Introduction to Internet Programming with Perl and HTML: Part 2 – Introduction to Programming Concepts
This article is Part 2 of our series on Introduction to Internet Programming series. See the Introduction and Part 1, Introduction to HTML. The Introduction to Programming covers basic programming concepts and teaches the reader to develop "pseudo-code" for various tasks.
2.1 What is a Program?
2.2 Basic Programming Concepts
2.3 Looping and Functions
2.4 Your Homework
2.1 What is a Program?
So, you have never written a program, don't know what a programming language is, and have not a clue where to start? Great -- this lesson is just for you!
A program is a sequence of commands that a computer executes in order to perform some actions, be it the calculation of the number of calories in your lunch or the trajectory the space shuttle will follow on its re-entry to Earth from space. The sequence of commands can be as short as just 1 command or as complex as you can imagine (millions of commands, or more). It is the job of the programmer to create and test the commands that will perform some predefined task. We shall teach you to be a programmer!
A programming language is a language in which you can specify the commands in a program. Just like there are innumerable ways to say "Hello" in the spoken languages of the world, there are innumerable ways to make a computer say "Hello", depending on what programming language you use. Different computer programming languages have different pros and cons to their use. Some are only available on certain operating systems (i.e. Visual Basic only works on Microsoft Windows), some are very complex (like C++ or Java), some are simple (Basic), and some are designed for specific tasks like mathematics (Fortran). It is partially up to the programmer to choose the appropriate language for his or her program. In this class we will be teaching your Perl. We will discuss Perl and its particular pros and cons in the next lesson.
So where do you begin? You can prepare for writing computer programs before you even learn your first programming language! You do this by specifying how you would go about writing a program to accomplish some specific task using pseudo-code, plain English statements describing what to do. In this lesson, we will teach you how to write pseudo-code along with many of the basic concepts behind computer programming languages.
2.2 Basic Programming Concepts
Statements: A statement is the simplest concept in a programming language. A statement is merely a single command to perform a single well-defined task. For example, "Stop the car", and "leave the house" are single statements describing single well-defined tasks. Note that even seemingly simple statements can often be broken down into many simpler statements. "Stop the car" could be broken down in to steps such as "Apply the brakes", "Downshift", "pull over to the curb", and "Apply the parking brake".
The number of steps you should supply and the level of detail you should use for describing their performance will be determined in part by the programming language you use -- in some languages "Stop the car" may be a single statement, on others, hundreds or thousands of statements! It will also be determined by how specific you wish to be in describing how to perform the action versus how much you want to leave up to the people who take your pseudo-code and write a program based upon it.
Variables: Statements only get you so far without additional concepts. Variables are perhaps the most ubiquitous entity in any programming language. You can think of a variable as a box in which you can store something -- a number, a date, someone's name, the price of a sweater, the number of people in your immediate family, the current speed at which your car is traveling, etc. The beauty of a variable is that you can store information in it and retrieve information from it any time you want!
Each variable "box" has a name. You refer to the different variables that you are using by their names. You can choose pretty much any name you wish, but it is advised to choose names that reflect their purpose or what is stored within them.
With variables, you can start having more interesting statements and start having the computer perform mathematics. The following is example pseudo-code for calculating the length of the hypotenuse of a right triangle.
Example 2.2.1: Pseudo Code For Length of the Hypotenuse
You see with this first example of pseudo-code how you can specify the actions required to perform a task in plain English without regard to any specific programming language. The assumption made in our example is that actions like "Ask the user for something" and "Calculate something" are simple enough that they require no further details. In truth, it will be your knowledge of the programming languages that the program may be written in, and the people who will be writing the code, that determine the level of detail that you need. For now, just follow your intuition.
Logic: One thing that computers do REALLY well is obey the rules of logic. In particular, they are really good at doing calculations and comparisons. For example "5 is less than 3" is a false statement, "A Ford Escort is a VW Jetta" is a false statement, and "2 + 2 = 4" is a true statement, are all straight-forward logical conclusions. "Yellow is a nice happy color" is not something that can be determined logically unless you have already specified what makes a color "nice and happy".
