Computer Science 1 Homework 2 Strings and Functions

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Part 1: Geometry and breathing!
This problem is a warm up of the use of functions. You will be asked to write a few simple functions.
A number of countries and companies are planning manned trips to Mars in the next few decades.
Let’s think ahead to one of the more important considerations for the trip – breathable air. Now,
the space station gets most of its oxygen from the electrolysis of water and I expect that a trip to
Mars will use a similar method, but what if we needed to carry all the oxygen needed along with
the astronauts? How much oxygen would we need to carry?
We will make some simplifying assumptions: the capsule is a sphere, 41% of the oxygen in the
capsule is used each day, and oxygen is stored in a set of cylindrical tanks (essentially SCUBA
tanks). These tanks are pressurized to 3000 psi, which means that each reservoir tank holds 210
times its internal volume of gas. Your problem is to determine the total volume of oxygen needed
for the trip, the amount of oxygen per tank, and therefore the number of oxygen tanks.
dimensions of the reservoir cylinder, again (radius, height), reading these through input. The
program will then produce the results as follows
 Calculate and output the total amount of oxygen needed during the journey to Mars. This
is the volume in the spherical capsule times the amount of oxygen in the air (21%) times the
percent of the oxygen used each day (41%) times 300 days for the one way trip.
Remember the volume of a sphere is given by pi* radius**3 * 4/3. You must write a
function volume_sphere(radius) to compute this and use it in your computation. You
must use the math module for the value of pi.
 Calculate and output the amount of oxygen held in each cylindrical tank with the given dimensions radius, height. Remember, the volume of a cylinder is given by pi* radius**2 * height.
Write a function volume_cylinder(radius, height) that finds and returns the volume of a
cylinder with the given measurements. (Here you are welcome to make use of the code from
the lecture notes!) Output the amount of oxygen the cylinder holds at 3000 psi (210 times
the volume of the cylindrical tank). You must use the math module for the value of
pi.
 Calculate and output the number of oxygen tanks the capsule will need to carry with it on
the journey. Always choose the next largest whole number, so 0.2 tanks becomes 1 tank,
while 1 tank remains 1 tank. Look at the ceil function in the math module to help you do
this.
As a final note, assume that all of the oxygen needed is contained in the reservoirs. In other words,
you do not need to account for the oxygen already in the capsule at the start of the journey, nor
do you need to be concerned with the amount of oxygen left in the tank at the end.
Here is example output of your program (how it will look on Wing IDE):
Radius of capsule (m) ==> 2.5
2.5
Radius of oxygen reservoir (m) ==> 0.093
0.093
Height of oxygen reservoir (m) ==> 0.633
0.633
Oxygen needed for the trip is 1690.570m^3.
Each cylinder holds 3.612m^3 at 3000 psi.
The trip will require 469 reservoir tanks.
Remember, you will need to use formating strings to generate the output in exactly the form we
have shown it above. This is necessary to match our output.
When you have tested your code, please submit it as Part 1 of HW 2.
Part 2: A simple trick (numerical functions)!
The original of this simple parlor trick is attributed to Albert Einstein, but it probably dates to
an earlier date! We’ve modified it a little to test a few more math functions and give a little more
challenge.
The trick: First, write the number 1089 on a piece of paper, fold it, and put it away. Next, ask
your friend to write down a five-digit number, emphasizing that the first and third digits must
differ by at least two. Don’t watch your friend doing the arithmetic. After your friend has written
down the five-digit number, ask them to reverse it, take the first three digits of the original number
and the last three digits of the reversed number, then subtract the smaller from the larger.
Example: 32156 reversed is 65123
321 – 123 = 198.
Tell your friend to reverse the new number.
Example: 198 becomes 891.
Example: 198 + 891 = 1089.
If all goes as planned, your friend will be amazed. The number you wrote down at the start –1089–
will always be the same as the end result of this mathematical trick.
Your job is to write a program that mimics this parlor trick by first predicting the outcome, then
requesting a five-digit number from the user, applying the steps above, outputing the steps as it
goes, and checking the result to make sure it is indeed 1089.
When looking at a problem like this, it is important to try to think about the central issue —
the most challenging question — and try to solve it first and check your solution. Here, the most
difficult question for us is reversing the digits in an integer. There are a number of ways to do this,
but we want you to think about it by making use of integer division and integer remainders (the
how to grab the first three digits:
>>> x = 14926
>>> x // 100 # Gets the first three digits
149
>>> x % 100 # Gets the last two digits
26
>>> y = 75 # We are going to reverse a two digit number
>>> tens = y//10 # Gets the tens digit
>>> tens
7
>>> ones = y%10 # Gets the ones digit
>>> ones
5
>>> ones*10 + tens # Gives us the reversed number
57
Start your programming effort by writing and testing a function called reverse3 that calculates
and returns the reverse of a three digit number. Test this function carefully. For example
>>> reverse3(123)
321
Next, write and test a function called reverse5 that calculates and returns the reverse of a five
digit number. Test this function carefully. For example,
>>> reverse5(26417)
71462
With these two functions done, write the complete program to mimic the parlor trick. The program
should the produce output given below by asking the user for a five-digit integer, and then carrying
out the computation outlined above, using the two reverse functions as needed. The program should
output the steps of the computation. Then, add a print statement at the end of your program to
say “You see, I told you”.
