CS173: Intro to Computer Science - 2D Arrays

Activity Goals

The goals of this activity are:
  1. To be able to create, access, and iterate a 2D array

Supplemental Reading

Feel free to visit these resources for supplemental background reading material.

The Activity


Consider the activity models and answer the questions provided. First reflect on these questions on your own briefly, before discussing and comparing your thoughts with your group. Appoint one member of your group to discuss your findings with the class, and the rest of the group should help that member prepare their response. Answer each question individually from the activity on the Class Activity Questions discussion board. After class, think about the questions in the reflective prompt and respond to those individually in your notebook. Report out on areas of disagreement or items for which you and your group identified alternative approaches. Write down and report out questions you encountered along the way for group discussion.

Model 1: 2D Arrays and Traversals

Java Visualizer Example of a 2D Array
Java Visualizer Example of a 2D Array
Manim 2D Matrix Animation


  1. How would you describe a 2D array in terms of usual 1D arrays?
  2. Does the first index indicate the row or the column of the 2D array?
  3. How might you create a 3-dimensional array?
  4. Run this code in the Java Visualizer. What is the output?
  5. Re-draw the arrays mat1 and mat2 as a square grid.
  6. Is it possible to re-size these arrays if you need to add additional elements later? If so, how, and if not, why not?
  7. Develop pseudocode to determine whether a 2D array is a "magic array", in which the sum of each column and row of a square array is the same value. A sample 3x3 magic array is: [ 4, 9, 2, 3, 5, 7, 8, 1, 6 ].

Embedded Code Environment

You can try out some code examples in this embedded development environment! To share this with someone else, first have one member of your group make a small change to the file, then click "Open in Repl.it". Log into your Repl.it account (or create one if needed), and click the "Share" button at the top right. Note that some embedded Repl.it projects have multiple source files; you can see those by clicking the file icon on the left navigation bar of the embedded code frame. Share the link that opens up with your group members. Remember only to do this for partner/group activities!

Reflective Journal Prompt

  1. How would you modify the above program to assign values to mat1, and then to multiply the two matrices together? To multiply matrices, each cell of the result is equal to the products of each element of the corresponding column of mat1 with each element of the corresponding row of mat2, added together. A triply-nested loop with a sum is required.


Submit your answers to the questions using the Class Activity Questions discussion board. You may also respond to questions or comments made by others, or ask follow-up questions there. Answer any reflective prompt questions in the Reflective Journal section of your OneNote Classroom personal section.

For Additional Practice

Feel free to visit these resources for additional practice exercises.