Remerciez-le!

Remerciez @Admin pour avoir partagé cet document gratuitement, de la manière la plus simple, en partageant sur les réseaux sociaux.

Pseudocode Guide for Teachers Cambridge IGCSE® Computer Science 0478 For examin

Pseudocode Guide for Teachers Cambridge IGCSE® Computer Science 0478 For examination from 2017 Cambridge International Examinations retains the copyright on all its publications. Registered Centres are permitted to copy material from this booklet for their own internal use. However, we cannot give permission to Centres to photocopy any material that is acknowledged to a third party even for internal use within a Centre. Version 2 Contents Introduction ...................................................................................................................................................... 2 How should teachers use this guide? ............................................................................................................ 2 1. Pseudocode in examined components ..................................................................................................... 3 1.1 Font style and size ................................................................................................................................... 3 1.2 Indentation ............................................................................................................................................... 3 1.3 Case and italics ....................................................................................................................................... 3 1.4 Lines and numbering ............................................................................................................................... 4 1.4 Comments ............................................................................................................................................... 4 2. Variables, constants and data types ......................................................................................................... 5 2.1 Atomic type names .................................................................................................................................. 5 2.2 Literals ..................................................................................................................................................... 5 2.3 Identifiers ................................................................................................................................................. 5 2.4 Assignments ............................................................................................................................................ 5 3. Arrays ........................................................................................................................................................... 7 3.1 Declaring arrays ....................................................................................................................................... 7 3.2 Using arrays ............................................................................................................................................. 7 4. Common operations .................................................................................................................................... 8 4.1 Input and output ....................................................................................................................................... 8 4.2 Arithmetic operations ............................................................................................................................... 8 4.3 Logic operators ........................................................................................................................................ 8 5. Selection ....................................................................................................................................................... 9 5.1 IF statements ........................................................................................................................................... 9 5.2 CASE statements .................................................................................................................................... 9 6. Iteration ....................................................................................................................................................... 11 6.1 Count-controlled (FOR) loops ................................................................................................................ 11 6.2 Post-condition (REPEAT UNTIL) loops ................................................................................................. 11 6.3 Pre-condition (WHILE) loops ................................................................................................................. 12 7. Index of symbols and keywords .............................................................................................................. 13 Introduction 2 Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers Introduction How should teachers use this guide? We advise teachers to follow this guide in their teaching and make sure that learners are familiar with the style presented here. This will enable learners to understand any pseudocode presented in examination papers and pre-release materials more easily. It will also give them a structure to follow so that they can present their algorithms more clearly in pseudocode when required. Teachers should be aware that learners are not required to follow this guide in their examination answers or any other material they present for assessment. By definition, pseudocode is not a programming language with a defined, mandatory syntax. Any pseudocode presented by candidates will only be assessed for the logic of the solution presented – where the logic is understood by the Examiner, and correctly solves the problem addressed, the candidate will be given credit regardless of whether the candidate has followed the style presented here. Using a recommended style will, however, enable the candidate to communicate their solution to the Examiner more effectively. 