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Algorithms
Algorithms
An algorithm is a set of steps you must complete in order to accomplish some task. For example, brushing your teeth requires that you:
- pick up toothbrush
- pick up toothpaste
- apply toothpaste to toothbrush
- put toothbrush in mouth against teeth
- scrub for two minutes (According to the American Dental Association)
- expectorate (spit)
The order of these events are important. For example if we spit as our first step instead of our last then after scrubbing our teeth we have a mouth full and no instructions as to what to do with it.
In addition, we need to be sure we're being specific enough with our instructions. The instruction "apply toothpaste to toothbrush" may seem somewhat obvious, but it does neglect some substeps that are necessary. If you look closely, we never opened the toothpaste! Also, how do we get the toothpaste out of the container? We need to be careful that we get all the correct, necessary steps in enough detail to make the algorithm work every time. That being said, we can, and often have to, make some assumptions about the level of previous knowledge.
Algorithms are an important part of computer science: They define the what a computer does and how it does it. As such, programmers have many ways of defining these algorithms visually because it allows us to translate the algorithms into our programming language of choice. We will be examining flowcharting in this course:
Flowchart
Flowchart
Flowcharts define the flow of a program from one step to the next, defining the algorithm with a few basic shapes that have particular meanings:
Terminator:
Terminator:
A terminator looks like a rounded rectangle. Its entire purpose is to tell us where the algorithm starts and where it ends. We denote this by putting "Start" and "End" in the middle of the terminator. To the right is an example of a blank terminator.
Process:
Process:
Process blocks are meant to represent actions performed by the computer that the user does not see. An example would be if the computer were adding 2 numbers together. The action performed is written inside of a rectangle. To the right is an example of a blank process block:
Decision:
Decision:
Decisions are how we create programs that have different results every time. Programs would be boring without them. Decisions are shown using a diamond. Within the diamond we write the condition, for example, x > 0. To the right is an example of a blank decision:
Input/Output:
Input/Output:
Communication with the user is done through input and output statements. Without this interaction a program simply runs and completes on its own without the user knowing about any progress or the result of the program. This symbol is used for two things:
- Allowing users to input information into the program
- Writing information out to the screen in some way that the user can view it
Flowcharts use parallelograms for input and output. To the right is a blank input or output symbol:
Flow:
Flow:
Flow determines the order of the instructions. We represent flow using arrows that connect two shapes together:
IMPORTANT:
- Only one arrow may leave Terminator, Process, Input/Output and On-Page Reference blocks
- At least two arrows must leave a Decision block
- There is no limit to the number of arrows that may enter a block
On-Page Reference:
On-Page Reference:
While designing algorithms, you may find that you have run out of space. In order to avoid having extra or long flow arrows, we use on-page references. These references are numbered in order to show the reader where the flow of the program goes after reaching the end of a page. The symbol used is a circle:
Pseudocode
Pseudocode
Pseudocode is a high-level description of an algorithm. It uses similar structures as actual programming languages, except that they read more like standard English to make it easier to understand. It is a very informal language that is used only to describe an algorithm.
Decisions are shown through:
if CONDITION then ACTIONSend if if CONDITION then ACTION 1else ACTION 2end if if CONDITION_1 then ACTION 1else if CONDITION_2 then ACTION 2end ifA CONDITION is some evaluable check that can be done based on data that is available.
ACTIONS are what you do if a CONDITION is true. If a CONDITION is not true, then the ACTION is not performed.
Example:
Example:
If we were to write some pseudocode for leaving your house in the morning, it may look something like this:
put books in bag if lunch available then put lunch in bagend if if is sunny then put on sunglasseselse put sunglasses in bagend if put on shoes if is cold then get winter jacketelse if is cool then get light jacketend if if is raining then get umbrellaelse if is going to rain later then get umbrellaend if get bag open door exit home lock doorWe can see what this would look like as a flow chart:
Review Work
Review Work
- Describe the process of preparing a party as a flowchart.
- Make a flowchart how to advice people to choose a university/profession. Alternatively, help them choose a pet.
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