12.1: Accessing Individual Elements
- Page ID
- 39278
We can use the len() function, which we’ve already learned two uses for, in yet a third way: to ascertain the number of key/value pairs in a series. Using the Figure 11.1.2 example:
Code \(\PageIndex{1}\) (Python):
print(len(alter_egos))
| 4
Accessing the value for a given key uses exactly the same syntax that NumPy arrays used (boxies), except with the key in place of the numeric index:
Code \(\PageIndex{2}\) (Python):
superhero = alter_egos['Peter']
print("Pssst...Peter is really {}.".format(superhero))
| Pssst...Peter is really Spidey.
This is why it’s important that the keys of an associative array be unique. If we type “alter_egos['Peter'],” we need to get back one well-defined answer, not an ambiguous set of alternatives.1 The values, on the other hand, may very well not be unique.
To overwrite the value for a key with a new value, just treat it as a variable and go:
Code \(\PageIndex{3}\) (Python):
alter_egos['Bruce'] = 'Batman'
print(alter_egos)
| Bruce Batman
| Peter Spidey
| Tony Iron Man
| Thor Thor
| dtype: object
This same syntax works for adding an entirely new key/value pair as well:
Code \(\PageIndex{4}\) (Python):
alter_egos['Diana'] = 'Wonder Woman'
print(alter_egos)
| Bruce Batman
| Peter Spidey
| Tony Iron Man
| Thor Thor
| Diana Wonder Woman
| dtype: object
It’s just like with ordinary variables, if you think about it. Saying “x=5” overwrites the current value of x if there already is an x, otherwise it creates a new variable x with that value.
Finally, to outright remove a key/value pair, you use the del operator:
Code \(\PageIndex{5}\) (Python):
del alter_egos['Tony'] ]
print(alter_egos)
| Bruce Batman
| Peter Spidey
| Thor Thor
| Diana Wonder Woman
| dtype: object
Bye bye, Iron Man.
Don’t get mad when I tell you that all of the above operations work in place on the Series, which is very different than some of the “return a modified copy” style we’ve seen recently. Hence all of these attempts are wrong:
Code \(\PageIndex{6}\) (Python):
alter_egos = del alter_egos['Tony'] <--- WRONG!
alter_egos = alter_egos['Bruce'] = 'Batman' <--- WRONG!
alter_egos = alter_egos['Diana'] = 'Wonder Woman' <--- WRONG!
You don’t “change a value and get a new Series”; you just “change it.”
Accessing by Position
One slightly weird thing you can do with a Pandas Series is ignore the key (index) altogether and instead use the number of the key/value pair to specify what value you want. This gives me the heebie-jeebies, because as I explained back on p. 57, there really isn’t any meaningful “order” to the key/value pairs of an associative array. In true All Things To All People™ fashion, however, Pandas lets you do this.
Accessing a Value by Position
You can ask for the value of (say) “the second” superhero. To do so, you use the bizarrely-named .iloc syntax:
Code \(\PageIndex{7}\) (Python):
a_hero = alter_egos.iloc[1]
print(a_hero)
| Spidey
This is occasionally useful, so I mention it for completeness. The .iloc numbers start with 0 (not 1) as is true throughout Python.
Accessing a Key by Position
Similarly, you can get the key (as opposed to the value) of the key/value pair at a particular position. To ask for the key of “the second” superhero, you use the .index syntax:
Code \(\PageIndex{8}\) (Python):
a_secret_hero = alter_egos.index[1]
print(a_secret_hero)
| Peter