Lecture 2 Variables, Expressions, Statements, and Data Types

Spyder Shortcuts:
ctrl + L: clear all the history in the console window
ctrl + 1: comment/uncomment the selected scripts or the entire line
%reset: clear all the variables in the memory
ctrl + Enter: change the line in console without executing the current code
ctrl + shift + Enter: start a new line in the script window inspite of the location of the cursor.
F5: Run the script.

1. Hello world
You can print a text ('string' type data) using either ' ', or " ".

2. Data Type
Not surprisingly, strings belong to the type str and integers belong to the type int. Less obviously, numbers with a decimal point belong to a type called float, because these numbers are represented in a format called floating-point.

3. Variables:

Programmers generally choose names for their variables that are meaningful. They document what the variable is used for.
The underscore character _ can appear in a name. It is often used in names with multiple words, such as my_name or price_of_tea_in_china.

Some illegal variable names:
7sixers: it doesn't begin with a letter
more$: contains illegal character
class: one of python keywords.

Python has twenty-nine keywords:

You might want to keep this list handy. If the interpreter complains about one of your variable names and you don’t know why, see if it is on this list.

4. Operators and operands
Operators are special symbols that represent computations like addition and multiplication. The values the operator uses are called operands. The following are all legal Python expressions whose meaning is more or less clear:

The symbols +, -, and /, and the use of parenthesis for grouping, mean in Python what they mean in mathematics. The asterisk (*) is the symbol for multiplication, and ** is the symbol for exponentiation.

When a variable name appears in the place of an operand, it is replaced with its value before the operation is performed. Addition, subtraction, multiplication, and exponentiation all do what you expect. However, '/' is division, '//' is integer division. Integer division always rounds DOWN, even in cases like this where the next integer is very close.

5. Order of operations
a. Parentheses have the highest precedence and can be used to force an expression to evaluate in the order you want. Since expressions in parentheses are evaluated first, 2 * (3-1) is 4, and (1+1)**(5-2) is 8. You can also use parentheses to make an expression easier to read, as in (minute * 100) / 60, even though it doesn’t change the result.
b. Exponentiation has the next highest precedence, so 2**1+1 is 3 and not 4, and 3*1**3 is 3 and not 27.
c. Multiplication and Division have the same precedence, which is higher than Addition and Subtraction, which also have the same precedence. So 2*3-1 yields 5 rather than 4, and 2/3-1 is -1, not 1 (remember that in integer division, 2/3=0).
d. Operators with the same precedence are evaluated from left to right. So in the expression minute*100/60, the multiplication happens first, yielding 5900/60, which in turn yields 98. If the operations had been evaluated from right to left, the result would have been 59*1, which is 59, which is wrong.

6. Operations on strings
Interestingly, the + operator does work with strings, although it does not do exactly what you might expect. For strings, the + operator represents concatenation, which means joining the two operands by linking them end-to-end. For example:

1. In the same row, all script after '#' will be commented (won't be executed)
2. Shortcut: ctrl + 1 (number 1, not L)

1. Print the following text separately:
2. Show the keyboard shortcuts for the following actions (to be used in Spyder): (Type these key combinations as comments in Spyder)
3. Complete the following calculation in Spyder:

(2) Type 'import math', then start a new line and type 'math.exp(1). Run the file (F5). Explain your results in a comment line in Spyder.
(3) If you understand what you got from the problem above, you should be able to do this in Spyder:

4. What are the results for the following scripts? (Answer these questions using comments in the same script file)



5. Are these variable names legal? (Answer these questions using comments in the same script file)

Send a SINGLE '.py' file to the email.