Performing logical operations, especially when a variable is involved, allows you to determine the truth or falsity of all kinds of things. This is known as "Boolean logic" as the result is the 2-valued truth or falsity of the statement.
Conditionals: Well, since we can determine if comparison is true or false, it would be nice if we could act on this information. This is where a conditional comes in. Consider the example of driving "If the stoplight is red then stop the car". This uses a little logic (is the stoplight red or not?) or determine whether or not we should stop the car. As you can imagine, we use these types of rules all the time in everyday life without ever realizing it! They are used all the time in computer programs too. Lets consider another mathematical example, computing the result of dividing one number into another...
Example 2.2.2: Pseudo Code For Dividing 2 Numbers
The trick here is that we use a conditional to check to be sure that the user provided us with valid number. Since you cannot divide by zero, we have to be sure that we don't try and that well inform the user of the error.
Conditionals can contain more complicated logical statements and can be chained, for example, if you are driving your car...
Example 2.2.2: Driving
Obviously, you could make these instructions arbitrarily complicated, but even this much shows us how you can chain "ifs" together so that if the first rule doesn't happen, then you can consider the second, and so on, until all the rules fail when you can perform some default action ("keep going straight"). It also shows you how you can have compound logical statements "the car is moving AND the road turns right". These are two separate statements that must both be true in order for the rule to take effect. You will often see multiple logical statements joined together with words like "and" and "or".
2.3 Looping and Functions
Looping: Many times in your daily life, you repeatedly do the same action. For example, when washing dishes by hand, you "grab a dirty dish, wash it, put it in the drying rack, and repeat until there are no more dirty dishes to wash." This is a kind of "loop".
Loops are essential in many programs, too. There are two general types of loops -- those where the condition is tested first, and those where the condition is tested last. In our dishes example above, the condition is tested last "repeat until there are no more dirty dishes" -- in other words, it assumes that there is at least 1 dirty dish since the first thing we do is grab one! This loop could be re-phrased with the condition first: "while there are dirty dishes, grab a dirty dish, wash it, put it in the drying rack, and repeat". This accomplishes the same task, but it also works if there are NO dirty dishes. For us humans, it wouldn't matter -- if there are no dirty dishes, we are not going to try to grab one (note the if statement there -- we do that unconsciously all the time). For a computer, if you tell it to, it WILL try to grab that dirty dish, even if there are none left! This could cause all sorts of problems -- so you have to be careful to be logical and take into account all the situations that could occur.
Another thing you should be careful of with loops is to make sure that they always end! If a loop never ends, it is called an "infinite loop" and almost always is not what you intended! If you run a program and it never stops, chances are good that you have an infinite loop somewhere.
In the following example, we are going to the grocery store with a list of items. We need to describe a method for shopping. A list is a kind of variable that is like a box with many numbered compartments -- each compartment behaves like a normal variable. You can refer to the contents of the compartments in your list (aka the list elements or items) by their "compartment number", i.e. item 1 of "shopping list", item 2 of "shopping list", etc.
Example 2.3.1: Grocery Shopping
1. Drive to the grocery store 2. Exit the car 3. Go up to the grocery store 4. If the store is open then 4.1 Enter the store 5. Otherwise, 5.1 Go home 5.2 Stop the program 6. Enter the grocery store and go to the area where carts are stored 7. If there is a free cart, get it 8. Otherwise, 8.1 wait until someone returns one 8.2 get the newly free cart 9. Let variable "n" be the number of the item in our shopping list that we are currently looking for 10. Set "n" equal to 1 11. while "n" refers to one of the items in our list, do 11.1 Look at every item in the store until we find a match for grocery list item "n". 11.2 If we find the item, add it to our cart 11.3 Otherwise, mark our list that it was not found. 11.4 Change "n" to "n+1". (i.e. lets look for the next item in the list) 12. Proceed to the checkout counters 13. Let "counter" store the number of the checkout counter we will proceed to. 14. Let "people" be the number of people currently in line at that counter. Set this to "99999999" 15. Let "items" be the number items in our cart. 16. Count the number of items in the cart and save it in "items". 17. For each checkout counter 17.1 Find out the maximum number of items that you can checkout with at that counter, store in "max". 17.2 If "max "is less than "items", go to the next counter 17.3 Find out how many people are currently in line, store in "p". 17.4 If "p" is less than "people" then 17.4.1 Set "counter" equal to the current counter number 17.4.2 Set "people" equal to "p" 18. Go to counter number "counter" 19. Check out 20. Take groceries to the car 21. Drive home 22. Stop program
This example is a good demonstration of the use of all the pseudo-code features we have so far mentioned. As you can see, many of the parts of this example could be extended with more detail, or simplified by suppressing detail. I.e. what do you do if you couldn't find ANY items on your list at the store?
