Lecture 10

Flow of Control: Loops (Part 2)

COMP1681 / SE15

Introductionto Programming

SE15: Loops(2) 10–2

Today’s Learning Objectives

Meet do-while loops Learning about tracing and debugging code For you to learn more about techniques for planning the

solution to a programming problem For you to recognise the importance of writing clear,

readable, well-documented code

SE15: Loops(2) 10–3

Lecture Outline

do-while loops Top tips for loops Example of a nested loop Tracing code Debugging in Drjava Top tips for writing code

SE15: Loops(2) 10–4

do-while Loops

int count = 0;

do

{

System.out.print(count + “,”);count++;

} while (count <= 10);

!warning: the loop body is always executed once!

SE15: Loops(2) 10–5

Hints

Watch out for extra semicolons with for loopsfor (i=0; i < 10; i++);

Avoid declaring variables inside loops Avoid break statements if possible You cannot use do-while unless you are certain that

the loop can iterate at least once. If you have computation that changes some numeric

quantity by some equal amount each time, consider a for statement

A while statement is always the safest

SE15: Loops(2) 10–6

More torn up code

Reconstruct the following fragments of code to produce the following output:

0 4

0 3

1 4

1 3

3 4

3 3

public static void main (String [] args)

for ( int x = 0; x < 4; x++)

class MultipleFors

for ( int y = 4; y > 2; y--)

System.out.println(x + " " + y);if(x == 1)

x++;

Head First Java, Sierra & Bates, O’Reilly

SE15: Loops(2) 10–7

What does the following code do?class LoopTest{ public static void main(String [] args) { int y = 7; for(int x = 1; x < 8; x++) { y++; if(x > 4) System.out.print(++y + " "); if(y > 14) { System.out.println("x = " + x); break; } } }}

SE15: Loops(2) 10–8

UML Activity Diagrams(Flow diagrams)

for(int i = 0; i < 10; i++)

{

System.out.println(i);

}

System.out.println(“done”);

Is i < 10?

false

true

Declare int iSet i = 0

Enter loopbody

Print the value of i

Increment i

print “done”

Initial node

Final node

Action node

SE15: Loops(2) 10–9

Top tips for writing programs

Put the scaffolding in place, then stop, think and plan If the problem seems complex, simplify it or break it

down into more manageable pieces Write down in English the steps you need to take For each of the main steps of your solution, write a

comment in your main method Use the comments to remind you what to do at each

point in the program

SE15: Loops(2) 10–10

Simplifying the Problem

Convert only from °C to °F Perform a single conversion Steps become

1. Read temperature in °C from keyboard

2. Calculate temperature in °F

3. Output result of calculation to screen

SE15: Loops(2) 10–11

Commenting Your Code

// Temperature conversion program// Written by Nick Efford, 2005-10-18

public class Temperature {

public static void main(String[] args) { // Read temperature in Celsius

// Convert to Fahrenheit

// Output result of calculation }

}

SE15: Loops(2) 10–12

Top Tips

Adopt a good coding style Add a small amount of code at a time Compile and run after each new addition of code

Fix errors before adding more code! Use temporary println statements to test for correct

behaviour, or run in the debugger

SE15: Loops(2) 10–13

Writing Readable Programs

Use a good coding style Descriptive names for classes, methods, variables… Sensible use of blank lines and indentation Consistency!

Use an appropriate level of commenting Derive them from your pseudocode ‘Comment as you go’, don’t add them all at the end!

SE15: Loops(2) 10–14

Iteration 1(Pseudocode)

Read a temperature in Celsius from the keyboardConvert temperature from Celsius to FahrenheitOutput Fahrenheit temperature to screen

SE15: Loops(2) 10–15

Iteration 1(UML)

Read temperature inCelsius from keyboard

Convert temperaturefrom Celsius to Fahrenheit

Output Fahrenheittemperature to screen

Initial node

Final node

Action node

SE15: Loops(2) 10–16

Iteration 2(Pseudocode)

Read a temperature from the keyboardRead temperature scale from the keyboardIf temperature scale starts with ‘C’ or ‘c’: Convert temperature from Celsius to Fahrenheit Output Fahrenheit temperature to screenOtherwise if temperature scale starts with ‘F’ or ‘f’: Convert temperature from Fahrenheit to Celsius Output Celsius temperature to screenOtherwise: Print an error message on the screen

SE15: Loops(2) 10–17

Iteration 2(UML)

Read temperature inCelsius from keyboard

Convert temperaturefrom Celsius to Fahrenheit

Output Fahrenheittemperature to screen

Convert temperaturefrom Fahrenheit to Celsius

Output Celsiustemperature to screen

Read temperaturescale from keyboard

[ starts with C or c ] [ starts with F or f ]

Print errormessage

Decisionnode

Guardcondition

SE15: Loops(2) 10–18

Iteration 3(Pseudocode)

Repeat: Read a temperature from the keyboard

Read temperature scale from the keyboardIf temperature scale starts with ‘C’ or ‘c’: Convert temperature from Celsius to Fahrenheit Output Fahrenheit temperature to screenOtherwise if temperature scale starts with ‘F’ or ‘f’: Convert temperature from Fahrenheit to Celsius Output Celsius temperature to screenOtherwise: Print an error message on the screenAsk user whether another calculation is required

While user's response starts with ‘Y’ or ‘y’

SE15: Loops(2) 10–19

Iteration 3(UML) Read temperature

Read temp. scale

Convert to °Fand output

Print errormessage

Another conversion?

Convert to °Cand output

[ Y or y ]

[ C or c ] [ F or f ]

Merge node

SE15: Loops(2) 10–20

Your Turn!

How would you simplify Coursework 1? What would you attempt to do in your first iteration? What would the pseudocode / activity diagram look like?

SE15: Loops(2) 10–21

Iteration 1:Read numbers, stopping at –1

Read a number from the keyboardWhile the number last read is not equal to –1: Read another number from keyboard

int score = keyboard.nextInt();while (score != -1) { // Do stuff here... score = keyboard.nextInt();}

Read number

Compare with –1

Read anothernumber [ equal ]

[ not equal ]

SE15: Loops(2) 10–22

Summary

We have Looked at do-while loops Looked at tracing programs and debugging Seen how solutions can be expressed as pseudocode Looked at an alternative, graphical representation for

solutions: the UML activity diagram Emphasised the importance of writing readable code

SE15: Loops(2) 10–23

Follow-up Work

Reading from Savitch Section 3.3 (using pseudocode to specify loops) Section 2.4 (commenting and coding style)

Apply what you've learned today to Assignment 1 Go to the SE15 Off-Site Resources web page and visit

How NOT to do a programming assignment