Arrays and file handling

Computer Science · Programming. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

An array stores many values of the same data type under one identifier. Each value is an element, and an index gives its position. A 1D array is a single list. A 2D array is a table with rows and columns, so each element needs two index values.

Arrays are useful because a loop can step through the index and process every element with the same few lines of code. One FOR loop handles a 1D array. Nested FOR loops handle a 2D array.

Data in variables and arrays is lost when the program ends. Storing data in a file keeps it, so the program can read it again the next time it runs.

Rules to remember

1D array: DECLARE Names : ARRAY[1:30] OF STRING gives 30 elements, Names[1] to Names[30].
2D array: DECLARE Grid : ARRAY[1:5, 1:8] OF INTEGER gives 5 × 8 = 40 elements. Grid[2, 7] is the element in row 2, column 7.
All elements of an array have the same data type.
Fill or read an array with a loop: FOR i ← 1 TO 30 ... INPUT Names[i] ... NEXT i.
OPENFILE "Data.txt" FOR READ or FOR WRITE. Opening an existing file FOR WRITE replaces its old contents.
READFILE "Data.txt", LineOfText reads one line into a variable. WRITEFILE "Data.txt", LineOfText writes one line to the file.
Always finish with CLOSEFILE "Data.txt". Order: open, read or write, close.

Common mistake

When a loop changes elements using other elements of the same array, students use the old values in their trace. Go one step at a time and use the values the array holds at that moment.

Worked example

An array Nums[1:5] holds 3, 8, 1, 6, 2. What is output?
Total ← 0
FOR i ← 2 TO 4
Total ← Total + Nums[i]
NEXT i
OUTPUT Total

The loop uses i = 2, 3 and 4 only, so Nums[1] and Nums[5] are not used.
Nums[2] = 8, Nums[3] = 1 and Nums[4] = 6.
Total = 0 + 8 + 1 + 6 = 15.

Answer: 15

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