Variables, Python Programming

Variables

We cannot  go very far without variables. A variable is a value related to a name that we can bind  to have a particular value  and  then  later use in an expression. When  a variable is calculated in an statement, it is computed by looking  to see to what  value  it is bound.

An interpreter keeps track of which  variables are bound to what  values  in binding environments. An environment defines a mapping between variable values  and names.  The values  may be integers, ?oating point characters, numbers, or pointers to more hard  entities  such as procedures or larger  collections of data.

Here is an example binding environment:

 

b

3

x

2.2

foo

-1012

 

Each row shows a binding: the entry  in the ?rst column is the variable name  and the entry  in the second  column is the value  it to which  it is bound.

 

When  you  start  up  the Python shell,  you  instantly start communicating with  a local binding environment.  You can include a binding or change  an existing  binding by evaluating an assignment statement of the form:

=

where is a variable name  (a string  of letters  or digits  or the character _, not starting with  a digit) and is a Python expression.14 Expressions are always  computed  in some environment.

We may  have the given interaction in a fresh Python shell:

 

>>> a = 3

>>> a

3

>>> b

 

Traceback (most recent call last):

File "", line 1, in

NameError: name 'b' is not defined

>>>

We started by assigning the variable value 3. That included a binding for a to the local environment.

Next, we computed the statement a. The value of a statement with one or more variable names in it cannot be determined unless we know with respect to what environment it is being computed. Thus, we will always speak  of evaluating expressions in an environment. In the mean while the program  of evaluating an expression in some environment E, if the interpreter takes a variable, it checks that  variable in E: if E contains a binding for the variable, then  the related value  is returned; if it does  not, then  an error  is provided. In the Python shell communication above,  we may see that the interpreter was  able to search a binding for a and  return a value,  but  it was  not able to ?nd  a binding for b.

 

Why do we bother de?ning values  for variables? They provide  us to reuse  an intermediate value in a calculation. We may  want  to calculate a formula in two parts, as in:

 

>>> c = 952**4

>>> c**2 + c / 2.0

6.7467650588636822e+23

They  will  also  play  a crucial  role  in abstraction and  the  de?nition of function.  By providing  a name  to a value,  we can isolate  the use of that  value  in other  calculation, so that  if we decide to modify  the value, we only have to change  the de?nition (and not change  a value several  places in the code).

It is ?ne to reassign the value  of a variable; although we use the equality symbol  = to stand for assignment, we are not  creating a mathematical equation of equality.  So, for program, we can write:

 

>>> a = 3

>>> a = a + 1

>>> a

4

 

Posted Date: 8/11/2012 1:34:02 AM | Location : United States







Related Discussions:- Variables, Assignment Help, Ask Question on Variables, Get Answer, Expert's Help, Variables Discussions

Write discussion on Variables
Your posts are moderated
Related Questions
#que Write a program that will take price data for stocks and print it graphically to the screen. Your program should begin by asking the user for the file name. It should then cre

Define a function that draws a pentagon of a size you provide when you call the function. Include a ''''''docstring'''''' as the first line of the function''s body — it tells the p


Non-local references So far, whenever we needed to compute a variable, there  was  a binding for that  variable in the 'local' environment (the environment in which  we were e

protocol rdt2.2 considers there is a bit errors between sender to receiver and also from receiver to sender. So now we have to consider checking bit errors introduced in reply from

List mutation and shared structure Lists are mutable data  structures, which defines  that  we can actually modifies  the values  stored in their components. We do this by usi

Structured data We will often  want  to work  with  large  groups of data.   Rather  than  providing  each  number its own  value,  we need  to manage the data  into natural s

I need how to write small kind of viruses...

For this assignment, you are to: 1.    Modify the output format of your Python script from Assignment 1 to match the input format of your C program from Assignment 2. 2.    W

Suppose the cover price of a book is $24.95, but bookstores get a 40% discount. Shipping costs $3 for the first copy and 75 cents for each additional copy. What is the tota