COMP1511 Week 06 Laboratory Exercises

02/04/2017

COMP1511 Week 06 Laboratory Exercises

COMP1511 17s1 (http://www.cse.unsw.edu.au/~cs151 (http://www.cse.unsw.edu.au/~cs1511/17s1/) 1/17s1/)

Week 06

Laboratory

Introduction to Programming (http://www.cse.unsw.edu.au/~cs1511/17s1/)

Exercises

Objectives In  In  this Lab, you will practice: simple input of strings & characters manipulating strings and characters

Preparation Before the lab you should re-read the relevant lecture slides and their accompanying examples. You should also have read the lab assessment guidelines (https://cgi.cse.unsw.edu.au/~cs1511cgi/l (https://cgi.cse.unsw.edu.au/~cs1511cgi/lab_assessment.html). ab_assessment.html).

Getting Started One member of your programming pair should login and run the following commands inside a Linux terminal Create a new directory for this lab called $

lab06   by typing:

mkdir lab06

Change to this directory by typing: $

cd lab06

Exercise: Devowelling Write a C program

devowel.c   which reads characters from its input and writes the same characters to its output, except it does

not write lower case vowels ('a', 'e','i', 'o', 'u'). Your program should stop only at the end of input. For example: $

./devowel

Are you saying 'Boo' or 'Boo-Urns'?

Ar y syng 'B' r 'B-Urns'? It was the best of times. It was the worst of times.

It ws th bst f tms. It ws th wrst f tms. Ctrl-d 

getchar to read characters don't use scanf or fgets . putchar   to output each character.

Hint: Hint: hint use Hint: Hint: use

As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 devowel.c

Exercise: Caesar Cipher Write a C program

caesar.c   which reads characters from its input and writes the characters to its output encrypted with a

Caesar cipher (https://en.wikipedia.org/wiki/Caesar_cipher). A Caesar cipher (https://en.wikipedia.org/wiki/Caesar_cipher) shifts each letter a certain number of positions in the alphabet. The number of positions to shift will be given to your program as a command line argument. Characters other than letters should not be encrypted. Your program should stop only at the end of input. Your program should contain at least one function other than main. For example:

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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COMP1511 Week 06 Laboratory Exercises

./caesar 1

This life well it's slipping right through my hands

Uijt mjgf xfmm ju't tmjqqjoh sjhiu uispvhi nz iboet These days turned out nothing like I had planned

Uiftf ebzt uvsofe pvu opuijoh mjlf J ibe qmboofe Ctrl-d  $

./caesar 10

abcdefghijklmnopqrstuvwxyz

klmnopqrstuvwxyzabcdefghij ABCDEFGHIJKLMNOPQRSTUVWXYZ

KLMNOPQRSTUVWXYZABCDEFGHIJ Ctrl-d  $

./caesar -42

Control well it's slipping right through my hands

Myxdbyv govv sd'c cvszzsxq bsqrd drbyeqr wi rkxnc These days?

Droco nkic? Ctrl-d 

Hint: handle upper and lower case letters separately Hint: use

%

Hint: use

atoi   to convert the command-line argument to an int.

As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 caesar.c

Exercise: Manually Cracking a Caesar Cipher This text was encrypted with a Caesarcipher. Naq jr'yy arire or eblnyf Vg qba'g eha va bhe oybbq Gung xvaq bs yhk whfg nva'g sbe hf Jr penir n qvssrerag xvaq bs ohmm

What is the plain-text? (hint: try various shifts with caesar.c)

Exercise: Letter Frequency Write a C program

letter_frequency.c   which reads characters from its input until end of input.

It should then print the occurance frequency for each of the 26 letters 'a'..'z'. The frequency should be printed as a decimal value and an absolute number in exactly the format below. Note upper and lower case letters are counted together. For example:

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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COMP1511 Week 06 Laboratory Exercises

./letter_frequency

Hello and goodbye. Ctrl-d 

'a' 0.066667 1 'b' 0.066667 1 'c' 0.000000 0 'd' 0.133333 2 'e' 0.133333 2 'f' 0.000000 0 'g' 0.066667 1 'h' 'i' 'j' 'k' 'l' 'm' 'n' 'o'

0.066667 0.000000 0.000000 0.000000 0.133333 0.000000 0.066667 0.200000

1 0 0 0 2 0 1 3

'p' 'q' 'r' 's'

0.000000 0.000000 0.000000 0.000000

0 0 0 0

't' 0.000000 0 'u' 0.000000 0 'v' 0.000000 0 'w' 0.000000 0 'x' 0.000000 0 'y' 0.066667 1 'z' 0.000000 0 $

