## Using Variables in Python

The "print" function is a Python instruction that will output variable values and results to the computer screen. This instruction will allow us to automatically output data results to the screen when running a program. This section reviews what we've covered so far and introduces examples of the print function. Try executing these instructions in the command line window to make sure you understand how to use the print function:

z=1.45
print(z)

Notice how the print function outputs the value of the variable z to the screen.

It is possible to use the print command in a slightly more sophisticated way. Type this set of commands in the command window:

temperature=40
print('Temperature outside = ', temperature)

Observe, it is possible to "dress up" the screen output by adding some extra descriptive text. We will see more examples of this later on as we introduce and develop expertise with the string data type. But, as a preview of using the print function with text, try typing this command:

print('Hello world!')

Congratulations! You have now executed what is probably the most-used example in just about every introductory programming course.

Read this page to see more examples of using the print function. Try typing some of those examples in the Repl.it IDE to be sure you are comfortable with the print function. Consider executing these instructions one after the other (that is, sequentially):

var1=22
var1=-35
var1=308

What value will the variable var1 contain after these instructions are executed? You can check your answer by using the print function.

### Variables

Any Python interpreter can be used as a calculator:

Python
3 + 5 * 4

Output
23

This is great but not very interesting. To do anything useful with data, we need to assign its value to a variable. In Python, we can assign a value to a variable, using the equals sign =. For example, to assign value 60 to a variable weight_kg, we would execute:

Python
weight_kg = 60

From now on, whenever we use weight_kg, Python will substitute the value we assigned to it. In layman's terms, a variable is a name for a value.

In Python, variable names:

• can include letters, digits, and underscores
• are case sensitive.

This means that, for example:

• weight0 is a valid variable name, whereas 0weight is not
• weight and Weight are different variables

### Types of data

Python knows various types of data. Three common ones are:

• integer numbers
• floating point numbers, and
• strings.

In the example above, variable weight_kg has an integer value of 60. To create a variable with a floating point value, we can execute:

Python
weight_kg = 60.0

Python
weight_kg_text = 'weight in kilograms:'

### Using Variables in Python

To display the value of a variable to the screen in Python, we can use the print function:

Python
print(weight_kg)

Output
60.0

We can display multiple things at once using only one print command:

Python
print(weight_kg_text, weight_kg)

Output
weight_kg_text = 'weight in kilograms: 60.0

Moreover, we can do arithmetic with variables right inside the print function:

Python
print('weight in pounds:', 2.2 * weight_kg)

Output
weight in pounds: 132.0

The above command, however, did not change the value of weight_kg:

Python
print(weight_kg)

Output
60.0

To change the value of the weight_kg variable, we have to assign weight_kg a new value using the equals = sign:

Python
weight_kg = 65.0
print('weight in kilograms is now:', weight_kg)

Output
weight in kilograms is now: 65.0

### Variables as Sticky Notes

A variable is analogous to a sticky note with a name written on it: assigning a value to a variable is like putting that sticky note on a particular value.

This means that assigning a value to one variable does not change values of other variables. For example, let's store the subject's weight in pounds in its own variable:

Python
# There are 2.2 pounds per kilogram
weight_lb = 2.2 * weight_kg
print(weight_kg_text, weight_kg, 'and in pounds:', weight_lb)

Output
weight in kilograms: 65.0 and in pounds: 143.0

Python
weight_kg = 100.0
print('weight in kilograms is now:', weight_kg, 'and weight in pounds is still:', weight_lb)

Output
weight in kilograms is now: 100.0 and weight in pounds is still: 143.0

Since weight_lb doesn't "remember" where its value comes from, it is not updated when we change weight_kg.

Source: The Carpentries, https://swcarpentry.github.io/python-novice-inflammation/01-intro/index.html