CS150 - Fall 2013 - Class 1
Discuss with your neighbor...
- What is computer science?
- computer science is more than programming
- study of computers
- solving problems using computers as the tool
Goals of the class
- introduction to computer science
- introduction to programming in Python
- design, code and debug medium-sized programs in Python by the end
- CS as a tool for the sciences
- data processing and analysis
- applications in math, engineering and the sciences
- examine a few other programming languages commonly used in scientific programming (Matlab and R)
- Administrative material
- need to get it (either online or at the bookstore). Make sure to get the *first* edition!
- Class M/W with a lab on Friday
- Assignments will generally be due Wed
- All handouts, labs, assignments, etc. will be posted on the course web site
- Late policy
- keep up with the reading
- do the daily homework problems
- Honor_code and collaboration (read the class policy!)
- computer use during class
Style of the course
- ask lots of questions
- be expected to do group work in class
- You'll be expected to be here in class and try particularly hard not to miss Friday class/labs
- I'll post my notes and examples online. You may still want to write some things down, but you don't have to write every word down
- I assume no prior computer science, programming or science background
- The pacing may be a bit slow for some early, but it will get harder
- Some example final projects I've done in the past
Wing IDE 101
- We'll be using Wing IDE for this course as our interface into Python
- What is an IDE?
- Integrated Development Environment
- Incorporates a text editor with other tools for running, debugging and navigating through the code
- When it starts, three parts of the window
- the main part is the editor
- the bottom right is the interpreter
- the bottom left has search, etc. which can be useful
- Python is an interpreted language
- you can type commands and get an immediate response back
- many programming languages require you to compile the program first and then run it
- Python makes a great calculator
- the ">>>" is the interpreter prompt where you type statements
- when you hit enter/return, Python executes the statements and gives you the answer
- Python has all of the standard mathematical operations
- What operations might you want?
- +, -, *, /
- ** (power or exponentiation)
- % (mod aka remainder)
>>> 11 % 7
- What is operator precedence?
- Python follows the normal operator precedence you're used to for math
- things in parenthesis get evaluated first
- ** is next
- %, * and / next
- + and - last
- What should be the answer for 11/2?
- Why 5?
- anything that represents a value (e.g. a number) is called an expression
- contrast this with a statement, which tells the computer something to do
- for example, "walk over there" is a statement in English
- or, for computers, something like, "print this out" or "draw that on the screen"
- All the things we've seen so far have been expressions
- In Python (and many programming languages) all expressions have a type
- 11 is an expression
- 2 is an expression
- the "type" of an expression determines how Python interprets and understands an expression
- Python is a "strongly typed" language: every expression in Python has a type
- what are the types of the expressions above?
- they are both numbers, but Python makes a distinction between integers and floating point numbers (i.e. numbers with decimals)
- Does this explain why 11/2 gave us 5?
- Because both 11 and 2 are integers, the result is also an integer
- Python rounds down (i.e. towards -infinity or to the smaller number) when doing integer division
- What should be the answer off 11/-2?
- We can make a number a float by adding a decimal point
- for a given operation, if one of the numbers is a float, then the other is converted to a float and the result will be a float
- what is the result of
- the parenthesis will get evaluated first (11/2) will give us the integer 5
- we'll then do floating point multiplication by 4.0, giving us a float of 20.0
- you're having a party and you're trying to figure out how many hot dogs to buy. Here's what you know:
- tim isn't a big fan of hot dogs, so he'll only eat 1
- amy generally eats 2
- todd always eats twice as many as amy
- brenda eats one less than todd
- mark eats half as many as brenda, but likes to take an extra on his way home
- try and do this on paper
- 13 (assuming that if someone eats half a hot dog, we still have to count the whole thing)
- how did you do it?
- how could you figure this out in Python?
- might be able to do it, but would require a lot of remembering (or writing things down)
- variables allow us to store things and then use them in other expressions
- a variable is storage for a value
- it holds a value
- we can change its value
- we change the value of a variable using '=' (also know as assigning to a variable)
- changing the value of a variable is a statement. It tells the interpreter to do something, but does NOT represent a value
tim = 1
amy = 2
todd = 2 * amy
brenda = todd - 1
mark = (brenda+1)/2 + 1
total_hotdogs = tim + amy + todd + brenda + mark
- When assigning to a variable, Python evaluates what is on the right hand side of the equals sign and then assigns this value to the variable
- if I then type amy = 4, what happens to todd?
- todd stays the same
- generally you want to give good names to variables (x and y are not good names unless they represent x and y coordinates)
- variables should be all lowercase
- multiple words should be separated by an '_' (underscore)
what if you get a text from amy and she now says she's planning on eating 4?
- we'd have to re-enter each of the lines (well except the first one)
programs in Python
- besides interacting with the shell, we can write statements in a file and then run them
- the top window in the Wing IDE allows us to do this
- on many levels, it behaves a lot like a text editor (e.g. Word). You can:
- create new files
- open files
- save files
- edit files
- I've typed in the same code we'd typed into the shell, but now in the text editing section
- I've saved it in a file called bbq.py
- we'll use the extension .py by convention to indicate a Python program
- Anything different from what I'd typed before into the shell?
- In a program (vs. typing it into the shell) if you want to see the value of a variable or an expression, you need to "print" it
- If I didn't include this, I wouldn't get any answer (in fact I wouldn't see anything)
- I've used whitespace
- you can use spaces without affecting how the code executes
- we use blank lines to make the code more understandable
- I've included "comments"
- comments in Python are designated using '#' (the pound sign)
- python ignores everything from the # to the end of the line
- you can put them on lines by themselves
- or if you have short comments, you can add them at the end of a line
- comments are VERY important
- they allow you to communicate in plain English (to others and to yourself when you look at the code later)
- you will be required to put them in your programs for this course
running the code
- With an IDE, not only can you edit code, you can also run it
- the green arrow, runs the program
- when you run a program, you get a brand new shell session (in the bottom right)
- Any variables, etc. you may have manually created in the shell window will NO LONGER EXIST
- it executes your program a line at a time from the top
- it's like you typed all of those commands in the shell
- the one difference is that you don't get line-by-line feedback
- for example, if you put 4+4 in a program on a line by itself, you won't see anything
- if you want to see the value of a variable or an expression, you need to "print" it
- if we run this program, we see 13 printed out, like we would expect