Combining Lists and Dictionaries¶

While the list and dictionary data structures are useful individually, they are even more useful in combination. One combination is a list each of whose elements is a dictionary. Another combination is a dictionary at least one of whose values is a list.

A List of Dictionaries¶

While a single instance (a book, a weather report, an earthquake) is of interest we are often more interested in a list of instances (all the books available on Amazon, the forecasted weather for a city, all earthquake reports in the past month). We have dealt extensively with lists of individual properties (a list of book prices, a list of forecasted temperatures, a list of earthquake magnitudes). We now want to see that each list item can be more than a simple basic type (int, float, string, Boolean), each list item can be a dictionary value.

A simple example of a list of dictionaries is shown in the following figure. This table visually illustrates the abstraction of the weather forecast for a given city. The weather abstraction has three properties: temperature, humidity and wind. Each row of the table represents a separate instance of the abstraction (i.e. the next day in the forecast).

Graphical Representation of the Weather Forecast Abstraction

Because the properties of city, temperature, humidity, and wind are conceptually different and involve different types we would use a dictionary to represent each instance. This involves breaking the abstraction table apart and representing each instance (each row in the abstraction table) by a dictionary. Our graphical representation of a dictionary is also a table. The figure below shows what this step looks like for the weather forecasts for the next five days.

Representing Each Instance by a Dictionary

The next step is representing the single collection of all instances of the abstraction. In this case we notice that all of the items we want to represent are of the same structure, that is, they are all dictionaries with the same set of keys.

Graphically, this list of dictionaries is shown in the following figure.

Representing All Instances of the Abstraction

Because the forecasted weather data is a list iteration can be used to access and manipulate each element of the list. The only difference is that the item selected from the list on each iteration is a complete dictionary. The following Blockly example shows how to print the wind speed values in each of the weather dictionaries.

Iterating Through List of Dictionaries

In this example, the forecast property used in the iteration has as its value the list item currently selected in the Blacksburg_Forecast list. The forecast value is a dictionary. In Blockly, the get key from dict block selects from the forecast dictionary the value associated with the key wind. This value is then printed.

If we examine the BlockPy Property Visualizer (see next figure) we can see that the value of the property Blacksburg_Forcast is of type List and has a value consisting of a sequence of weather dictionaries. It can also be seen that the forecast property is of type Dictionary and its value is the value that it had on the last step of the iteration (i.e., the last dictionary in the list).

Abstraction-Blockly-Property-Visualizer-List-Dictionaries.png

A List of Dictionaries

Dictionaries with List Values¶

So far our examples dictionaries have key-value pairs where the value is a single basic type. For example, the temperature property is a single whole number, the crime rate is a single number with a decimal point, the name of a city is a single string. However, more intricate arrangements are possible - and needed.

We will now see that a dictionary’s key can have a list as its value. A simple example illustrating a property with a list value is the abstraction of a temperature forecast that separates the high temperatures that are forecast from the low temperatures that are forecast. This abstraction has two properties: highs and lows as shown in the following figure. The value associated with the highs property is a list of forecasted high temperatures. The lows property is a list of forecasted low temperatures. Each temperature is represented by a single whole number (i.e., an int type).

Dictionary Keys with List Values

In Blockly, the block in the Weather named “get highs and lows in <city>” returns the high and low temperatures for the selected city. The following figure show a Blockly algorithm to create a line drawing of the high and low temperatures for a given city on the same plot.

A Blockly Example of a Dictionary with List Values

The textual representation of a dictionary with list values can be seen by examining the output from the above program. The output is shown in the figure below. Notice that the value associated with the key ‘highs’ is a list of numbers. The same is true of the value associated with the key ‘lows’.

The Output from the Blockly Example of a Dictionary with List Values

More Complex Combinations¶

Having seen that lists and dictionaries can be combined individually we now want to see that these two data structures can be combined in multiple ways to represent an abstraction. This idea will be explored in more depth in the next section (see Layers of Abstraction).

Consider the example shown in the following figure that is that abstraction of a national weather forecast. In this abstraction, each instance is an abstraction of the forecast for a particular city. The city forecast abstraction has two properties: city and forecast. The forecast property gives the forecasted temperatures for that corresponding city.

Multiple Combinations of Lists and Dictionaries

To represent this abstraction as a data structure we will need:

a list each element of which is an instance of the city forecast abstraction
a dictionary to represent each city forecast abstraction; the dictionary has the two keys city and forecast*
a list to represent the forecasted temperatures for a given city; this list is the value of the key forecast in the dictionary representing the city forecast abstraction.

Notice that with this organization we can talk meaningfully say:

The national weather forecast abstraction is a list of city forecasts abstractions
Each city forecast abstraction contains a list of temperatures

The following Blockly example shows an algorithm that will will create a line drawing of the forecast for each city in the national weather report. Notice that on each iteration, the value of the property City Report is a dictionary with the keys city and forecasts. In this algorithm the list of temperatures is accessed using the forecasts key. This list of temperatures is plotted. The name of the city is accessed using the city key. The name of the city is printed.

Multiple Combinations of Lists and Dictionaries in Blockly

The output from this Blockly program can be seen in the figure below. On each iteration, the name of the city is printed and the list of temperatures is plotted.

The Outputs from the Blockly Program