Your output must match the following (this is how it will look on Wing):
Enter a 5 digit number whose first and third digits must differ by at least 2.
Enter a value ==> 29467
29467
Here is the computation:
29467 reversed is 76492
492 – 294 = 198
198 + 891 = 1089
You see, I told you.
When you have tested your code, please submit it as Part 2 of HW 2.
Part 3: Find the hidden message! (string functions and an if)
Write a program that either encrypts or decrypts a string, depending on the choice that the user
makes. First, the program should ask the user whether they want to encrypt (’E’) or decrypt
(’D’), then it should ask the user for a sentence using (input) written in a cipher to decrypt, or in
clear text to either encrypt (if ’E’ was input) or decrypt. The program should then carry out the
encrypt or decrypt operation using the rules described below and print the resulting sentence. For
both encrypt and decrypt, the program should print the difference in length between the cipher
and clear text versions. The difference should always be printed as a positive number. The ’E’
for encrypt, or ’D’ for decrypt should be case insensitive, which means that ’e’ means encrypt,
just like ’E’ does. If something other than ’E’ or ’D’ is entered in respone to the first query, just
print “I didn’t understand … exiting” and exit the program. Processing the input will require
an if/elif/else block. We will cover if/elif/else on Monday. Until then, you can write the
encrypt and decrypt functions and apply both to any string that is given. Do not let the fact
that we have not covered if keep you from getting a jump on this homework. All of
parts 1 and 2 can be completed as well as most of part 3.
Here is a series of examples running of the program (what you will see on Wing):
Run 1:
Enter ‘E’ for encrypt or ‘D’ for decrypt ==> d
d
Enter cipher text ==> wh*%\$ s7654 s2(*2(ri7654@@@s
wh*%\$ s7654 s2(*2(ri7654@@@s
Deciphered as ==> why so serious
Difference in length ==> 14
Run 2:
Enter ‘E’ for encrypt or ‘D’ for decrypt ==> e
e
Enter regular text ==> back off man I am a scientist
back off man I am a scientist
Encrypted as ==> back 7654ff m-? I%4%m%4% sci2(*2(ntist
Difference in length ==> 9
Run 3:
Enter ‘E’ for encrypt or ‘D’ for decrypt ==> D
D
Enter cipher text ==> wh*%\$ s7654 s2(*2(ri7654@@@s
wh*%\$ s7654 s2(*2(ri7654@@@s
Deciphered as ==> why so serious
Difference in length ==> 14
Run 4:
Enter ‘E’ for encrypt or ‘D’ for decrypt ==> E
E
Enter regular text ==> back off man I am a scientist
back off man I am a scientist
Encrypted as ==> back 7654ff m-? I%4%m%4% sci2(*2(ntist
Difference in length ==> 9
Run 5:
Enter ‘E’ for encrypt or ‘D’ for decrypt ==> a
a
I didn’t understand … exiting
The encryption rules are based on a set of string replacements, they should be applied in this order
exactly:
‘ a’ => ‘%4%’ replace any a after a space with %4%.
‘he’ => ‘7!’ replace all occurrences of string he with 7!
‘e’ => ‘9(*9(‘ replace any remaining e with 9(*9(
‘y’ => ‘*%\$’ replace all occurrences of string y with *%\$
‘u’ => ‘@@@’ replace all occurrences of string u with @@@
‘an’ => ‘-?’ replace all occurrences of string an with -?
‘th’ => ‘!@+3’ replace all occurrences of string th with !@+3
‘o’ => ‘7654’ replace all occurrences of string o with 7654
‘9’ => ‘2’ replace all occurrences of string 9 with 2
For example the cipher for methane is m2(*2(!@3-?2(*2(+. Here is how we get this:
>>> ‘methane’.replace(‘e’,’9(*9(‘)
‘m9(*9(than9(*9(‘
>>> ‘m9(*9(than9(*9(‘.replace(‘an’,’-?’)
‘m9(*9(th-?9(*9(‘
>>> ‘m9(*9(th-?9(*9(‘.replace(‘th’,’!@+3′)
‘m9(*9(!@+3-?9(*9(‘
‘m9(*9(!@+3-?9(*9(‘.replace(‘9’, ‘2’)
‘m2(*2(!@+3-?2(*2(‘
Decrypting will involve using the rules in reverse order.
Your program must use two functions:
 Write one function encrypt(word) that takes as an argument a string in plain English, and
returns a ciphered version of it as a string.
 Write a second function decrypt(word) that does the reverse: takes a string in cipher and
returns the plain English version of it.
Both functions will be very similar in structure, but they will use the string replacement rules in
different order. You can now test whether your functions are correct by first encrypting a string,
and then decrypting. The result should be identical to the original string (assuming the replacement
rules are not ambiguous).
Use these functions to implement the above program. When you have tested your code, please
submit it as Part 2 of HW 2.
Finished and still want extra challenges to sharpen your programming skills? These
are not extra credit but for you to do additional practice.
 Revise part 2 so that if the person enters a number that is not valid (i.e. first and third
digit do not differ by two), you immediately print a statement informing the user and do not
compute anything. Otherwise, you carry out the above computation.
 Revise part 3 so that it takes an input sentence, encrypts it and then decrypts it, and then
checks whether the result is the same. It should print “same” if they are the same and
“different” otherwise.