1. Pseudocode in examined components Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers 3 1. Pseudocode in examined components The following information sets out how pseudocode will appear within the examined components and is provided to allow you to give learners familiarity before the exam. 1.1 Font style and size Pseudocode is presented in a monospaced (fixed-width) font such as Courier New. The size of the font will be consistent throughout. 1.2 Indentation Lines are indented by four spaces to indicate that they are contained within a statement in a previous line. Where it is not possible to fit a statement on one line any continuation lines are indented by two spaces. In cases where line numbering is used, this indentation may be omitted. Every effort will be made to make sure that code statements are not longer than a line of code, unless this is absolutely necessary. Note that the THEN and ELSE clauses of an IF statement are indented by only two spaces (see Section 5.1). Cases in CASE statements are also indented by only two places (see Section 5.2). 1.3 Case and italics Keywords are in uppercase, e.g. IF, REPEAT, PROCEDURE. (Different keywords are explained in later sections of this guide.) Identifiers are in mixed case (sometimes referred to as camelCase or Pascal case) with uppercase letters indicating the beginning of new words, for example NumberOfPlayers. Meta-variables – symbols in the pseudocode that should be substituted by other symbols – are enclosed in angled brackets < > (as in Backus-Naur Form). This is also used in this guide. Example – meta-variables REPEAT <Statements> UNTIL <condition> Where the pseudocode contains descriptions rather than code symbols, keywords or literals, these are written in italics. Example – descriptions REPEAT <Statements> UNTIL all seats are allocated 1. Pseudocode in examined components 4 Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers 1.4 Lines and numbering Where it is necessary to number the lines of pseudocode so that they can be referred to, line numbers are presented to the left of the pseudocode with sufficient space to indicate clearly that they are not part of the pseudocode statements. Line numbers are consecutive, unless numbers are skipped to indicate that part of the code is missing. This will also be clearly stated. Each line representing a statement is numbered. However when a statement runs over one line of text, the continuation lines are not numbered. 1.4 Comments Comments are preceded by two forward slashes // . The comment continues until the end of the line. For multi-line comments, each line is preceded by //. Normally the comment is on a separate line before, and at the same level of indentation as, the code it refers to. Occasionally, however, a short comment that refers to a single line may be at the end of the line to which it refers. 2. Variables, constants and data types Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers 5 2. Variables, constants and data types 2.1 Atomic type names The following keywords are used to designate atomic data types:  INTEGER : A whole number  REAL : A number capable of containing a fractional part  CHAR : A single character  STRING : A sequence of zero or more characters  BOOLEAN: The logical values TRUE and FALSE 2.2 Literals Literals of the above data types are written as follows:  Integers : Written as normal in the denary system, e.g. 5, -3  Real : Always written with at least one digit on either side of the decimal point, zeros being added if necessary, e.g. 4.7, 0.3, -4.0, 0.0  Char: A single character delimited by single quotes, e.g. ꞌxꞌ, ꞌCꞌ, ꞌ@ꞌ  String: Delimited by double quotes. A string may contain no characters (i.e. the empty string) e.g. "This is a string", ""  Boolean: TRUE, FALSE 2.3 Identifiers Identifiers (the names given to variables, constants, procedures and functions) are in mix case. They can only contain letters (A–Z, a–z) and digits (0–9). They must start with a letter and not a digit. Accented letters and other characters, including the underscore, should not be used. As in programming, it is good practice to use identifier names that describe the variable, procedure or function they refer to. Single letters may be used where these are conventional (such as i and j when dealing with array indices, or X and Y when dealing with coordinates) as these are made clear by the convention. Keywords identified elsewhere in this guide should never be used as variables. Identifiers should be considered case insensitive, for example, Countdown and CountDown should not be used as separate variables. 2.4 Assignments The assignment operator is ←. Assignments should be made in the following format: <identifier> ← <value> The identifier must refer to a variable (this can be an individual element in a data structure such as an array or an abstract data type). The value may be any expression that evaluates to a value of the same data type as the variable. 2. Variables, constants and data types 6 Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers Example – assignments Counter ← 0 Counter ← Counter + 1 TotalToPay ← NumberOfHours * HourlyRate 3. Arrays Cambridge IGCSE Computer Science 0478 – Pseudocode Guide for Teachers 7 3. Arrays 3.1 Declaring arrays Arrays are considered to be fixed-length structures of elements of identical data type, accessible by consecutive index (subscript) numbers. It is good practice to explicitly state what the lower bound of the array (i.e. the index of the first element) is because this defaults to either 0 or 1 in different systems. Generally, a lower bound of 1 will be used. Square brackets are used to indicate the array indices. One-dimensional arrays are declared as follows (where l is the lower bound and n is the upper bound): DECLARE <identifier> : ARRAY[<l>:<n>] OF <data type> Example – array declaration DECLARE StudentNames : ARRAY[1:30] OF STRING DECLARE NoughtsAndCrosses : ARRAY[1:3] OF CHAR 3.2 Using arrays In the main pseudocode statements, only one index value is used for each dimension in the square brackets. Example – using arrays StudentNames[1] ← "Ali" NoughtsAndCrosses[2] ← 'X' StudentNames[n+1] ← StudentNames[n] Arrays can be used in assignment statements (provided they have same size and data type). The following is therefore allowed: Example – assigning an array SavedGame ← NoughtsAndCrosses A statement should not, however, refer to a group of array elements individually. For example, the following construction should not be used. StudentNames [1 TO 30] ← "" Instead, an uploads/S4/ 0478-pseudocode-guide.pdf