./letter_frequency

Hey! Hey! Hey! I don't like walking around this old and empty house So hold my hand, I'll walk with you my dear Ctrl-d 

'a' 0.072289 6 'b' 'c' 'd' 'e' 'f'

0.000000 0.000000 0.084337 0.084337 0.000000

0 0 7 7 0

'g' 0.012048 1 'h' 0.096386 8 'i' 0.072289 6 'j' 0.000000 0 'k' 0.036145 3 'l' 0.084337 7 'm' 0.036145 3 'n' 0.060241 5 'o' 0.084337 7 'p' 0.012048 1 'q' 0.000000 0 'r' 0.024096 2 's' 0.036145 3 't' 'u' 'v' 'w' 'x'

0.048193 0.036145 0.000000 0.036145 0.000000

4 3 0 3 0

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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COMP1511 Week 06 Laboratory Exercises

'y' 0.084337 7 'z' 0.000000 0 Hint: use an array of counts. As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 letter_frequency.c

Exercise: Substitution Cipher Encoding Write a C program substitution.c  which reads characters from its input and writes the characters to its output encrypted with a Substitution cipher (https://en.wikipedia.org/wiki/Substitution_cipher). A Substitution cipher (https://en.wikipedia.org/wiki/Substitution_cipher) maps each letter to another letter. The mapping will be given to your program as a single command line argument. This command line argument will contain 26 characters: an ordering of the letters 'a'..'z'. Characters other than letters should not be encrypted. Your program should stop only at the end of input. Your program should contain at least one function other than main. For example: $

./substitution qwertyuiopasdfghjklzxcvbnm

I was scared of dentists and the dark

O vql leqktr gy rtfzolzl qfr zit rqka I was scared of pretty girls and starting conversations

O vql leqktr gy hktzzn uoksl qfr lzqkzofu egfctklqzogfl Ctrl-d  $

./substitution abcdefghijklmnopqrstuvwxyz

The identity cipher!!!

The identity cipher!!! $

./substitution bcdefghijklmnopqrstuvwxyza

The Caesar cipher is a subset of the substitution cipher!

Uif Dbftbs djqifs jt b tvctfu pg uif tvctujuvujpo djqifs! Ctrl-d 

As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 substitution.c

Exercise: Substitution Cipher Decoding Write a C program decode.c  which decrypts text encrypted by substitution.c For example: $

./decode qwertyuiopasdfghjklzxcvbnm

O vql leqktr gy rtfzolzl qfr zit rqka

I was scared of dentists and the dark O vql leqktr gy hktzzn uoksl qfr lzqkzofu egfctklqzogfl

I was scared of pretty girls and starting conversations Ctrl-d  $

./decode abcdefghijklmnopqrstuvwxyz

The identity cipher!!!

The identity cipher!!! $

./substitution bcdefghijklmnopqrstuvwxyza

Uif Dbftbs djqifs jt b tvctfu pg uif tvctujuvujpo djqifs!

The Caesar cipher is a subset of the substitution cipher! Ctrl-d 

As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 decode.c

Exercise: Manually Cracking a Substitution Cipher https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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This English text was encrypted with a Substitution cipher. Ndwu fdw sojfauw xafwl dvsq Vuq vcxafaoul vsw yon Vuq fdw swlwufcwuf saqwl datd Xjf wcofaoul nouf tson Vuq nwsw kdvutaut ojs nvrl, Fviaut qappwswuf sovql Fdwu yomw, yomw nayy fwvs jl vbvsf vtvau

What was the original text? Hint: use letter_frequency.c  on the encrypted text and compare the frequencies to English letter frequencies (https://en.wikipedia.org/wiki/Letter_frequency) and then try your guesses with decode.c

Challenge Exercise: Automatically Cracking Caesar Ciphers Write a C program crack_caesar.c   which decrypts text encrypted by an unknown Caesar cipher. Your program should make no assumptions about the language of the original text - don't assume its English. Your program will be given as a command-line argument the name of a file containing a large amount of unencrypted text in the same language as the encrypted text. Your program will be given the encrypted text on standard input. You may read it all before printing the decryption. You may assume the encrypted text contains at most 10000 characters. For example:

/web/cs1511cgi/17s1/tlb/06unsw.txt   contains 42000 characters of English text (UNSW students on facebook) For example, here is some English text encrypted with a Caesar cipher with an unknown shift: Kyzj zj fli crjk xffuspv Z yrkv kf wvvc kyv cfmv svknvve lj uzv Slk zk'j fmvi Aljk yvri kyzj reu kyve Z'cc xf Pfl xrmv dv dfiv kf czmv wfi Dfiv kyre pfl'cc vmvi befn

