Today's Topics

Elements of a first program

At the end of class on Wednesday, we wrote our first full python program using the atom editor. You can use any editor you want to modify the file, but to simplify the course discussion, we will be using atom throughout the course. 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.

 My first python program
 A. Danner
 January 2019

def main():
    print("Welcome to cs21.")


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

text in here is ignored by python
but could be helpful to a human
reading the code

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 will 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 defintion.

Quick practice

Variables and types

We use computer programs to process data, but 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 stores a value of a particular type. In the example below, place is a variable that stores the name of a place. The data type currently stored in place is a string containing a sequence of characters between quotes.

place = "Okavango Delta"

The syntax

<var> = <value>

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, e.g.,

numStudents = 35
year = 2019
diff = -6

and the floating point (float) data type for decimal data, e.g,

tempF = 34.1
gravity = 9.8

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

Another good way to experiment is just to open a python shell and try a few one liners.

$ python3
Python 3.6.7 (default, Oct 22 2018, 11:32:17)
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> 3+2
>>> "hello"-7
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for -: 'str' and 'int'
>>> "hello"+"there"
>>> 7.16-4
>>> x=4
>>> y=3
>>> x+y
>>> x=y
>>> y=7
>>> x
>>> y

Converting between types.

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 these examples in the python shell.


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

Getting input from the user.

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

A simple greeting program
A. Danner
January 2019

def main():
  name = input("What is your name?: ")


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

Designing a full program

Suppose we are given the following problem: Have the user enter the current year as a number and the year he or she plans to graduate and print how many more college years he/she has left. We want to design an algorithm that can solve this problem (this one is a bit easy) and then implement and test our solution in python.

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

  1. First think about how you would solve the problem. no keyboards, mice, or even writing utensils. Just think.
  2. Write or sketch a solution on paper. Discuss you solution with a friend or neighbor. This solution does not have to be in python. pseudocode is fine.
  3. Write your solution in python using your editor (atom).
  4. save, run, and test your solution. Go back and make changes if needed

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 little brothers. Always do step 4 if you do step 3.

cd cs21/inclass/w01-intro
atom ./
Enter the current year: 2019
Enter your graduation year: 2022
You have 3 year(s) until graduation