Lists

Lists#

Learning Outcomes#

  • Creating lists and dictionaries

  • Extract information from them

Prerequisites#

You should be able to:

Lists#

A list (list) is a collection of any number of values. To declare a list, write different values in between square brackets [] and separated by commas.

elements = ['H', 'He', 'Li']
print(elements)
print(type(elements))

Lists can contain more lists. A list containing lists (which themselves could contain more lists) is called a nested list. To make them more legible, we often write them over several lines.

# A list of molecules where each molecule is represented by a list of atoms
molecules = [["H","H","O"],["H","H","H","C"]]

# The list can identically be written like this
molecules = [["H","H","O"],     # water
             ["H","H","H","C"]] # methane

print(molecules)
Exercise: Nesting lists

Writing lists of lists requires a lot of brackets, which can lead to typos. What follows is a broken list which should contain two lists: the first with the numbers 0.1, 0.2, and 0.3, and the second with 1.0, 2.0, and 3.0. Find the errors in this list and fix them:

coordinates = [[0.1, 0.2, 0.3[]1.0, 2.0 3.0]]
print(coordinates)
Click to view answer

You should keep close track of both the commas inside each sublist and in between each sublist:

coordinates = [[0.1, 0.2, 0.3],[1.0, 2.0, 3.0]]
print(coordinates)

Dictionaries#

A dictionary (dict) also contains multiple values, like a list. However, a dictionary always contains entries in the form of pairs. The first part of the pair is called the “key”, and represents the known value (like a word in a real dictionary). The second part is the “value” and represents the unknown value (like the definition). To declare a dictionary, write pairs of “keys” and “values” in between curly braces {} like this: {key_1: value_1, key_2: value_2}.

atomic_weights = {
  "Hydrogen": 1.008,
  "Helium": 4.002602,
  "Lithium": 6.941
}

print(atomic_weights)
print(type(atomic_weights))
Exercise: Writing a dictionary

The pKa of methanol is 15.3. For ammonia, it’s 32.5. Write a dictionary where the names of these molecules are the keys, and the values are their pKas.

Click to view answer

Here is the correct syntax. Make sure that your colons : and commas , are in the right places!

pKas = {
'methanol' : 15.3,
'ammonia' : 32.5,
}

Exercise: Combining dictionaries and lists

Can you work out what information is contained in data?

data = {
'H' : [[1.007825, 99.9885],[2.014102, 0.0115]],
'C' : [[12.0, 98.93],[13.003355, 1.07]],
'N' : [[14.003074, 99.632],[15.000109, 0.368]],
'O' : [[15.994915 , 99.757],[16.999132, 0.038],[17.999160,0.205]]
}
Click to view answer

Here, we can clearly see why descriptive variable names could be useful. The word data is not helping us at all.

In fact, this is a dictionary containing lists of lists of two numbers. The keys are clearly element symbols. As for the values, if we look at each pair of numbers carefully, the first is always close to an integer, much like an atomic mass, and the second is always just under 100 or just above 0.

With these clues, we could guess that this dictionary contains a list of the atomic masses and abundances of different isotopes for each of four elements.


Accessing parts of a list or dictionary#

Indexing lists#

It can be handy to store sequences of data into a list. For example, if we want to store an absorption spectrum of 100 data points, we just need one list for absorbance and another for wavelength, instead of 200 variables containing different floats.

However, we can find ourselves wanting to access only one part of the list at a time. To this end, we can use indexing. To indicate that we are only referring to a specific element of a list, we follow the name of the list by a number in square brackets:

elements = ['H', 'He', 'Li']
print(elements[1])

In this example, we used the number 1, which returned the second element of the list. This is because the Python programming language counts the position of an element in a list starting from 0. This position is called the index.

Exercise: Indexing

Print the first element of this list:

elements = ['H', 'He', 'Li']
Click to view answer

All you have to do is to use the index 0.

elements = ['H', 'He', 'Li']
print(elements[0])

If you want to count indices from the end instead of the beginning, use negative numbers and start from -1:

elements = ['H', 'He', 'Li']
print(elements[-1])

Slicing#

You may also need to access ranges of values within a list. In this case, instead of writing one index, write two of them separated by a colon :. The resulting list will be the range of values between the two numbers, including the first value and excluding the last. This process is called slicing.

elements = ['H', 'He', 'Li', 'Be']
print(elements[1:3])

Note how Be was not printed, despite the fact that it corresponds to elements[3].

Exercise: Slicing

Indexing and slicing gets a little more delicate when dealing with nested lists. Try to extract the list [2,3] from the following nested list:

elements = [[1,2,3,4],
           [5,6,7,8]]
Click to view answer

It may help to work step by step. First, we want to isolate the first sublist ([1,2,3,4]). Since it’s the first list, we can use the index 0:

elements = [[1,2,3,4],
           [5,6,7,8]]
print(elements[0])

Thanks to this, we can verify that elements[0] indeed corresponds to the first sublist, so we can further slice this list by adding more square brackets:

elements = [[1,2,3,4],
           [5,6,7,8]]
print(elements[0][1:3])

Exercise: Slicing a nested list

Print the list containing Li and Be by slicing the following list

elements = ['H', 'He', 'Li', 'Be']
Click to view answer

We should change the indices accordingly.

elements = ['H', 'He', 'Li', 'Be']
print(elements[2:4])

Accessing dictionary values#

You can also use access values inside dictionaries but the syntax is different than for lists. Here, instead of writing a number in square brackets, you write a key. In return, you will obtain the corresponding value:

atomic_weights = {
  "Hydrogen": 1.008,
  "Helium": 4.002602,
  "Lithium": 6.941
}

print(atomic_weights["Helium"])
print(type(atomic_weights))
Exercise: Accessing dictionaries

Print the atomic weight of lithium, using the dictionary provided:

atomic_weights = {
 "Hydrogen": 1.008,
 "Helium": 4.002602,
 "Lithium": 6.941
}
Click to view answer

Simply use "Lithium" as the key:

atomic_weights = {
 "Hydrogen": 1.008,
 "Helium": 4.002602,
 "Lithium": 6.941
}
print(atomic_weights["Lithium"])

Learning outcomes#

  • Creating lists with square brackets and dictionaries with braces.

  • Extract information from them by indexing, slicing, and using keys for dictionaries.

Summary#

  • Lists can store multiple values in one same variable. They use [].

  • Dictionaries can do this too but are only useful when we will later want to access values by using keys. They use {}.

  • You can use indexing or slicing by writing the name of a list followed by square brackets containing numbers: masses[3:5].

  • For a dictionary, instead of numbers, write the key inside the square brackets: masses['Li'].

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