COMP1511 Week 06 Laboratory Exercises

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COMP1511

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

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

Objectives

In

In this Lab, you will practice: this Lab, you will practice:

simple input of strings & characters simple input of strings & characters manipulating strings and 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 Before the lab you should re-read the relevant lecture slides and their accompanying examples. You should also have read the lab assessment guidelines

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 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

Create a new directory for this lab called

lab06

by typing: by typing: $

$

mkdir lab06

Change to this directory by typing: 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 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').

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

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 Ctrl-d Hint

Hint: hint use: hint use

getchar

to read characters don't useto read characters don't use

scanf

oror

fgets

.. Hint

Hint: use: use

putchar

to output each character. to output each character. As usual autotest is available to test your program. 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 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).

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. 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.

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.

Characters other than letters should not be encrypted. Your program should stop only at the end of input. Your program should stop only at the end of input.

Your program should contain at least one function other than main. Your program should contain at least one function other than main. For example:

For example:

Week

Week 06

06 Laboratory

Laboratory

Exercises

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$

./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.

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$

./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' 0.066667 1

'i' 0.000000 0

'j' 0.000000 0

'k' 0.000000 0

'l' 0.133333 2

'm' 0.000000 0

'n' 0.066667 1

'o' 0.200000 3

'p' 0.000000 0

'q' 0.000000 0

'r' 0.000000 0

's' 0.000000 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' 0.000000 0

'c' 0.000000 0

'd' 0.084337 7

'e' 0.084337 7

'f' 0.000000 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' 0.048193 4

'u' 0.036145 3

'v' 0.000000 0

'w' 0.036145 3

'x' 0.000000 0

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'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!!!

$

./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!!!

$

./substitution bcdefghijklmnopqrstuvwxyza

Uif Dbftbs djqifs jt b tvctfu pg uif tvctujuvujpo djqifs!

The Caesar cipher is a subset of the substitution cipher!

Ctrl-d

~cs1511/bin/autotest lab06 decode.c

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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..

~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.

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$

./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.

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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).