# Homework Assignment #3 CS5004 solved

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## Description

5/5 - (1 vote)

Problem 1 [30pts]. According to historians, Julius Caesar used the following encryption
technique for sending messages. Given a string s and an integer k, replace every letter by
the letter than comes k positions later in alphabetical order. For example the string “hello”
with key 5, is encrypted into “mjqqt” since letter ‘m’ is the fifth letter after ‘h’ and so on.
Note that ‘z’ in this case is mapped to ‘e’ (since after ’z’ we go back to ’a’). To decrypt the
message one needs to know the key; for our example, just “shift” each letter by -5 positions.
In this problem you will write a program called Cipher.java which does both encryption
and decryption. Some examples follow
— Enter the text below
Hello
— Enter E for Encryption, D for Decryption, X to exit: E
— Enter key: 5
— The encrypted text is below
Mjqqt
Now another example
1
— Enter the text below
Mjqqt
— Enter E for Encryption, D for Decryption, X to exit: D
— Enter key: 5
— The decrypted text is below
Hello
Another example
— Enter the text below
Hello
— Enter E for Encryption, D for Decryption, X to exit: F
— Input not recognized. Try again.
— Enter E for Encryption, D for Decryption, X to exit: X
NOTE: You can assume that the input contain only alphabet letters, both lower case and
upper case, space, and the usual punctuation marks {., !?}. Non alphabet characters are to
be left unchanged. Also, you must keep the case; i.e. a lower case letter should be mapped
to a lower case letter.
NOTE: The key k is a positive integer. But, it can be a large number. In this case, you
must wrap it around. For example if the key is 27 then ’a’ is mapped to ’b’ etc. Your code is
expected to work with any non-negative integer. Please check that input key is valid (that
is key ≥ 0; if not print an error message and exit).
Problem 2 [10pts]. In discrete math, the n-the Harmonic number is defined as
Hn =
Xn
k=1
1
k
=
1
1
+
1
2
+ … +
1
n
Write a program that takes as input an integer n > 0 and outputs Hn. An example:
— Enter n: 100
5.187377517639621
If the input is not valid (e.g. -1), your program should print an error message and exit.
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