So for example: $

./crack_caesar /web/cs1511cgi/17s1/tlb/06unsw.txt

Kyzj zj fli crjk xffuspv Z yrkv kf wvvc kyv cfmv svknvve lj uzv Slk zk'j fmvi Aljk yvri kyzj reu kyve Z'cc xf Pfl xrmv dv dfiv kf czmv wfi Dfiv kyre pfl'cc vmvi befn Ctrl-d 

This is our last goodbye I hate to feel the love between us die But it's over Just hear this and then I'll go You gave me more to live for More than you'll ever know Hint: extract the letter frequencies from the file of unencrypted text. Use these frequencies as estimate of the probability each letter will be used. Test all possible shifts using these probabilities to see which is the most likely.. As usual autotest is available to test your program. $

~cs1511/bin/autotest lab06 crack_caesar.c

Challenge Exercise: Automatically Cracking Substitution Ciphers Write a C program crack_substitution.c  which decrypts text encrypted by an unknown s cipher. Your program should make no assumptions about the language of the original text - don't assume its English. Your program will be given as a command-line argument the name of a file containing a large amount of unencrypted text in the same language as the encrypted text. Your program will be given the encrypted text on standard input. You may read it all before printing the decryption. You may assume the encrypted text contains at most 10000 characters. For example:

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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COMP1511 Week 06 Laboratory Exercises

./crack_substitution /web/cs1511cgi/17s1/tlb/06unsw.txt

M'ka paat dra qegbu, ueta md xbb Rxu vw fxya teq Umxvetup, ogmbbmxtd, mt Oab-Xmg teq Red psvvag tmlrdp, vmu Jsbw Qrat wes xtu M qaga negakag qmbu Dra fgxzw uxwp, fmdw bmlrdp Dra qxw wes'u cbxw qmdr va bmya x frmbu Qmbb wes pdmbb beka va Qrat M'v te betlag westl xtu oaxsdmnsb? Qmbb wes pdmbb beka va Qrat M'ka led tedrmtl osd vw xfrmtl pesb? M yteq wes qmbb, M yteq wes qmbb M yteq drxd wes qmbb Qmbb wes pdmbb beka va qrat M'v te betlag oaxsdmnsb? M'ka paat dra qegbu, bmd md sc Xp vw pdxla teq Frxbbatlmtl xtlabp mt x taq xla teq Red psvvag uxwp, gefy t gebb Dra qxw wes cbxw neg va xd wesg preq Xtu xbb dra qxwp, M led de yteq Wesg cgaddw nxfa xtu abafdgmf pesb Ctrl-d 

I've seen the world, done it all Had my cake now Diamonds, brilliant, in Bel-Air now Hot summer nights, mid July When you and I were forever wild The crazy days, city lights The way you'd play with me like a child Will you still love me When I'm no longer young and beautiful? Will you still love me When I've got nothing but my aching soul? I know you will, I know you will I know that you will Will you still love me when I'm no longer beautiful? I've seen the world, lit it up As my stage now Challenging angels in a new age now Hot summer days, rock n roll The way you play for me at your show And all the ways, I got to know Your pretty face and electric soul Hint: you will need to look at the probabilities of sequences of 2 or perhaps 3 letters occurring or the probabilities of words. An autotest is available to test your program but because this is a difficult problem it is possible very good attempts at the problem won't pass the autotests. $

~cs1511/bin/autotest lab06 crack_caesar.c

Submission/Assessment When you are satisfied with your work, ask your tutor to assess it. You also need to submit your work electronically by typing (run this command in the $

lab06   directory):

give cs1511 lab06 devowel.c caesar.c letter_frequency.c

Submit the challenge exercises only if you attempt them. If you are working at home, you may find it more convenient to upload your work via give's web interface (https://cgi.cse.unsw.edu.au/~give/code/login.php?app=/~give/Student/give.php&debug=get_authuser).

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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Remember the lab assessment guidelines (https://cgi.cse.unsw.edu.au/~cs1511cgi/lab_assessment.html) - if you don't finish the exercises you can finish them in your own time, submit them by Monday 11:00am using

give   and ask your tutor to assess them

at the start of the following lab. Either or both members of a programming pair can submit the work (make sure each program lists both of you as authors in the header comment).

https://cgi.cse.unsw.edu.au/~cs1511/17s1/lab/06/questions

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