IGCSE2.2.2Paper 2 — Algorithms, Programming and Logic

Arrays

One name holding many values, so a loop can work through them.

YOU SHOULD BE ABLE TO

Declare and use one-dimensional and two-dimensional arrays.
Read from and write to an array using an index.
Use a loop to process every element of an array.
Perform a linear search and a bubble sort on an array.

RUN THE PSEUDOCODE

Write the pseudocode your syllabus uses and press Run. Assignment is the arrow — type <- and it works the same as ←. Both syllabuses are accepted, so OUTPUT and PRINT both work; where a word differs from the one the exam prints, the runner says so.

PROGRAM

DECLARE Name : STRING
DECLARE Age : INTEGER

Name <- "Bat"
Age <- 14

OUTPUT "Hello ", Name
OUTPUT "Next year you are ", Age + 1

INPUT — ONE VALUE PER LINE

OUTPUT

Press Run to see what it does

TRACE TABLE

Total ← 0
FOR Count ← 1 TO N
    Total ← Total + Count
NEXT Count
OUTPUT Total

STARTN = 5

Count	Total	OUTPUT

One row per pass through the loop. Leave a cell blank if it does not change on that pass.

//Declaring arrays in Cambridge pseudocode

You state the bounds and the type. A 2-D array is indexed [row, column].

DECLARE Scores : ARRAY[1:30] OF INTEGER
DECLARE Grid   : ARRAY[1:5, 1:5] OF CHAR

Scores[1] ← 78
Grid[2, 3] ← 'x'

FOR i ← 1 TO 30
   OUTPUT Scores[i]
NEXT i

//Linear search

Check each element in turn until you find the target or run out. Simple, works on unsorted data, but slow on large arrays.

Found ← FALSE
FOR i ← 1 TO 30
   IF Scores[i] = Target THEN
      Found ← TRUE
      Position ← i
   ENDIF
NEXT i

//Bubble sort

Repeatedly compare neighbouring pairs and swap them if they are the wrong way round. After each pass the largest remaining value has 'bubbled' to the end.

FOR Pass ← 1 TO 29
   FOR i ← 1 TO 29
      IF Scores[i] > Scores[i+1] THEN
         Temp ← Scores[i]
         Scores[i] ← Scores[i+1]
         Scores[i+1] ← Temp
      ENDIF
   NEXT i
NEXT Pass

IN THE EXAM

A swap needs a THIRD variable. Writing A ← B then B ← A loses the original value — a classic trap.

CHECK YOURSELF

1.An array is DECLARE A : ARRAY[1:10] OF INTEGER. How many values can it hold?

2.Why does swapping two array elements need a third variable?

Programming Concepts File Handling

RUN THE PSEUDOCODE

PROGRAM

DECLARE Name : STRING
DECLARE Age : INTEGER

Name <- "Bat"
Age <- 14

OUTPUT "Hello ", Name
OUTPUT "Next year you are ", Age + 1

INPUT — ONE VALUE PER LINE

OUTPUT

Press Run to see what it does

Count

Total

OUTPUT

//Bubble sort

Repeatedly compare neighbouring pairs and swap them if they are the wrong way round. After each pass the largest remaining value has 'bubbled' to the end.

FOR Pass ← 1 TO 29
   FOR i ← 1 TO 29
      IF Scores[i] > Scores[i+1] THEN
         Temp ← Scores[i]
         Scores[i] ← Scores[i+1]
         Scores[i+1] ← Temp
      ENDIF
   NEXT i
NEXT Pass