Unit 2 Lab 2R: Making Lists, Page 2

On this page, you will plan a railroad crossing data collection system by creating an abstract data type to organize crossing activation data with timestamps and speed.

What happens when a train approaches a railroad crossing?
Think about what data a railroad crossing system might need to collect. Consider sensors, timing, and safety information. Brainstorm 3-5 different types of data points that would be important to track.
Read the following section about organizing railroad crossing data. (Don't build these in Snap!. You'll build your own data structures later in this lab.)

: Sublist

A sublist is a list used as an item of another list.

(The word sublist is also used to refer to some subset of a list.)

Example: Railroad Crossing Data Structure
Railroad Data Structure

Without abstraction, accessing this data looks messy:
Crossing without Abstraction

But code like Crossing Code Snippet

is hard to read and understand. A better approach is to use abstraction to organize the crossing data. The abstraction hides the complex list and item of blocks, making your code much easier to write, read, and debug.

With Abstraction: Cleaner Railroad Crossing Code
Crossing with Abstraction

Now the code becomes much more readable:
More Readable Code

: Data Types

A data type is what kind of data something is (number, text string, list, etc.). For example, number is the data type for the first input to and list is the the data type for its second input.

AAP-1.A.3 second sentence

Each programming language provides some primitive data types (data types that are built-in). For example, Snap! provides numbers, text (words and sentences), Booleans (true or false), lists, and some you haven't yet used as data (such as sprites and costumes). This menu shows all of Snap!'s primitive types.

: Abstract Data Types

AAP.1.D.1

The word "abstract" in computer science means making something easier to understand, not harder! ADTs help you create cleaner, more readable code.

An abstract data type (ADT) is a custom data type that's meaningful to your program. It's not built into the language; you develop it as you code. This is an abstraction because it hides the details inside the constructor and selectors so that programmers working with crossing record think about timestamps and events, not list indices.
The block is the constructor; it constructs one example of the data structure. By naming the pieces of the structure, the constructor ensures you build the structure consistently every time.
The selector blocks like , , , of crossing record, are the selectors; they each select one piece of the data structure.

The constructor and selector together implement the crossing record abstract data type.

Data abstraction is the creation and use of abstract data types in a program.

💡 Real-World Connection: Railroad Safety Systems

Modern railroad crossings use sophisticated data collection systems to ensure safety. These systems track:

Train detection sensors - magnetic, infrared, or track circuit sensors
Timing data - when trains approach, occupy, and clear the crossing
Gate and signal status - position of crossing gates and warning light states
Traffic data - vehicle detection and pedestrian presence
System diagnostics - equipment health and maintenance alerts

Reshma Saujani (born 1975) founded the nonprofit organization Girls Who Code in response to the historical decline in the number of women employed in computer science fields. According to their website, in 1995, 37% of computer scientists were women. As of 2023, it’s only 24%. Reshma and Girls Who Code aim to fix this imbalance by running clubs that teach girls CS. In addition to being an activist, Reshma was the first Indian American woman to run for Congress and has served as New York City’s Deputy Public Advocate, where she created innovative partnerships to support DREAMers and promote campaign finance reform, among other initiatives.

Video: Teach girls bravery, not perfection

Planning a Railroad Crossing Data Collection System

💡 Real-World Connection: Railroad Safety Systems