Arrays and records

Overview

Recall questions on one, two and three-dimensional arrays, calculating element addresses, why arrays give direct access, the limitations of arrays, records, and how arrays differ from records and are combined into arrays of records.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is an array?

   Model answer

   An ordered, indexed collection of elements that are all of the same data type, held in a block of consecutive memory locations. It has a size fixed when it is created, and each element is reached by its index (position number) rather than by searching for it.

2. 2

   A one-dimensional array is a row of values. What does a two-dimensional array give you, and how do you get at a single element?

   Model answer

   A two-dimensional array is a table: rows and columns. You reach a single element with two indices, conventionally the row first and the column second, for example grid[2][5] meaning row 2, column 5. It is the natural structure for anything grid-shaped, such as a spreadsheet, a board game, a seating plan or a black-and-white image.

3. 3

   Describe what a three-dimensional array is and give a use for one.

   Model answer

   A three-dimensional array is effectively a stack of two-dimensional tables, so it needs three indices, for example cube[layer][row][column]. Uses include a three-dimensional game world or voxel model, a colour image where the third index selects the red, green or blue channel, or exam marks stored as [student][subject][term].

4. 4

   How many elements does an array declared as 2 by 3 by 4 hold?

   Model answer

   24. The sizes of the dimensions multiply: 2 x 3 x 4 = 24 elements.

5. 5

   Explain why an array can jump straight to element 500 without looking at the other 499.

   Model answer

   Because the elements are all the same size and stored contiguously, the computer can calculate the address of any element arithmetically rather than searching for it: address = base address + index x size of one element. That single calculation gives constant-time random access, which is the array's main strength.

6. 6

   An array of 4-byte integers begins at address 2000 and is indexed from 0. What is the address of element 6?

   Model answer

   2024. Address = 2000 + (6 x 4) = 2000 + 24 = 2024. Note that if the array were indexed from 1 instead, you would use (6 - 1) x 4, giving 2020 - always check the base index.

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