Subtopic Notes

I - Subroutine

9-11. Data Types and Structures, Programming

Subroutine

Subroutine is a reusable block of code that is given a name
Designed to perform a specific task
Often called within a program to avoid repetition.
Can be tested/improved independently of program
Easy to share procedures/functions with other programs
Create routines that can be called like built-in command
Header: The first statement in subroutine definition
- Includes name of the subroutine
- Includes parameter passed along with data type
- Includes return data type if it exists

Procedure

Subroutine that does not return a value
Procedure calls are standalone statements

Function

Subroutine that returns a value.
Function calls are always made as part of an expression
The keyword RETURN is used as one of the statements within the body of the function to specify the value to be returned.
Normally, this will be the last statement in the function definition, however, if the RETURN statement is in the body of the function its execution is immediate and any subsequent lines of code are omitted.
Because a function returns a value that is used when the function is called, function calls are not complete program statements
When the RETURN statement is executed, the value returned replaces the function call in the expression and the expression is then evaluated

Parameter

Variables used in a subroutine to accept input values, allowing data to be passed into the subroutine.
A subroutine may or may not have parameters.
When calling a function, the parameters used must be of correct data type and of the same sequence as the definition
Parameters should not be passed by reference to a function.
Unless otherwise stated, it should be assumed that parameters are passed by value.
Parameters can be passed either by reference or by value (not supported in python)
Pass by value: Data copied into procedure so variable not changed outside procedure
Pass by reference: Link to variable provided so variable changed after going through procedure

Argument: The actual value or data passed to the subroutine when it is called

Local Variable: Variables defined within a function or block, accessible only within that specific scope. These variables are not accessible outside the subroutine

Global Variable: Variables defined outside all functions or blocks, accessible throughout the entire program.

Procedure/Function Interface: The way a procedure or function interacts with the rest of the program. It includes its name, parameters (inputs), and, for functions, the return value.

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PSEUDOCODE

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To specify whether a parameter is passed by value or by reference, the keywords BYVAL and BYREF precede the parameter in the definition of the procedure.

If there are several parameters passed by the same method, the BYVAL or BYREF keyword need not be repeated.

Example – passing parameters by reference

PSEUDOCODE

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Library Routines

A library routine is a pre-written, reusable function or procedure provided by a programming library to perform common tasks, such as mathematical operations or input/output handling.
IDEs typically include a standard library of functions and procedures
Programming language development systems often offer library routines that can be easily integrated into programs
Some examples of library routines in pseudocode are: MOD, DIV, ROUND, RANDOM

Best Coding Practices

Use meaningful identifiers name for variables, constants, arrays, procedure, functions
Use commenting feature wisely
Divide the program into different modules using procedures and functions

String Manipulation

Strings are conceptually like arrays of characters. This means many operations done in arrays can be done in string. In pseudocode, the first character index may be 0 or 1, but for the functions of the insert it is not zero indexed.

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Slicing/Substring: Returns a range of characters by specifying a start and end index (exclusive)
Concatenation: Combining two or more strings together

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Pseudocode String Functions

The Cambridge International AS & A Level syllabus (9618) specifically requires candidates to know string manipulation functions in their chosen programming language. Pseudocode string manipulation functions will always be provided in examinations (In Insert). Some basic string manipulation functions are given here.

An error will be generated if a function call is not properly formed or if the parameters are of an incorrect type or an incorrect value.

String and character functions

A string of length 1 may be considered to be either of type CHAR or STRING
A CHAR may be assigned to, or concatenated with, a STRING
A STRING of length greater than 1 cannot be assigned to a CHAR
These functions are not zero-indexed

String and character functions (These will be given in insert)
`LEFT(ThisString : STRING, x : INTEGER) RETURNS STRING` returns leftmost `x` characters from `ThisString`
`RIGHT(ThisString : STRING, x : INTEGER) RETURNS STRING` returns rightmost `x` characters from `ThisString`
`LENGTH(ThisString : STRING) RETURNS INTEGER` returns the integer value representing the length of `ThisString`
`MID(ThisString : STRING, x : INTEGER, y : INTEGER) RETURNS STRING` returns a string of length `y` starting at position `x` from `ThisString`
`UCASE(ThisChar : CHAR) RETURNS CHAR` returns the character value representing the upper-case equivalent of `ThisChar`
`TO_UPPER(ThisString : STRING) RETURNS STRING` returns a string formed by converting all characters of ThisString to uppercase
`LCASE(ThisChar : CHAR) RETURNS CHAR` returns the character value representing the lower-case equivalent of `ThisChar`
`TO_LOWER(ThisString : STRING) RETURNS STRING` returns a string formed by converting all characters of `ThisString` to lowercase
`NUM_TO_STR(x : <datatype1>) RETURNS <datatype2>` returns a string representation of a numeric value (supports -ve). `<datatype1>` may be `REAL` or `INTEGER`, `<datatype2>` may be `CHAR` or `STRING`
`STR_TO_NUM(x : <datatype1>) RETURNS <datatype2>` returns a numeric representation of a string. `<datatype1>` may be CHAR or STRING, `<datatype2>` may be `REAL` or `INTEGER`
`IS_NUM(ThisString : <datatype>) RETURNS BOOLEAN` returns `TRUE` if `ThisString` represents a valid numeric value. `<datatype>` may be `CHAR` or `STRING`
`ASC(ThisChar : CHAR) RETURNS INTEGER` returns an integer value (the ASCII value) of character `ThisChar`
`CHR(x : INTEGER) RETURNS CHAR` returns the character whose integer value (the ASCII value) is `x`
In pseudocode, the operator & is used to concatenate (join) two strings. Example: `"Summer" & " " & "Pudding"` produces `"Summer Pudding"`

Where string operations (such as concatenation, searching and splitting) are used in a programming language, these should be explained clearly, as they vary considerably between systems.

Where functions in programming languages are used to format numbers as strings for output, their use should also be explained.

Numeric Functions

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Pseudocode Numeric Functions

Pseudocode Numeric Functions
`INT(x : REAL) RETURNS INTEGER` returns the integer part of `x`
`RAND(x : INTEGER) RETURNS REAL` returns a real number in the range 0 to `x` (not inclusive of `x`). Example: `RAND(87)` could return `35.430729`

Date Functions

There is not built in data type of date, however we can import a module to make it work
Date format pseudocode is assumed to be DD/MM/YYYY unless otherwise stated.

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Pseudocode Date Functions

Pseudocode Date Functions
`DAY(ThisDate : DATE) RETURNS INTEGER` returns the day number from `ThisDate`
`MONTH(ThisDate : DATE) RETURNS INTEGER` returns the month number from `ThisDate`
`YEAR(ThisDate : DATE) RETURNS INTEGER` returns the year number from `ThisDate`
`DAYINDEX(ThisDate : DATE) RETURNS INTEGER` returns the day index number from `ThisDate` where Sunday = 1, Monday = 2 etc.
`SETDATE(Day, Month, Year : INTEGER) RETURNS DATE` returns a value of type `DATE` with the value of `<Day>/<Month>/<Year>`
`TODAY() RETURNS DATE` returns a value of type `DATE` corresponding to the current date.

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