Transcript PrimesFactory Lab

PrimesFactory Lab
Laugh if you get the pun
Poll: Which of these is not a prime number?
Prime Numbers
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A prime number (or a prime) is a natural
number greater than 1 that has no positive
divisors other than 1 and itself.
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43,
47, 53, 59, 61, 67, 71, 73, 79, 83, 89…
If a number p is prime, then the only cases
where p % t == 0 are when t==1 or t==p
◦ e.g. If the number 23 is prime, then the only cases
where 23 % t == 0 are when t==1 or t==23
Poll: Which one of these is not a factor of 45...
Factors
In mathematics, a divisor of an integer n,
also called a factor of n, is an integer
which divides n without leaving a
remainder.
 Factors of 24
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◦ 1, 2, 3, 4, 6, 8,12, 24
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For an integer n, the factors t are
numbers that satisfy the rule n % t == 0
Poll: Which of these is not a prime factor of ...
Prime Factors
The prime factors of a positive integer
are the prime numbers that divide that
integer exactly.
 Factors of 24
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◦ 1, 2, 3, 4, 6, 8,12, 24
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Prime factors of 24 are 2 and 3
PrimesFactory
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Three important methods
1. Calculate the prime factorization of any
number
2. Print a list of all prime numbers
3. State whether or not a given number is a
prime number
PrimesFactory
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Three important methods
1. getPrimeFactors(int num)
 //returns an ArrayList with all of the prime factors
of num
2. listPrimesUpTo(int num)
 //returns a comma separated list of all prime
numbers less than num
3. isPrime(int num)
 //return true if num is a prime number
Arrays
ArrayList
String[] arr = new String[10];
…
//insert Strings into array
…
for(int i=0; i<arr.length; i++)
{
System.out.println(arr[i]);
}
ArrayList<String> arrList =
new ArrayList<String>();
…
//insert Strings into ArrayList
…
for(int i=0; i<arr.size(); i++)
{
System.out.println
(arrList.get(i));
}
Arrays vs. ArrayList
Arrays
ArrayList
String[] arr = new String[10];
…
//insert Strings into array
…
for(String x : arr)
{
System.out.println(x);
}
ArrayList<String> arrList =
new ArrayList<String>();
…
//insert Strings into ArrayList
…
for(String x : arrList)
{
System.out.println(x);
}
Arrays vs. ArrayList
Arrays
ArrayList
Fixed length, set when it is
created
Shrinks and grows as needed
Must keep track of last slot if
array is not full
Last slot is always
arrList.size()-1
Must write code to shift
elements if you want to insert
or delete
Insert with just
arrList.add(object)
Delete with just
arrList.remove(objectIndex)
or arrList.remove(object)
Arrays vs. ArrayList
ArrayList methods
add(Object elem)
 remove(int index)
 remove(Object elem)
 contains(Object elem)
 isEmpty()
 indexOf(Object elem)
 size()
 get(int index)
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Generics
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ArrayList class is generic, which means it
has a type parameter
◦ Class header  public class ArrayList<E>
 <E> is placeholder for any non-primitive type
◦ ArrayList<String>  stores Strings
◦ ArrayList<Integer>  stores ints
List is restricted to particular data type
 Built-in safety
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The Jokes Get Pun-nier
What kind of music do Santa’s elves listen
to the most?
 What kind of musicians are Integers and
Doubles?
 Wrap music/Wrappers :P
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Auto-Boxing and -Unboxing
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ArrayList must contain objects
◦ NO PRIMITIVES
 Objects usually start with upper case letter
 String, Egg, Pokemon, Dog, etc.
 Primitives usually start with lower case letter
 int, double, boolean, etc.
◦ Numbers must be boxed—placed in wrapper
classes like Integer or Double—before
insertion into an ArrayList
Auto-Boxing and -Unboxing
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Auto-boxing
◦ Automatic wrapping of primitive types in their
wrapper classes
◦ Use intValue() or doubleValue() to retrieve
the numerical value
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Auto-unboxing
◦ Automatic conversion of a wrapper class to
its corresponding primitive type
Auto-Boxing and –Unboxing
Example
ArrayList<Integer> arrList = new
ArrayList<Integer>();
arrList.add(4); //auto-boxing
//4 is an int, wrapped in an
//Integer before insertion
int n = list.get(0);
//auto-unboxing
//Integer is retrieved
//and converted to int