Flow of Control: Loops

Lecture 9: Supporting Material

Dr Kathryn Merrick

Tuesday 31st March, 2009

Overview

Writing programs that can repeat operations

for loops

while loops

Reference: Text book Ch 4.

A Motivating Example

Recall the robot_vision_simulator?

It could calculate the service of one cadet, based on their uniform colour.

What if we want a robot to monitor its surroundings or act repeatedly for some period of time…?

Loops

Computer programs can repeat operations using loops

This saves the programmer (you!) from writing out the same code multiple times

Many programming languages have two or three types of loops. MATLAB has two:

for loops while loops

for Loops

Overview: for Loops

“Used when you know how many repetitions are required.”

Example: Data Processing

% loop to print 10 numbers and their squares

for x=1:10

x_squared = x*x;

fprintf(‘%d squared is %d\n’, x, x_squared);

end

Nested Loops

% code to print all the times-tables from% the 1-times-table up to the 12-times-table

for x=1:12 % repeat for each times-table

fprintf(‘\nThe %d-times-table:\n’, x); fprintf(‘-------------------\n’);

for y=1:12 % repeat for each entry in table x_times_y = x*y;

fprintf(‘%d times %d is %d\n’, x, y, x_times_y); endend

Beware! Time Complexity

Often want to optimse the time it takes code to run

For one loop the time complexity is equal to the number n of iterations (repetitions) of the loop

Written O(n)

In a nested loop the time complexity is number of iterations of the first loop multiplied by the number of iterations of the nested loop:

Eg: O(n2) This gets really slow if:

n is large there many levels of nesting

Demo 1:

Using for Loops in Functions

A Robot Simulator for Movement

Syntax: for Loops

for ( counter_variable = range )code to be repeated goes here

end

Programmer or function user know the range to use a for loop

while Loops

Overview: while Loops

“Used when you don’t know how many repetitions are required,

but

you know some condition about when to stop repeating.”

Demo 2:

Using while Loops in Functions

A Robot Simulator for Finding Food

Problem Solving With while Loops

The bouncing height of a ball is determined by its starting height and elasticity:

ht+1 = ht x elasticity2

Where elasticity is between 0 and 1.

How many times will the ball bounce between given starting and stopping heights?

From Problem to Program…

1. Understand the problem ‘in English’

2. Develop a rough solution on paper

3. Implement the solution in MATLAB

4. Test your solution

Demo 3:

Using while Loops in Functions

Bouncing Ball

Syntax: while Loops

while ( Boolean condition goes here )

code to be repeated goes here

end

Beware! The Infinite Loop

while true

fprintf(‘This is a never ending loop\n’);

end

Beware! You could end up with an infinite loop without such an obvious condition

This blatant example is the basis of a simple computer virus or denial of service attack

Some More Interesting Robots…

Summary

After today’s lecture you should be able to:

Write functions that can repeat operations

Use a for loop

Use a while loop