1.4 If Statements - IP
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>> Hello everyone and welcome back to intro to
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Python here on Cybrary on-demand.
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I as always, I'm your instructor
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Joe Perry and in this lesson today,
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we're going to be learning about if statements.
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In Lesson 3, programming basics.
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Lesson 3, our objectives are going to be;
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Understand and define the term flow control,
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understand and apply if statements,
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and learn the components of if statements.
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Note this has a slightly Python bent.
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The inclusion of ellipse,
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which we'll talk about in a minute,
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is something that Python uses that
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not all other programming languages use.
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But this isn't going to be actual Python code yet.
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It's just going to be bent toward Python.
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All flow control actually is,
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is execution decisions made
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based upon some internal logic.
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I want to say that again, and I'm going to say it
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again throughout this course.
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It is an execution decision
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made based upon some internal logic.
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A program, for example, might perform
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different operations based on the keys you press.
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Almost all programs do.
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A Word processor is going to print the letter
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of the key you press or the symbol.
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If you use Shift, it'll print
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a capital. Things like that.
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That's a great example of
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flow control because it's not something
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people generally think of as flow control.
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Flow control, it's
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a weird phrase to keep saying over and over again,
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but it's something that people often
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get wrapped around the axle with,
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get a little bit confused,
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don't really understand, and really,
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it's because it's being over-complicated.
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The simple fact of the matter is that any decision
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your program makes based on some internal logic,
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some evaluation of truth or falsehood,
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that's all flow control.
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That's all it is. Don't get too far into the weeds on it.
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Just understand that it is
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a decision your program is making.
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If statement specifically in this lesson,
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going back to the Boolean logic we
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talked about in our earlier lesson,
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if is implied in all of those statements.
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All of those statements can be framed and
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generally are framed as if then.
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If A equals one and B equals one,
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then A and B equals one or if A equals one,
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or B equals one,
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then A or B equals one.
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It's really easy to understand,
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or it's not really easy to understand,
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that's not a fair thing to say.
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It's very straightforward.
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All you're doing with if,
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is you're just using it to evaluate a condition.
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You'll often hear this referred to
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as a conditional and in
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the help documentation that we provide,
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it is referred to as a conditional statement.
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Some examples of if from everyday life,
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if it is raining, wear a raincoat.
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If the subway is late,
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yell at the sign.
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That doesn't help but if you've
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never lived in Washington DC,
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I don't know how to tell you how much better it makes you
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feel to shout at a subway sign,
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this has done nothing wrong.
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But it gets out a little bit of
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your aggression and in DC, that's very important.
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If the number X is divisible by 3,
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print yes to the screen.
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That's a more programmatic example.
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That's something that's going to often show up
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in different programs,
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different tools, having menus that are
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based on a control number that is actually a number,
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an integer of some kind.
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That's a very common paradigm used to control a program.
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You'll see the graphic above me is a flowchart.
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Flowcharts are a great way of
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learning to understand if statements,
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I'm aware that's probably pretty hard to read.
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That's okay. It's actually just an xkcd comic.
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I mentioned before and I'll mention again,
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you're going to see a lot of those in this series,
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and then basically everything I do
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because xkcd is glorious.
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Elif, the next statement is part of the if paradigm,
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where we have, if, Elif, and else.
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Elif is a little bit less
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common like I mentioned it's used in Python,
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it's used in some other languages,
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but it's not used in all of them.
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The way Elif works is it is
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actually its own if statement.
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It's a hybrid of else and if.
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If the first condition,
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so whatever your if statement was,
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evaluates to false,
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then consider this condition and evaluate it.
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Which is to say that, for example,
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for the menu example I gave a moment ago,
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if the number is one,
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add, say it's a calculator.
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If the number is one, perform the add operation.
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If Elif the number is two,
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perform the subtract operation.
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Elif the number is three, so on and so forth.
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The idea here is just that
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Elif is an if statement that only
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gets checked in the case that the
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preceding if statement was,
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or the preceding Elif statement was false.
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Now you can have many Elif statements in a row.
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There's not really a serious upper limit on it,
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though generally speaking, you're going to want to avoid
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too many of them just for code readability.
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The final part of this paradigm is the else statement.
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Else only ever exists in the context of an if,
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Elif, else statement.
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It exists also for a type of loop,
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but we'll talk about that later.
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The point is it only exists in
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the context of evaluation of
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a truth or falsehood determination of flow control.
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If the previous evaluation was false,
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then perform this operation.
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Now unlike Elif,
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there is no new conditional being evaluated with else.
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It's simply, if A perform this behavior,
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else in all other cases perform a different behavior.
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That's really all there is to if, Elif, and else.
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But we're going to take a couple of examples here and
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make sure that we understand and can break this down.
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Here are the facts of the situation.
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X is a number.
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If that number is less than 10,
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you will print small.
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Elif that number is greater than 100,
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you will print large,
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else you will print medium.
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We're going to try it out with a few examples.
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X in this first case,
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we're going to set equal to 999.
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Now, if that number is less than 10 if
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is going to be the first statement we evaluate always.
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In most programs within certain contexts,
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your statements are going to be
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evaluated from top to bottom.
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Functions will mess with that a little bit.
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We'll talk about why and how later.
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But generally speaking, you're going to
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evaluate from top to bottom.
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But even in cases where that might
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be messed up for some reason,
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you've got complex go-tos or whatever, if, Elif,
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and else will always be
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evaluated in the order of if, Elif,
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and else and if you don't have an if,
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you can't have an Elif or an else.
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If that number is less than 10, print small.
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Well, 999, of course, is larger than 10,
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so we're not going to print small
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that is evaluated to false,
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therefore we are going to
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>> consider the second statement.
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>> Elif that number is greater than 100, print large.
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Well, the last time I checked,999 is larger than 100,
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therefore, we print large.
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Now, what happens if X equals 15?
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Well, if that number is less than 10,
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we print small, 15 is still larger than 10.
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Elif that number is greater than 100, print large.
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It's smaller than 100,
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so we can't perform that operation either.
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Else print medium.
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Now again, there's no new evaluation when I'm saying
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if it's some set of numbers,
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if it's within some range, simply if neither of
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the previous two conditions are met, just print medium.
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We have X equals one.
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If the number is less than 10, print small.
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Of course, that's going to be our first evaluation
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because that's the first line we check,
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and sure enough, it is small.
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That's all there is for Lesson 3.
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In this lesson, again we talked
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about the concept of flow control,
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which is an execution decision
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based upon some internal logic.
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Then we went ahead and we talked about
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if statements, if, Elif,
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and else and we understood the way that
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those three statements fit together and how they work.
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Join me in the next lesson,
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which is going to be discussing for loops.
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That will be Lesson 4 worked
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out. Did not mean to do that.
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I actually did not realize that that worked
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out that way until just now.
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I have been recording this course for
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two weeks and I just realized the unintentional pump.
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Anyway, I'll see you back in Lesson 4, for loops.
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As always, I'm your instructor Joe Perry and
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thank you for joining us on Cybrary on-demand.
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