The use of arrays (or equivalent) when solving problems, including both…

The idea

An array stores several values of the same data type under one identifier, each reached by an index. A 1D array is a single list, such as scores[0] to scores[4] holding five test marks. A 2D array is a table of rows and columns, such as grid[2][3], where the first index selects the row and the second the column. You use arrays to avoid many separate variables, to loop through data with a counter, and to store grids for games or tables of records. You must be able to declare an array, assign and read elements, traverse it with a for loop, and process every element, for example totalling a 1D array or searching a 2D array row by row.

What earns the marks

• Declares an array of a stated size and data type, for example scores as five integers, and explains that all elements share one identifier.
• Reads and writes individual elements using correct indices, remembering that indexing commonly starts at 0 so the last index is size − 1.
• Traverses a 1D array with a for loop whose counter runs from the first to the last valid index, processing each element once.
• Uses two indices for a 2D array, the first for the row and the second for the column, and nests two loops to visit every cell.
• Applies standard array processing such as finding a total, maximum or minimum, or searching for a value and recording its position.
• Chooses an array over separate variables when several related values of the same type must be stored and processed together.

Common mistakes and how to put them right

• Using an index equal to the array length, for example scores[5] on a five-element array.

  Put it right: The error is an out-of-range access, so the correction is to loop while the counter is less than the length.

• Swapping the row and column indices in a 2D array.

  Put it right: The error is reading the wrong cell, so the correction is to fix the convention that the first index is the row and the second is the column.

• Mixing data types in one array, such as names and ages together.

  Put it right: The error breaks the same-type rule, so the correction is to use separate arrays or a record structure.

Exam tips

• Sketch the array as boxes with their index numbers before writing code, and mark the first and last valid indices.
• For a 2D array, label the rows and columns on your sketch and trace one complete inner loop before writing the nested loops.
• Check every loop bound against the declared size, since off-by-one errors are the most common lost mark in array questions.