Lists and dictionaries

One sensor reading is useful, but sometimes we want to look at several readings together. Instead of inventing a new variable for each one, we can put them in a list. We’ll also meet dictionaries, which let us find values by a name.

A list stores items in order: readings = [False, False, True]. Square brackets create the list; commas separate its items. An index selects one item, starting from zero: readings[0] is the first. readings[-1] selects the last. An index outside the list raises IndexError.

len(readings) gives its length. readings.append(False) adds an item to the existing list. The dot accesses an operation belonging to that object; this operation is called a method. A for reading in readings: loop visits each item directly, so you often don’t need to manage indexes yourself.

A dictionary stores values under named keys: inputs = {"request": 0.5, "enabled": True}. Curly braces create it; each colon separates a key from its value. inputs["request"] retrieves the matching value. An absent key raises KeyError. Use a list for an ordered sequence and a dictionary when meaningful names make the data clearer.

Read it before running it¶

Put this worked example in student-work/foundations/lists_and_dictionaries.py. Read it once and write down what you think it prints. Then save and run python student-work/foundations/lists_and_dictionaries.py using your environment’s Python.

lists_and_dictionaries.py

readings = [False, False, True]
detected_count = 0
for detected in readings:
    if detected:
        detected_count = detected_count + 1
print("Samples:", len(readings))
print("Detected samples:", detected_count)

inputs = {"request": 0.5, "enabled": True}
print("Requested output:", inputs["request"])

The output reports three samples, one detected sample, and request 0.5. Append another True reading and predict which outputs change. Notice that counting true samples does not count separate physical pieces: one piece might remain detected for many samples. Choose names that match what your program actually measures.

Temporarily misspell "request" when looking up the key. Compare that KeyError with the NameError from earlier. One is a missing dictionary key; the other is a missing Python name.

Try it yourself¶

A place to start

Start with an empty result list: high_requests = [].

A more specific hint

Inside the loop, append the current request only when it passes the comparison.

Solution to Exercise 1

The filtered result is [0.8, 0.9], length 2; the original remains length 4. A value exactly 0.5 does not pass > 0.5.

lists_and_dictionaries.py

requests = [0.2, 0.8, 0.4, 0.9]
high_requests = []
for request in requests:
    if request > 0.5:
        high_requests.append(request)
report = {"limit": 0.5, "high_requests": high_requests}
print(requests, len(requests))
print(high_requests, len(high_requests))
print(report)

Before moving on¶

Explain why the first item uses index zero and why a dictionary key uses quotes here. Revisit your loop accumulator: what happens if you reset the result list inside the loop?