Functions: There are often computations or operations that need to be performed at multiple places in your program. Instead of repeating the statements over and over, it is usually better to extract these statements and place them into a "function" or "subroutine". These are named sets of statements that perform a specific action, like "calculate the average of a list of numbers" or "drive home". Then, in your program, you can just refer to the function wherever you need to perform its action, simplifying your work.
Example 2.3.2: Finding the Average Height
1. Make a list called "height" with enough items for every person in your class. 2. For each person in your class 2.1 Calculate Person's Height [A Function] 2.2 Store the height in the "height" list in an item labeled by that person's name. 3. Make a variable "height_total" and initialize it to zero. 4. For each item in the "height" list 4.1 Add the height to variable "total_height" 5. Make a variable "height_average" 6. Set "height_average" equal to "height_total" divided by the number of people in the class 7. Display the contents of "height_average" Function "Calculate Person's Height" 1. Stand the person up straight 2. If the person's shoes are on, then 2.1 have them take his/her shoes off 3. Use a yard stick to measure the height in inches 4. Record the height and return it
This example shows a function that performs some set of actions and returns a result. That means that back in the main program, after you executed the function, you get back anything that it "returns", in this case, you will know the person's height.
Breaking up your programs into functions that perform repeated tasks is a very good thing as it makes your programs smaller, reduces the number of errors you might make, and makes it easy to update and change your program later. Use functions.
You will also notice one new concept that we introduced above. We stored the students' heights in an array where the items were labeled by the students' names and not some number. You can do this in many programming languages (especially Perl). It is technically called a "hash", but you can just think of it as labeling a list by a name vs a number; depending on the situation one way may be simpler or more appropriate than the other.
2.4 Your Homework
You are a "pollster". You have a list of questions each with a list of acceptable answers. You need to write pseudo-code for the following process: Calling a "poll taker". Asking all of the questions in your poll. Getting all the responses. Making sure that the responses are acceptable. Recording all the responses. Of course, you need to figure out and describe in detail a good method for doing this in your pseudo-code! (I.e. this could be used to train a new pollster how to give a poll to someone over the phone).
Your pseudo code should convert your common-sense idea of of how to give a poll into instructions. Try to be clear, concise, and logical.
You would be graded on how well you implement your ideas in pseudo code and how clear and logical they are. Your should absolutely avoid writing a narrative description of the poll giving process!
The idea behind pseudo code is that you should be able to hand it to anyone and they should be able to write a computer program based on your code that implements your ideas. Hence, you should limit ambiguity where possible and consider all "corner" and "unlikely" situations that may occur.
Similar Posts:
- Introduction to Internet Programming with Perl and HTML: Part 3 – Introduction to Perl
- Introduction to Internet Programming with Perl and HTML: Part 4 – Basic Perl Flow-Control Syntax
- Introduction to Internet Programming: Part 5 – Perl Data Part I – Lists
- Introduction to Internet Programming Part 6: Perl Data Part II – Hashes
- Introduction to Internet Programming with Perl and HTML: Part 1 – Introduction to HTML
Tags: functions, loops, perl, program, programming, pseudocode, variables
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