Review (CSCI-UA.0002 - Summer 2017)

Previously on …

Values

What's a value? →

A value is just data!

it can be stored in a variable
it can be used an an expression
it can't be evaluated further (2 + 3 is not a value; it can be evaluated further)

Types

What's a type? →

A type is just a kind or category of values. Name the types that we learned about. →

str (string)
int (integer)
float (floating point number)
complex

Basic Data Types - Strings

str - strings

What's a string? How do we know a literal value is a string? →

ordered sequence of characters
examples: 'string', "string", """string""
escape special characters using backslash or alternate delimiter

String Operators

Name two operators that can be used with strings →

describe what they do
name the types of operands that they work with

+ - concatenation (addition)
- joins strings together
- both operands must be strings
* - multiplication or repetition
- repeats a string
- one operand must be a string, the other, an int

String Operators Continued

What do the following lines of code output (error is a possible answer)? →

print("three" + " blind" + " mice")
print(3 + " strikes")
print(3 * "ring ")
print("hey" * "three")

three blind mice

TypeError: unsupported operand type(s) for +: 'int' and 'str'

ring ring ring

TypeError: can't multiply sequence by non-int of type 'str'

Basic Data Types - Integers

int - integers

What's an int? →

whole number - negative or positive
examples: 1751, -95
"unlimited"

Basic Data Types - Floating Point Numbers

float - floating point numbers

What's a float? How do we know a literal value is a float? →

real number - contains decimal point
10.00, 491.545532111
may overflow
small and large numbers expressed in scientific notation

Operators for Numeric Types

Name 7 operators that can be used on floats and/or ints. →

addition: +
multiplication: *
subtraction: -
division: / (results in a float even if two integers - ex 10 / 2 → 5.0)
integer division: // (closest integer to the left - ex -23 // 3 → -8)
modulo: % (ex 10 % 7 → 3)
exponentiation: ** (ex 2 ** 3 → 8)

Operators for Numeric Types Continued

The following block of code will cause a run time error. Why? →

a = "20"
print(a + 5)

A Type Error will occur… the operator, plus, doesn't support int and str. What can be done to fix this? →

One fix is to convert the variable, a, into an integer using the int function:

a = "20"
print(int(a) + 5)

Type Conversions: a Closer Look

The following functions will attempt to convert the parameter(s) passed in into the type that the function is named after:

int
- converts "int like" parameters to ints
- int("54") → 54
- int(" 54 ") → 54
- int(54.2) → 54
- int("54.0") → results in ValueError
- int("54.2") → results in ValueError

Variables

Variables allow us to bind names to values so that the variable name can be used in place of the literal value. In code, how would we assign the value 12 to a variable called n? →

n = 12

use the assignment operator (just one equals sign): =
name to the left, value to the right

Valid Variable Names

names can only be alphanumeric (numbers and letters) and/or the underscore
the first character has to be a letter (uppercase or lowercase) or an underscore
however, names are case sensitive - case matters
can't be a keyword or reserved word

and       del       from      not       while
as        elif      global    or        with
assert    else      if        pass      yield
break     except    import    print
class     exec      in        raise
continue  finally   is        return 
def       for       lambda    try

Values, Types, Variables Review