Python variables and types
Contents
Python variables and types¶
Python uses many data types. Here we cover the most basic ones with particular emphasis on numerical data types.
Numerical data types¶
We have already played with two kinds int and float.
int stands for integer and float stands for floating point number (an approximation to a real number).
You can use the type() function to see the type of something.
Here are some int’s:
type(1)
int
type(2)
int
And here are some float’s:
type(1.24235)
float
type(1e-5)
float
type(1.0)
float
type(2.)
float
When you add or multiply two int’s, you get an int:
type(1 + 2)
int
type(2 * 4)
int
Same thing for two float’s:
type(1.0 + 2.3)
float
type(2.1 * 3.5)
float
But what happens when you mix int’s and float’s?
Let’s try it out:
type(1 + 2.4)
float
Adding an int to float gave you a float. Let’s try another example:
type(2 * 3.4)
float
Multiplying an int with a float gave you a float.
This is a general rule: “When you have an operation that involves both int’s and float’s, the int’s are promoted to float’s and then the operation is carried out. Thus, the result is always a float.
What do we mean by promoted. We mean that when python sees:
2 + 2.4
it changes it first to
2.0 + 2.4
and then carries out the addition.
Python can do this by using the float() function.
Here is an example:
float(1)
1.0
type(float(1))
float
Similarly, there is an int() function that can change a float to an int:
int(23.4353)
23
type(int(23.33453))
int
Let’s see what type is math.inf and math.nan:
import math
type(math.inf)
float
type(math.nan)
float
Questions¶
First guess and then try out, the type of the following expressions:
1 / 2.02 // 32.0 // 35 % 35.0 % 3int(5.0 % 3)
# Your code here
Python variables¶
You can use variables to store the values of things. Here is a variable:
x = 1
Here is another variable:
y = 2
Now you can do:
z = x + y
and you can look at z:
z
3
Questions¶
In the code block provided below evaluate the angle between the two vectors: $\( \vec{r}_1 = 4\hat{i} + 3.5\hat{j} + 2.5\hat{k}, \)\( and \)\( \vec{r}_2 = 1.5\hat{i} + 2.5\hat{j}. \)\( Remembet that the angle between two vectors is: \)\( \theta = \frac{\vec{r}_1\cdot \vec{r}_2}{|\vec{r}_1||\vec{r}_2|}. \)$ Hint: Define variables
x1,x2,y1,y2,z1,z2and usemath.acos().
# Your code here
Boolean types¶
Boolean types are either True or False.
As a matter of fact True and False are special Python keywords.
Here it is:
True
True
False
False
type(True)
bool
type(False)
bool
What can you do with True and False?
We will learn more about them when we talk about conditionals (in another hands-on activity), but here are some examples:
True and True
True
So and is another Python keyword…
True and False
False
True or False
True
False or False
False
So or is another Python keyword…
Here is the final example:
not True
False
not False
True
So not is another Python keyword…
You can also have longer expressions with bool’s:
True and (False or True)
True
And just like for numerical types, there is a bool() function that can turn something into a bool type. For example:
bool(1)
True
bool(0)
False
bool(1.0)
True
bool(0.2242)
True
The NoneType¶
Python has a special type called the NoneType.
It is a special construct that is used to represent the “nothing.”
Here it is:
None
Notice that when you trye to see None you do not see anything. This is because it is representing “nothing.” You cannot see the “nothing.”
What is the type of None?
type(None)
NoneType