CS52 - Fall 2015

CS52 - Fall 2015 - Class 1

Example code in this lecture

Lecture notes

PERM list, etc.

CS51, CS52 and CS62: how it all fits together
   - Who should be here?
      - taken CS51
      - taken AP CS
      - something similar and have gotten approval from a CS faculty member

course overview
"The goal of this course is to give you a broad view of Computer Science, including the topics of algorithms and complexity, computer architecture and organization, programming languages with a solid understanding of functional programming and recursion, and finite automata and computability."

Course web page: http://www.cs.pomona.edu/classes/cs52/
   - Use Piazza (I'll have you all registered by the end of the day)
   - Mentor sessions
      - Use them to get help *if* you get stuck
      - These are NOT required
      - Work before coming to the session
      - The earlier sessions will be *much* less crowded and you'll get better support
   - Course readings
      - No official book, but many you can use to supplement (You should find one that you like)
      - Won't be extensive reading, but do read the handouts as I suggest
   - Course schedule

Administrative: http://www.cs.pomona.edu/classes/cs52/administrivia.html

Academic honesty and collaboration
   - I take it very seriously!
   - Read the administrative handout
   - Read the CS department policy
   - Understand what is appropriate and what is not!
   - Some things to watch out for:
      - Don't procrastinate!
         - One of the common ways I've seen people get into trouble is by not leaving enough time to work on the assignment.
         - If you find yourself in this situation, just turn in what you have and take it from there.
      - Do not give your work to anyone!
         - It doesn't help them.
         - It doesn't help you (and can get you into trouble).
      - Understand what type of collaboration is ok and what type is not.
         - You MAY:
            - Use/copy ANY of the examples covered in class or in the readings (in fact, you're encouraged to!)
            - Look at and work through examples from class with a partner or group
            - Talk about issues raised in the reading with a partner or group
            - Talk at a high-level (i.e. NO CODING) about the assignments with a partner or group
            - Help someone with a syntax problem (i.e. code won't compile)... this is the only time you should look at someone else's code
            - Help someone understand an error message

         - You may NOT:
            - look at anyone else's code to help yourself
            - give or show your code to anyone else (except to help diagnose a syntax problem)
            - search online for answers, solutions, etc.
            - sit next to someone in the class that you are talking with about the assignment (ideally, just don't sit next to anyone who's in the class)

Assignment 0
- Posted and due Friday at 5pm
- If you've never taken a CS class, you won't have an account yet. Wait until Wednesday to start the assignment (though you can look at it now)

Two ways of interacting with SML
   1) the interpreter
      - Type commands and get back results
      - Nice for testing things out and doing simple things
      - I'll use this a lot in class
   2) by editing a file and then running in the interpreter
      - Much more useful when writing actual programs
      - Same as if you'd typed the commands into the interpreter

To start SML you first open Terminal, which is a shell
   - What can you do in the shell?
      - almost everything you can do with the GUI (i.e. the windows environment)
      - navigate the file system
      - edit files
      - run programs
      - ...
   - then just type sml and you'll see:
      Standard ML of New Jersey v110.69 [built: Mon Jun 8 23:24:21 2009]
      \-

      The dash is the statement prompt from SML

Your first SML function
   > fun add1 x = x + 1;
   val add1 = fn : int -> int

   - The syntax for declaring a function is:
      fun <name_of_function> <arguments> = <function_body>;

      - it looks a bit more like declaring a mathematical function
   - What do you think: "val add1 = fn : int -> int" is?
      - This is the response from SML saying that it has defined a new value
         - The responses can always be broken down as follows:

            val <variable_name> = <value> : <type>
      - The name of the value is add1
      - fn tells us it's a function
      - int -> int is the type of the function
   - This already tells us some interesting things about SML:
      - functions are "first-class entities" in SML. They are treated like any other *value* in the language (ints, strings, etc.)
         - they can stored to variables
         - they can be passed as arguments to other functions
         - they can be returned from functions
      - int -> int
         - this says that the function takes an integer as a parameter and returns an integer
         - How did SML know this?
            - Everything in SML has a type
               - this is called a "strongly-typed" language
               - values do
               - functions, therefore, also do
               - operators (like +, -, *, etc.) do
            - SML has a very good type inferring system
               - + we know is used on numbers
               - therefore, x must be a number
               - therefore the function both takes and returns a number

Using a function
   - Once we've declared a function, we can use it
   > add1 10;
   val it = 11 : int

   - As before, the response from SML can be understood:
      - name of the variable: it
         - what do you think "it" is?
            - name for the previous statement when we don't assign it to a value
               - can actually use it if you'd like in the next statement
      - value is 11
      - type of the value is int
      - get used to looking at the response and understanding it!

   - Anything interesting about how we called the function?
      - no parentheses!
         - we could have added them, but for style reasons we won't put them in
      - like java, we will terminate all statements with a semi-colon

The power function
   - We'd like to write a function called power that takes two numbers, n and k, and produces n^k
   - Write this in Java:
      public static int power(int n, int k){
         if( n == 0 ){
            return 1
         }else{
            n * power(n, k-1)
         }
      }

   - look at the power function in power.sml code
      - now I'm using Aquamacs (an Emacs editor) to view the file
      - like before, we use fun to define a new function
      - what's different?
         - we're not using if/else like in Java
      - SML uses "patterns" to

Using the function
   - We can type this (or copy and paste this) into the SML window and we get the following response:
      val power = fn : int * int -> int

   - Anything new here?
      - the input type to the function is "int * int"
         - this is called a "tuple"
            - a tuple is a combination of two or more values
            - they are defined by putting comma separated values inside parentheses
               > (1, 2);
               val it = (1,2) : int * int
         - the function only has one input
            - in fact, all functions in SML only have one input!

   - to call it we give it a tuple:
      > power (10, 2);
      val it = 100 : int

Any problems with our function?
- Doesn't handle negative numbers
- How could we fix this?

Look at the power2 function in power.sml code
   - We now have three patterns in the function
      1) for when k = 0
      2) for when n = 0 (strictly, this could be handled by the third case)
      3) the general case

   - the if-then-else statement is used to handle the case where k < 0
      - notice that if-then-else represents a value
         - each branch has a value of the same type
         - no matter what branch we go down, we obtain a value
         - we'll see this type of behavior A LOT