Last week you read through the syllabus (CS21 webpage) to learn about CS21, and some of the course structure and design and, and you worked on Lab 0 to learn some of the tools that you will use this semester.

Today, we are going to revisit some the course content, resources, talk about what CS21 is about (and what it is not about) and start writing our first Python program!

First ssh into a CS lab machine (use your username in place of tnas):

Then run update21. If update21 worked for you, it should have created an empty welcome.py file in your ~/cs21/inclass/w02-intro directory. Let’s practice some basic Linux commands to:

Inside the vim editor, we are going to implement our first Python program. Type in the following content (make sure you are in insert mode), and use your name instead of A. Student to take credit for your work.

Save the file and exit vim by entering vim ESC mode, and then typing :wq.

You should be back at the command prompt and you can run the program by typing the following in the terminal:

Did it work? If not, what errors did you observe?

At the end of class on Monday, we wrote our first full python program using the vim editor. Let’s look at a few of the components of our first program and explore python syntax and semantics. A programming language’s syntax describes the rules for valid placement of symbols and overall program structure. Semantics describes the meaning of the overall program as it relates to the syntax.

At the top of our program, we see a comment in triple block quotes:

Next we see the definition of our main function. For the first few weeks of the semester, our programs will always have this def main(): syntax. Later in the semester, around week 4, we’ll write other functions of our own besides main. A function is a reusable piece of code that can potentially accept some input data and produce some output. For now, we simply place what we want our program to do inside the body of the main function by indenting. So far, we only have a single print (print() is a built-in function), but we will soon add more.

The function definition is merely that: a definition. It does not actually run the code inside immediately. To see the output, we need to call the function by specifying the function’s name (without the def or the trailing :) and including any required input in between the parentheses. main does not require any input, so a simple main() as our last line will suffice. Note that this line is not indented or inside the function definition.

We use computer programs to process data, so a program must have a way of storing information and referring to it later. Our primary way of doing this is through the use of variables. A variable is a named container that refers to value of a particular type. In the example below, place is a variable that refers to the value of the name of a place. The value that the variable place refers to is string containing a sequence of characters between double quotes ("Paris").

The syntax

assigns the variable name on the left hand side the value of the expression on the right hand side. It is illegal, or a syntax error to have a constant value on the left and a variable name on the right.

In addition to the string type (str), we will initially focus on the integer (int) data type for whole numbers, for example:

and the floating point (float) data type for decimal data, for example:

Let’s try out a program that assigns some variable of different types and using the print() function to print out their values.

First let’s open in vim to see that the program does:

Then let’s run it using the python3 interpreter:

You can also try modifying the program code by changing the values assigned to variables, or by using the + operator to assign variables the values of some simple arithmetic expressions. Then, save your changes in vim and re-run in python3 and see what happens. You can combine two numeric values with + and also two strings with +. Try out some examples like this and see what happens (remember to save your changes to your file in vim with :wq before running):

We also encourage you to read Chapter 2 of the online text book for a longer discussion on topics regarding

One of the most helpful built-in functions in the first few weeks of the course will be the input() function. When provided a string prompt as input argument, this function prints the prompt to the screen and waits for a user to type something. When the user presses the Enter key, their response is returned as a value by the input() function as a string. A sample usage might be

The program above is pretty terse. How could you modify it so it prints "Hello" followed by the entered name on the same line?

Sometimes you may find it helpful or necessary to change a value of one type to another type. For example the value "7" is a string because it enclosed in quotes. But the contents of the string, the character 7 is also a valid integer. You can convert a value to an integer using the int() built in function. Try out some examples in the types.py program.

First open in vim:

Try out some of these and see what happens:

Similarly, you can use float() to convert a value to a float, or str() to convert a value to a string.

Suppose we are given the following problem: Have the user enter the current year as a number and the year they plan to graduate and print how many more college years they have left. We want to design an algorithm that can solve this problem and then implement and test our solution in python.

Here are some steps a computer scientist might use to solve the problem:

The real innovation is in steps 1 and 2. Steps 3 and 4 are sometimes skipped, argued logically/mathematically, or handed off to new hires, grad students, or younger siblings. Always do step 4 if you do step 3.

Mathematical calculations are one of the most common uses of computer programming. Below is a quick summary of python3 math syntax and semantics. I encourage you to try examples in using the operators.py program after the quick summary, and ask questions about strange cases.

You can also combine many operators in one statement e.g. 2*x+10, but note you must explicitly include the * to multiply. The algebra form 2x+3 produces a syntax error.

Take a minute or two, look at the example operators.py program in vim, and try running it. Then play aournd with modifying the program with other operators (remember to save your changes to the file, :w in vim). Do they do anything unexpected? Try to combine them in larger expressions. What happens?

Some math operators can be used with strings:

Some other math operators can also be used with strings, but in somewhat more complicated ways. We will explore these operators a little later in the semester. Stay tuned!