RepetitionMichael Heron

Introduction• The first of the real programming structures we are going to

look at is the repetition family.• Used to repeat blocks of code.• Used to repeat blocks of code.

• Haha, did you see what I did there??

• They’re somewhat more versatile than copy and paste.• And also play to the fundamental traits of programmers.

The Programmer Mindset• Good programming is based on one basic truth about

programmers.• We’re horribly lazy.

• Anything that the computer can be doing, it should be doing.• The less we have to do, the better.

• Repetition structures let us do that to a degree.

An Example• Imagine a program that would give you the average of five

numbers. • Prompt the user• Pull in the number• Add it to a total• Prompt the user• Pull in the number• Add it to a total• Etc

The Program#include <iostream>

using namespace std;

int main() { int total = 0; int tmp; cout << "Enter number 1" << endl; cin >> tmp; total = total + tmp; cout << "Enter number 2" << endl; cin >> tmp; total = total + tmp; cout << "Enter number 3" << endl; cin >> tmp; total = total + tmp; cout << "Enter number 4" << endl; cin >> tmp; total = total + tmp; cout << "Enter number 5" << endl; cin >> tmp; total = total + tmp; total = total / 5; cout << "Average is " << total;}

Problems• Not easy to read.• Fine for five numbers, more difficult for five hundred.

• Prone to error• How do you know you’ve repeated the code enough?

• Hard to expand• What if we now want 500 numbers?

• Difficult to fix• What if there’s a typo?

• Fix every line of code!

• Can’t adapt to runtime situations.

The Repetition Structure• Repetitions are used to manage these problems.• We provide a piece of code that says to the compiler ‘repeat

this section of code five times’• We move responsibility for the repetition onto the compiler.• Where it belongs.

Repetitions• The loop we are going to talk about today is the for loop.• It’s an example of a bounded loop• We use it when we know, at the time the program is running,

how many times we want to loop.• There are several parts to this structure.• We’ll go through each in turn.

• The braces on the structure are important too.• They denote ownership.

Parts of a For Loop• A for loop works on a counter.• The counter tells us how many times we have repeated.

• Or to use the correct jargon, iterated.

• In the structure of the for loop, we have three parts.• The initialisation

• This occurs at the start of the loop and never again• The continuation condition

• We check to see this after ever loo• The upkeep

• We do this after each time the code has been executed.

Parts of a For Loop• A for loop then has the following structure:

int counter; // Declaration of the counter variable

for (initialization ; continuation ; upkeep) { // Code to be executed }

The Braces• Braces are used to indicate ownership.• Not quite, but it’s a good enough explanation for now.

• The code in the braces belongs to the for loop.• If no braces are provided, the next statement only belongs to

the for loop.• This is not good practice.

• For loops don’t end with a semi-colon.• A semi-colon in C++ is considered a statement all by itself.

The Continuation• The continuation is what determines whether or not a loop

keeps iterating.• For a for loop, we use the special symbol <• It means less than.

• This is a comparison operator, and we’ll see more of these as we go along.

• Usually we base our continuation on the counter variable.• is the counter less than a set number?

• counter < 5

Rewritten Program#include <iostream>

using namespace std;

int main() { int total = 0; int tmp; int counter;

for (counter = 0; counter < 5; counter++) { cout << "Enter number " << counter + 1 << endl; cin >> tmp; total = total + tmp; } total = total / 5; cout << "Average is " << total; }

Benefits• Easier to read.• You can tell at a glance how many times the loop will repeat.

• Easier to maintain.• Just one section of code to alter.

• Easier to expand.• If we need to average 500 numbers, we have much less code to

change.• No transcription errors.

Program Style• Our first structure brings with it our first aesthetic issue.• Program layout

• One of the things most neglected by beginner programmers is the layout of their code.• At their own great cost!

• There are many different ways to layout programs.• This module adopts one, but does not require its use.

• It does require the use of a layout though.

Module Style• Braces on the same line as the structure.• Indent one level per level of ownership.• Indent the statements that belong to main by one level.• Indent the statements that belong to the for loop that belongs to

main by two levels.• Judicious use of white space!• You’ll see this style in all of the lecture notes.• It is not enforced by the compiler, but leads to more readable

programs.

This Program Does Not Work - Why?

#include <iostream>

using namespace std;

int main() {int total = 0;int tmp, i;for (i = 0; i < 5; i++) {cout << "Enter number " << i + 1 << endl;cin >> tmp;total = total + tmp;total = total / 5;}cout << "Average is " << total; }

Program Style• The more structures you have, the harder a program becomes

to read if you don’t adopt a style.• Readable programs are programs that you can debug.

• The layout of the code gives you syntactical clues.• It shows you where statements belong in the body of the

program.

Program Style• Comments are also hugely important.• These are text encoded in a program that gets ignored by the

compiler.• They are notes for the developer.

• You can indicate a single line to be commented using two slashes:• // This is a comment.

• Or a block of code using /* and */• /* This is also a comment */

Comments• Comments should be used to explain complex code.• It should be used to explain what you intend, not what the code

actually does.

int i; /* This for loop modifies a temporary variable by the counter.*/

int temp = 10; for (int i = 1; i < 10; i++) { temp = temp + i; }

int i;/* This for loop gives the power of temp to ten*/

int temp = 10; for (int i = 1; i < 10; i++) { temp = temp + i; }

Bad Better

Comments• There are many heuristics about comments.• Such as X% of your code should be comments

• I don’t adhere to any of these.• I find indiscriminate commenting actually detracts from

readability.• Some guidelines:• Comment intention where your intention is not blindingly

obvious.• Later when you start to use functions.

• Document what the function does.• Document parameters• Document return values

Comments• Comments help gain big picture understanding.

• Don’t comment the obvious.

• Consider your audience.• It’s not going to be your relatives who read your code.• It’s going to be fellow programmers who understand the syntax.

• Comment your code like the person who ends up maintaining it is a violent psychopath who knows where you live.• Damian Conway

Program Style• Good programs are readable programs.• Good code is its own documentation.

• You should always make use of meaningful names for your variables. • Again, not enforced by the compiler.

• Bad variable names inhibit understanding:• av = (av / 100) * 102.15;

• Good variable names increase understanding:• accountValue = (accountValue / 100) * 102.15;

Program Style• Avoid the use of magic numbers.• Numbers embedded in your code with no explanation.

int interestRate = 102.15; int percent = 100;

accountValue = (accountValue / percent) * interestRate;

Program Style• I don’t want to harp on about this.• But I’m going to anyway.

• The single worst thing you can do in a program is ignore the aesthetics.

• The single best thing you can do is adopt a strict coding style.• There are holy wars of formatting about which style is best.• They’re all roughly as good as each other. It doesn’t matter what

style you adopt, it only matters that you adopt one and stick to it.

Summary• Repetition is a powerful programming tool.• Repetition is a powerful programming tool.• Repetition is a powerful programming tool.• If a joke is funny once it’s funny every time, right?• Some jokes aren’t funny the first time.

• Program style is important!• Don’t neglect it!

• Comment your code• Where appropriate