IGCSE2.2.1Paper 2 — Algorithms, Programming and Logic

Programming Concepts

The building blocks every program is made of: data types, the three structures, and named blocks of code.

YOU SHOULD BE ABLE TO

Use the data types integer, real, char, string and boolean.
Use variables and constants, and explain the difference.
Use sequence, selection and iteration (count-controlled, pre-condition and post-condition).
Use the operators + − * / and integer division, MOD and DIV.
Use nested statements.
Write and call procedures and functions, with and without parameters.
Use local and global variables appropriately.
Apply validation and verification checks.
Use library routines and maintain readable code.

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.

//The three loops, and when to use each

Count-controlled when you know how many times. Pre-condition (WHILE) when it might not run at all. Post-condition (REPEAT) when it must run at least once.

FOR i ← 1 TO 10        known number of times
   ...
NEXT i

WHILE Answer <> "y" DO  may run ZERO times
   ...
ENDWHILE

REPEAT                  always runs AT LEAST once
   ...
UNTIL Answer = "y"

//DIV and MOD

DIV gives the whole-number part of a division; MOD gives the remainder. MOD 2 is how you test for even or odd.

17 DIV 5 = 3      17 MOD 5 = 2
IF Number MOD 2 = 0 THEN OUTPUT "Even"

//Validation vs verification

Validation asks 'could this be sensible?' — range, length, type, presence, format and check digit. Verification asks 'was it entered correctly?' — double entry, or a visual check by a human. They are not the same thing and exams test that you know it.

//Local vs global

A local variable exists only inside its procedure and cannot be seen outside — safer, because nothing else can change it by accident. A global variable can be used anywhere, which is convenient but makes bugs harder to find.

KEY TERMS

FLASHCARDS1 / 5

CHECK YOURSELF

1.A birth year of 3025 is entered. Which check would reject it?

2.Typing a password twice to make sure it matches is:

3.What does 17 MOD 5 give?

Pseudocode and Trace Tables Arrays

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

//The three loops, and when to use each

Count-controlled when you know how many times. Pre-condition (WHILE) when it might not run at all. Post-condition (REPEAT) when it must run at least once.

FOR i ← 1 TO 10        known number of times
   ...
NEXT i

WHILE Answer <> "y" DO  may run ZERO times
   ...
ENDWHILE

REPEAT                  always runs AT LEAST once
   ...
UNTIL Answer = "y"