t i o n ( sample )
s t d e v = format ( stdev , ’ , . 4 f ’ )
p r i n t ( ” The s t a n d a r d d e v i a t i o n i s ” , stdev , ” . ” , sep = ’ ’)
3. Assume that a file containing a sequence of names (as strings) is titled names.txt and
exists on the computer’s hard disk. Write a program that displays each name in the file
and the total number of names stored in the file.
# SOLUTIONS : Homework 7 − Problem 3
i n f i l e = open ( ’ names . txt ’ , ’ r ’ )
count = 0
line = i n f i l e . readline ()
w h i l e l i n e != ’ ’ :
p r i n t ( l i n e , end = ’ ’)
count += 1
line = i n f i l e . readline ()
i n f i l e . close ()
p r i n t ( ” There were a t o t a l o f ” , count , ”name ( s ) . ” )
4. Write a program that writes a sequence of random numbers to a file. Each number
should be between 1 and 100, and the program should let the user specify how many
random numbers the file will hold.
# SOLUTIONS : Homework 7 − Problem 4
from random import r a n d i n t
o u t f i l e = open ( ’ random numbers . txt ’ , ’w’ )
p r i n t ( ” This program w r i t e s numbers between 1 and 100 t o a f i l e
random numbers . t x t ” )
num = i n t ( i n p u t ( ”How many random numbers do you want w r i t t e n t o
the f i l e ? ”) )
f o r x i n r an ge (num) :
random number = r a n d i n t ( 1 , 1 0 0 )
random number = s t r ( random number ) + ’ \ n ’
o u t f i l e . w r i t e ( random number )
outfile . close ()