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Variables

Introduction
In Math class, you learned that a variable is a symbol for a number that you don't know yet.  In Computer Science, a variable is information that is stored in a program.
Variables in Python
A variable name refers to a particular object.
A variable is a location in memory where a programmer can store a value.  
Variables are created when first assigned.  
The type of the variable is determined by Python automatically.
A Variable is data that can change. 
Python Data Types
Data Type          
Explanation
Examples
int
positive or negative whole numbers
1, 2, 3, 4, 5
float
floating point numbers (numbers with a decimal)
1.1, 2.2, 3.3
string
a set of characters (letters, numbers, and/or special characters) enclosed by single or double quotation marks.
"hamster", 'gerbils'
boolean
Boolean values are either True or False
True, False          
 Variable names can be any length.  However, stick to variable names that are short, sweet and to the point.  Good variable names in Python will make your code easy to read and understand.
Variable Name Rules
Rule
Examples
Must begin with a letter or underscore.
x, y, points, robux, money, friends
Other characters can be letters, numbers or underscore.
x2, roblox_player, jail_break
Are case sensitive
student is different than Student
Python has keyword/reserved words that you cannot use as variable names.
False, class, finally, is, return, None, continue for, lambda, try, True, def, from, nonlocal, while, and, del global, not, with, as, elif, if, or, yield, assert, else, import, pass, break, except, in, raise
Can be any length greater than or equal to 1 but...
you’re the one who has to remember what it is!
Variable names are case sensitive.
Assigning a value to a Variable
To assign a value to a variable, use the = sign.
x = 3
name = "Sam"
robux = 100.50
Variable Roles
Variables have typical uses that are called variable roles.  Variable roles are not the same as types of variables (integer, float, string, boolean, etc.) since the concept of roles is a classification telling what kind of a task a variable has in a certain program.
Role
Pattern
Example
Constant
Assigned once                      
Assigned at the beginning of a program or at the head of a code block.  By convention, make this variable name in all caps. 
​
Example:
RATE = 7
Most recent                   
Fluctuating
Assigned user input.
​
Example:
name = input(“What’s your name? “)
Accumulator
Running total
1. Initialize
2. Accumulate
3. Print

Example:
sum = 0 # initialize to 0.
for x in range(3):
   num = int(input("Give me a number: "))
   sum = sum + num
print(sum)
Best-so-far
Record holder
Keep track of the record holder while iterating.

Pattern:
1. Initialize to worst-possible value before the loop or first value in a sequence of values.
2. During iteration, compare to best-so-far and assign to the new value, if needed.
3. After the loop, best-so-far will be the best-so-far.

Example:
x = 0
while x < 3:
     num = int(input("Give me a number: "))
     if x == 0:
         max = num
     else:
         if num > max:
            max = num
     x = x + 1 
One-Way Flag                        
Variable will be reset once condition is met

Pattern:
1. Initialize the flag so the condition has not been met.
2. With each iteration, check for the condition and change the flag if true.

Example:
done = False
while not done:
     done = True
Stepper
Step through a pre-determined sequence of values
Iterates a specific number of times.
​
Example:
x = 0
while x < 5:
    print(x)
    x = x + 1
​
for x in range(5):
    print(x)
Walker
Elements of iterator
Elements of a list during iteration.

Example:
for x in friends:
   print(x)

Elements of a dictionary during iteration.

Example:
for key in d:
    print(key, d[key])
Temporary
Needed only for a short period of time.
The tax is computed into the temporary tax 
​
Example:
income = float(input("Income: "))
TAXRATE = .25
tax = TAXRATE * income
​

Output Mistakes

Variable Data Type Examples

Last modified 4yr ago

Data Type	Explanation	Examples
int	positive or negative whole numbers	1, 2, 3, 4, 5
float	floating point numbers (numbers with a decimal)	1.1, 2.2, 3.3
string	a set of characters (letters, numbers, and/or special characters) enclosed by single or double quotation marks.	"hamster", 'gerbils'
boolean	Boolean values are either True or False	True, False

Rule	Examples
Must begin with a letter or underscore.	x, y, points, robux, money, friends
Other characters can be letters, numbers or underscore.	x2, roblox_player, jail_break
Are case sensitive	student is different than Student
Python has keyword/reserved words that you cannot use as variable names.	False, class, finally, is, return, None, continue for, lambda, try, True, def, from, nonlocal, while, and, del global, not, with, as, elif, if, or, yield, assert, else, import, pass, break, except, in, raise
Can be any length greater than or equal to 1 but...	you’re the one who has to remember what it is!

Role	Pattern	Example
Constant	Assigned once	Assigned at the beginning of a program or at the head of a code block. By convention, make this variable name in all caps. Example: RATE = 7
Most recent	Fluctuating	Assigned user input. Example: name = input(“What’s your name? “)
Accumulator	Running total	1. Initialize 2. Accumulate 3. Print Example: sum = 0 # initialize to 0. for x in range(3): num = int(input("Give me a number: ")) sum = sum + num print(sum)
Best-so-far	Record holder	Keep track of the record holder while iterating. Pattern: 1. Initialize to worst-possible value before the loop or first value in a sequence of values. 2. During iteration, compare to best-so-far and assign to the new value, if needed. 3. After the loop, best-so-far will be the best-so-far. Example: x = 0 while x < 3: num = int(input("Give me a number: ")) if x == 0: max = num else: if num > max: max = num x = x + 1
One-Way Flag	Variable will be reset once condition is met	Pattern: 1. Initialize the flag so the condition has not been met. 2. With each iteration, check for the condition and change the flag if true. Example: done = False while not done: done = True
Stepper	Step through a pre-determined sequence of values	Iterates a specific number of times. Example: x = 0 while x < 5: print(x) x = x + 1 for x in range(5): print(x)
Walker	Elements of iterator	Elements of a list during iteration. Example: for x in friends: print(x) Elements of a dictionary during iteration. Example: for key in d: print(key, d[key])
Temporary	Needed only for a short period of time.	The tax is computed into the temporary tax Example: income = float(input("Income: ")) TAXRATE = .25 tax = TAXRATE * income