What do Interpreted Programming Languages have in Common? Part II

I begin this tutorial a few weeks ago with Part I and received some very nice comments correcting my (fortunately) minor errors. This isn't a tutorial about how to program in a specific language or even really about how to program. I wanted to show the common structure of interpreted programming languages in the hopes of revealing some common threads, rather than focusing on the ins and outs of one language. I've heard it said that if you learn one langauge, it makes learning the next one easier. My problem is I get lost in the nuances of the language in question and lose track of the basic structure of programming. I've created this tutorial series to try and correct that. This tutorial is for my education as much as anyone else's so I welcome comments but, as I said before, be polite. This is about learning.

Speaking of Languages, code examples are presented in JavaScript, Python, and Ruby. I figured this represented a healthy cross-section of commonly used interpreted languages. Now, on with the show.

Controlling Program Flow, Part I

In our last episode (sorry, couldn't resist), we left off with Arrays and Booleans. The tutorial picks up with methods of flow control. According to Wikipedia, flow control is "a statement whose execution results in a choice being made as to which of two or more paths should be followed. For non-strict functional languages, functions and language constructs exist to achieve the same result, but they are not necessarily called control flow statements." In other words, it's a way of making decisions within the program given different conditions. Speaking of which:

Conditions: These statements in a program act as a decision tree or like a set of intersections in a city. The decisions or route are dependent on what conditions are or are not true (speaking of Booleans) or where you're trying to go. The basic structure of a conditional statement looks something like this:

if (condition)
{
     conditional code;
}

That's a very general example, of course, but it gives you a place to start.

if statements:
If statements are more or less the same in all three of the languages we've been working with:

JavaScript Sample Code:

var value = 0;
if (value > 0)
{
     alert("Greater than zero");
}

Python Sample Code:

n = 0
if n > 0:
     print ("Greater than zero")

Ruby Sample Code:

n = 0
if n > 0
     return "Greater than zero"
end

Of course, that's not much of a decision tree. Each program has different conditional statements that are similar but not exactly the same, to deal with a decision that is either this or that:

if-else statements:
The following examples are equivalent and look almost the same, but not quite:

JavaScript Example Code:

var name = "Jim";
if (name == "Jim")
{
     alert("Your name is Jim.");
}
else {
     alert("You are not Jim.");
}

Python Example Code:

name = "Jim"
if (name == "Jim"):
     print ("Your name is Jim")
else:
     print ("Your name is not Jim")

Ruby Example Code:

name = "Jim"
if name == "Jim"
     print "Your name is Jim"
else
     print "Your name is not Jim"
end

That takes care of "the fork in the road", so to speak, but what if there's more than one decision to make? It can't be just "Jim" or "no Jim" all the time. What about Jim, Bill, and Heather, for example?

else-if statements:
Depending on the program, this statement is called else-if, or elif, or elsif, but you get the idea.

JavaScript Code Example:

if (name == "Jim")
{
     alert ("Good Morning, Jim.");
}
else if (name == "Bill");
{
     alert ("Good Morning, Bill.");
}
else if (name == "Heather");
{
     alert ("Good Morning, Heather.");
}
else
{
     alert ("You're not on the list.");
}

Python Example Code:

if (name == "Jim"):
     print ("Good Morning, Jim.")
elif (name == "Bill"):
     print ("Good Morning, Bill.")
elif (name == "Heather"):
     print ("Good Morning, Heather.")
else:
     print ("You're not on the list.")

Ruby Example Code:

if name == "Jim"
     return ("Your name is Jim.")
elsif name == "Bill
     return ("Your name is Bill.")
elsif name == "Heather"
     return ("Your name is Heather.")
else
     return ("You're not on the list.")
end
print name

As you can see, there really isn't much difference between now each language expresses conditional statements. The details are quite minor and I hope this all illustrates the common factors in such statements, which is the important piece to learn in this lesson.


Time limitations are forcing me to make Part II shorter than I originally intended, but at least I've got it up and on the blog. Pick through everything and see if I left any holes. If so, let me know. If not, then all is good, at least for Part II. See you next time for Part III: Loops.