• What is a computer?
  A computer is a machine that stores and manipulates data under the control of a program. A computer is essentially a universal machine. It can be made to perform any task, if given the appropriate program. It is truly an amazing tool! This is why it is useful to any student in any major. You can make it solve problems that are of most interest to you.
• What is an algorithm?
  An algorithm gives a step-by-step description of how to solve a problem. Writing clear and concise algorithms to solve problems will be a focus of our work together. Typically we will first write an algorithm to understand how to tackle a problem. Then we use the algorithm as a guide to implementing a computer program.
• What is a computer program?
  A program is a set of detailed instructions, written in a programming language, for telling a computer exactly what to do. There are many different programming languages that exist. We will be learning Python. Programming languages are different than human languages. Every statement in a programming language has a specific syntax and semantics.
• What is syntax?
  The syntax of a programming language is the rules for how to properly format statements within that language. The computer will only execute programs that are syntactically correct. The computer will generate syntax errors whenever it finds syntactic mistakes. You can use these error messages to help you fix your program.
• What is semantics?
  The semantics of a programming language defines the meaning of the statements within that language, in other words how the computer behaves when executing a particular statement. You might have a syntactically correct program that executes, however, it may not solve the problem because you made a semantic error. These kinds of errors can be harder to find and correct.

Every piece of data in a program is of a particular type. Three basic types of data in Python are:

• integer
  An integer is a whole number, such as 1, 2, and 3.
• float
  A float is a real number, such as 1.5, 3.14, and 2.718.
• string
  A string is data that you want to treat literally, often textual information, and is enclosed in quotes, such as "Swarthmore College" or "USA".

What you can do to data depends on its type. As shown below, you can add two integers and get their integer sum. Similarly you can add two floats and get their float sum. You can also add an integer and a float. The result of mixing integers and floats will always be a float. However, you cannot add an integer to a string. This results in a syntax error. Perhaps surprisingly, you can add two strings, which performs concatenation.

>>> 1 + 2
3

>>> 1.2 + 1.3
2.5

>>> 1.2 + 4
5.2

>>> 1 + "testing"
Traceback (most recent call last):
  File "", line 1, in 
      TypeError: unsupported operand type(s) for +: 'int' and 'str'
      
>>> "test" + "this"
'testthis'

Using a variable allows us to name a piece of data and store it in the computer's memory. You can assign a value to a variable using the equal sign.

Syntax of assignment:

  <variable> = <expression>

When describing syntax, anything enclosed between < > indicates that it is a placeholder. A <variable> is a name that is given by a string of characters that starts with a letter and contains no spaces. An <expression> is any set of operations (such as addition, subtraction, multiplication, or division) applied to data that produces a value.

Below are some examples of assignment. The first line assigns x to be the sum of 4 and 3, and we can see that x is now 7. Then we re-assign x to be 10. Finally we re-assign x to be its current value plus 1, which makes x be 11.

>>> x = 4+3
x

>>> x
7

>>> x = 10
x

>>> x
10

>>> x = x + 1
x

>>> x
11
  

Semantics of assignment:

The line, x = x + 1, might look a little weird to you, because it seems mathematically incorrect. But in Python, the semantics of an equal sign is to assign the value of the expression on the right-hand side to the variable on the left-hand side. Python first evaluates x + 1 by looking up the current value of x (which is 10) and adds 1 to it, making 11. Then Python associates the new value (which is 11) with the variable. These examples demonstrate that a variable can be re-assigned many times. It only retains the value of the most recent assignment.