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Welcome back. So, we've seen how to assign variables to different values,

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for example, on line one here,

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we have code that assigns the variable x to the value six.

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What that does is,

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you can imagine a variable's values table,

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that says x now points to six.

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So, what that means is that,

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after we've assign x to six on line one,

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if we print out the value of x on line two,

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then that should print out six,

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because Python asks what's the value of x?

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It looks in this variable's values table,

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it sees that x points at six and it gets six.

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Now, what happens if we say something like on line three,

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x equals x plus one?

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Now, on first blush,

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this might look almost like a contradiction,

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but the way that Python evaluates this expression is,

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it first computes the value of this expression

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and then it takes the value of this expression and assigns it to the new value for x.

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So, in other words, what Python is first going to do is,

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it's going to ask what's the value of x?

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Python is going to get six,

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and then it takes that and adds one to it.

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So, the value of this expression,

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x plus one is going to be seven.

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So, all Python does,

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is it computes the value of x plus one to which we get

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seven and then it puts that as the new value for x.

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So, x is no longer six,

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x is now seven.

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So, now when we print x on line four,

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then this prints out seven.

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So, if we run our code,

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you will see that when we print out the value of x on line two we get six, but then,

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after reassigning x to be x plus one on line three,

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then when we print out x on line four,

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then we print out seven.

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Now, this kind of operation where we say x equals x plus one

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or whatever x's previous value was,

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we want to add one to it and reassign that to x.

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That's actually really common and for that reason,

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Python includes a shortcut for incrementing and decrementing like that.

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So, let's run through this code.

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So, on line one,

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we assign x to have the value six and then we

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print out the value of x which is going to print out six.

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On line three, we have this special syntax where we say x plus equals three.

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X plus equals three is a shortcut for saying x equals x plus three.

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Now, it allows us to just say the name of the variable x only once here.

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So, it's a little bit shorter in

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less variables especially if we have much longer variable names.

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So, again, this increments x by three and reassigns it.

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So, if we have our variables values table here.

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On line one we have assigned x to be six and then we print out that.

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On line three, we say x equals x plus three by saying x plus equals three.

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So, x is no longer six x instead becomes nine.

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Then, when we print out x this prints out nine,

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then we can do the same thing with subtraction.

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So, we can say x minus equals one as a shortcut for saying x equals x minus one,

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and so that's going to take x from nine and it's going to

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reassign it to the value eight and now when we print out the value of x,

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then we are going to print out eight.

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So, you can see x goes from six to nine to eight.

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So, we can do reassignment as many times as we want.

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So, we can say s equals one,

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and then we can add two to it,

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then we can add three to that,

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and four to that etc.

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If I run this code, you'll see the different values of s. So,

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s starts out as one,

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when we add two to that it becomes three,

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when we add three to that it becomes six,

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when we add four to that it becomes ten and so on.

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Later on in this course,

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you will find a much shorter way to actually do something like this.

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Now, let's run through some multiple choice questions,

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what gets printed out when the following statements execute?

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So, here, we assign x to be 12.

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So, x has the value 12 and then we say x equals x minus one.

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So, the value of this expression we replace x with 12 and we

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subtract one from 12 to get 11 and we take 11 and that's x's new value.

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So, x is no longer 12,

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x is now 11.

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So, when we print out x we are going to print out 11 or C. Next question,

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what gets printed when the following statements execute?

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So, here we first assign x to be 12.

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Then we say, x equals it's previous value minus three,

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sin x goes from 12 to nine.

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Then we say it's previous value plus five.

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So, x goes from nine to 14.

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So, then we say x equals x plus one,

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so it goes from 14 to 15.

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So, x ends up with the value 15 or C. Next question,

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construct code that will result in the value 134 being printed.

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So, the first thing that we want to do,

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is we want to assign an initial value for bank balance.

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If I try to put this first,

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then Python when trying to evaluate the value of

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this expression would say that it didn't find a variable named mybankbalance.

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So, I know that this line can't come first.

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Here, I can do the assignment first,

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so I can assign my bank balance to 100 and then I can reassign it like I do here.

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So, this looks valid and by the end of running these two lines,

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then mybankbalance is going to have the value 134,

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and then if we print out the value of my bank balance on line three,

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then we're going to print out 134.

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Next question, which of the following statements are equivalent?

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So, I see this statement reassigns x to it's previous value plus y.

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This statement assigns y instead of x.

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So, I know that this can't be equivalent to

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statement A. X plus equals x plus y looks like it's similar to statement A,

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but what that's really saying because we have plus equals here,

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is x equals x plus x plus y.

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So, here we have that extra x plus,

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so that's not equivalent to statement A,

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but statement D is.

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Then statement E just won't work.

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So, statement's A and D are equivalent.

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That's all for now until next time.