11.1.4 Input, Output and Built-in Routines

11.1.4 Input, Output and Built-in Routines

A useful program needs a way to receive data, process it and communicate results. Pseudocode represents these interactions using INPUT and OUTPUT.

Programs can also call ready-made routines instead of recreating every operation. These routines may measure a string, extract characters, change letter case, truncate a real value or generate a pseudo-random number.

Input, Processing and Output

Many short algorithms can be understood as a flow of data through three stages.

Receiving Keyboard Input

The INPUT command waits for a value and stores it in a variable. The variable must already have been declared with an appropriate data type.

Pseudocode pattern

INPUT <Identifier>

Example

DECLARE CrateCount : INTEGER

OUTPUT "Enter the number of crates: "
INPUT CrateCount

The prompt is produced using OUTPUT. The INPUT statement then stores the entered value in CrateCount.

Input must have a destination

Producing Console Output

The OUTPUT command displays information. It can display literal text, stored values, expression results or values returned by functions.

Pseudocode pattern

OUTPUT <Value or values>

Building a Complete Input–Output Sequence

A short interaction normally introduces the required variables, displays a prompt, receives data, performs processing and then labels the result clearly.

Original example: equipment labels

CONSTANT LABEL_PRICE = 1.75

DECLARE LabelCount : INTEGER
DECLARE TotalCost : REAL

OUTPUT "Enter the number of labels required: "
INPUT LabelCount

TotalCost ← LabelCount * LABEL_PRICE

OUTPUT "Labels ordered: ", LabelCount
OUTPUT "Total cost: $", TotalCost

Using Ready-Made Routines

A programming environment provides routines for operations that programmers frequently need. Calling a ready-made routine avoids writing its internal algorithm each time.

A function call is normally used wherever its returned value is required.

NameLength ← LENGTH(DeviceName)
OUTPUT "Final four characters: ", RIGHT(DeviceName, 4)

In the first statement, the returned integer is assigned to a variable. In the second, the returned string is sent directly to output.

Reading a Function Signature

A supplied signature explains how a function must be called. It identifies the expected arguments and the type of value returned.

RIGHT(Text : STRING, Count : INTEGER) RETURNS STRING

Valid use

Suffix ← RIGHT("SENSORGRID", 4)

The function receives a string and an integer. It returns "GRID".

Calls that do not match the signature

RIGHT(42, 4)
RIGHT("SENSORGRID", "four")
RIGHT("SENSORGRID")

These calls use the wrong argument type or the wrong number of arguments.

String Functions and Operations

String routines can count, extract or change characters. The following pseudocode functions illustrate the operations students may need to apply.

Positions used by MID

Consider the string "ALGORITHM".

MID("ALGORITHM", 3, 4) starts at position 3 and returns four characters: "GORI".

Numeric Routines

Taking the integer part

The INT function returns the integer part of a real value.

WholeUnits ← INT(14.87)

WholeUnits receives 14. The fractional part is removed; the value is not rounded to 15.

Generating a pseudo-random real value

RAND(Limit) returns a real value from zero up to, but not including, the supplied limit.

RandomReading ← RAND(20)

The result may be a value such as 7.43, satisfying:

0 <= RandomReading AND RandomReading < 20

Generating an integer from 1 to 8

RandomNumber ← INT(RAND(8)) + 1

1. RAND(8) produces a real value from 0 up to, but not including, 8.
2. INT produces an integer from 0 to 7.
3. Adding 1 shifts the possible results to 1 through 8.

Programming-Language Extension

Real programming languages may use different function names and indexing rules. These language-specific forms are useful for implementation, but they should not replace the required pseudocode notation in a pseudocode answer.

Python slicing

Text[start:end]

Python indexes begin at zero. The start index is included and the end index is excluded.

Converting input in a programming language

Some languages initially receive console input as text. A conversion routine may therefore be needed before arithmetic can be performed.

QuantityText = input("Enter quantity: ")
Quantity = int(QuantityText)

Worked Example: Asset-Label Generator

A laboratory system receives an asset name and a zone letter. It creates a short label using the final four characters of the asset name and an uppercase version of the zone.

Step 1: declarations

DECLARE AssetName : STRING
DECLARE ZoneLetter : CHAR
DECLARE ShortCode : STRING
DECLARE NormalisedZone : CHAR
DECLARE CharacterCount : INTEGER

Step 2: input

OUTPUT "Enter the asset name: "
INPUT AssetName

OUTPUT "Enter the zone letter: "
INPUT ZoneLetter

Step 3: use the routines

ShortCode ← RIGHT(AssetName, 4)
NormalisedZone ← UCASE(ZoneLetter)
CharacterCount ← LENGTH(AssetName)

Step 4: display labelled results

OUTPUT "Generated label: ", NormalisedZone, "-", ShortCode
OUTPUT "Characters in asset name: ", CharacterCount

Trace using sample input

Interactive: Input–Process–Output Tracer

Use the existing tracer to follow data from user input through calculation to console output. Change the input values and inspect the program state after each statement.

Interactive: Built-in Routine Explorer

The string and numeric modes reinforce the general idea of supplying arguments and receiving a return value. The slicing and conversion modes are programming-language extensions.

Implementation Note: Readable Code

Comments can explain why a less-obvious operation is being performed. They are intended for human readers and do not form part of the program’s output.

// Produce an integer from 1 to 8
RandomNumber ← INT(RAND(8)) + 1

Common Mistakes and Misconceptions

Practice

TAIL(Text : STRING, Count : INTEGER) RETURNS STRING

DECLARE WorkshopName : STRING
DECLARE NameLength : INTEGER

// Add the missing prompt, input, function call and output.

DECLARE Age : INTEGER

INPUT "Enter age: " Age
OUTPUT "Age"
LastPart ← RIGHT(Age, 3)

OUTPUT "Enter the number of robots: "
INPUT RobotCount

OUTPUT "Available spaces: ", SpacesAvailable

OUTPUT "Zone: ", ZoneLetter,
       ", shelf: ", ShelfNumber,
       ", items: ", ItemCount

DECLARE WorkshopName : STRING
DECLARE NameLength : INTEGER

OUTPUT "Enter the workshop name: "
INPUT WorkshopName

NameLength ← LENGTH(WorkshopName)

OUTPUT "Workshop: ", WorkshopName
OUTPUT "Number of characters: ", NameLength

Review