CS 1112: Spring 2019

Variables and assignment

Introduction

There are many forms of abstraction. The one we are concerned with now is the use of symbols to represent values.

Often when digital problem solving (think programming) we need to remember a value for future usage. The value might be something acquired (gotten) from the user of the program or it might be the result of a computation.

To be remembered, a value must be stored in the memory of the computer running the program.

Most programming languages support the remembering of values in computing memory with variables and assignment statements.

A variable is a symbolic name for some computer memory being reserved by the the program to remember a particular value. In Python, a variable name is always an identifier. As you should recall, an identifier starts with an alphabetic character and can then be followed by one or more alphanumeric (alphabetic or numeric) characters.

An assignment statement consists of two parts, separated by a single =. We call the = operator, the assignment operator. The things on either side of the = are called operands.

The first use of a variable must be as the left operand of an assignment statement.

When an assignment assignment is encountered (executed / ran) by a Python interpreter, the right operand is evaluated first to determine its value.

For now, the left operand will be always be an identifier. Python will reserve memory to store the evaluation, and then associate the identifier with that memory; that is, the identifier is now a variable whose value can be used in future Python statements.

Now take out a piece of paper and draw a horizontal lines across after the first one-third of the paper and the second one-third of the paper.

Think of a number. In the top section, write an assignment statement that assigns a variable n with that number. Along with a print() statement that echos (prints) n. If I was doing it, I would be using 3 as the number.

n = 3
print( "n =", n )

Next pretend you are the computer and execute the code. To simulate the assignment statement, using the middle section of the paper draw a box and label it n. Store your number in the box. To simulate the print() statement, copy the label literal string and the value in the n box in the bottom section of the paper.

After carrying out these two statements, my paper looks like this.

Now below the previous code, let's write an assignment statement that assigns a variable s to be twice the value of n. And also, a print() statement that echos the value of s.

s = 2 * n
print( "s =", s )

Next pretend you are the computer and execute the new code. To simulate the assignment statement, using the middle section of the paper draw a box and label it s. Get the value in the n box and multiply that value by 2 and store your the product in the s box. To simulate the print() statement, copy the label literal string and the value in the s box in the bottom section of the paper.

After carrying out these two statements, my paper looks like this.

Now below the previous code, let's write an assignment statement that assigns a variable t to be value of s and 4. And also, a print() statement that echos the value of t.

t = s + 4
print( "t =", t )

Next pretend you are the computer and execute the new code. To simulate the assignment statement, using the middle section of the paper draw a box and label it t. Get the value in the s box and add 4 to that value and store your the sum in the t box. To simulate the print() statement, copy the label literal string and the value in the t box in the bottom section of the paper.

After carrying out these two statements, my paper looks like this.

Now below the previous code, let's write an assignment statement that assigns a variable u to be one half of t using integer division. And also, a print() statement that echos the value of u.

u = t // 2
print( "u =", u )

Next pretend you are the computer and execute the new code. To simulate the assignment statement, using the middle section of the paper draw a box and label it u. Get the value in the t box and integer divide that value by 2 and store your the quotient in the u box. To simulate the print() statement, copy the label literal string and the value in the u box in the bottom section of the paper.

After carrying out these two statements, my paper looks like this.

Now below the previous code, let's write an assignment statement that assigns a variable v to be the difference of u and n. And also, a print() statement that echos the value of u.

v = u - n
print( "v =", v )

Next pretend you are the computer and execute the new code. To simulate the assignment statement, using the middle section of the paper draw a box and label it v. Get the values in the u and n boxes and get their differene and store your the result in the v box. To simulate the print() statement, copy the label literal string and the value in the v box in the bottom section of the paper. I bet your value for v is 2. Can you figure out why?

After carrying out these two statements, my paper looks like this.

Why are variables called variables

Variables are call variables because we are allowed to update them in other assignment states.

Now below the previous code, let's write an assignment statement that assigns variable u to be -5. And also, print() statement that echos the value of u.

u = -5
print( "u =", u )

Next pretend you are the computer and execute the new code. To simulate the assignment statement, using the middle section of the paper change the value in the u box to -5. To simulate the firstprint()statement, copy the label literal string and the value in theu` box in the bottom section of the paper.

After carrying out these two statements, my paper looks like this.

Now below the previous code, let's write a print() statement that echos the value of v.

print( "v =", v )

To simulate the `print() statement, copy the label literal string and the value in the v box in the bottom section of the paper. Observe that the most recent assignment of u did not affect v. An assignment only effects the target of the assignment statement.

After carrying out the statement, my paper looks like this.

Altogether now

Putting it altogether, we have the below, where ??? is the number you picked. The code can be found in program variety.py. The code will be made available during class.

n = ???
print( "n =", n )

s = 2 * n
print( "s =", s )

t = s + 4
print( "t =", t )

u = t // 2
print( "u =", u )

v = u - n
print( "v =", v )

u = -5
print( "u =", u )

print( "v =", v )

Variables and assignment

Links

Introduction

Why are variables called variables

Altogether now