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have two copies of the number one-- in a and in temp.
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Then, I change the value of a to equal b.
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So at this point in the story, a should have a value of 2.
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b should have a value of 2.
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Which could be problematic, except I've saved the original value 1
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in my temporary variable.
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And so, indeed I change b to equal that temporary value.
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So indeed, I can't do this magical switcheroo.
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Because if I simultaneously took one value and one value
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and put them in each other's places, I would essentially
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risk clobbering one or the other.
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Because the computer typically can only do one thing
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at a time underneath the hood, even though it's super fast.
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So I run the risk of somehow getting two copies of that same value,
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unless I'm careful.
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I really want to take one value, put it in a temporary space.
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Then, copy one value over.
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And then, do that switcheroo.
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But it doesn't seem to work.
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And yet, this is a good opportunity for one more tool.
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Let me do a further check.
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Let me go ahead and run debug 50 on no swap.
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And you know what?
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I'm going to set a break point right at the beginning of this program on line
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10 by simply clicking in the so-called gutter of the line number here.
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A red dot appears, so that now when I run debug 50,
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notice my right hand bar pops out.
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Notice that the line 10 in question is highlighted, because execution is
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paused at this line.
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1.2
Let's take a look at what's going on the right hand side of my debugger.
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Notice that x has a value of 1 and y has a value of 2.
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4.1
And that's what I intend.
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1.15
Now, let's begin to step over these lines.
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So let's step over the printf.
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And we see on my terminal window that x is 1.
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Step over the next printf and we see that y is 2.
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Now, let's step over that and we see indeed swapping dot dot dot.
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But rather than step over my call to the swap function, let's step into it.
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So I can actually see maybe my logic is buggy.
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So let's go ahead and step into swap.
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And notice that debug 50 immediately jumps down to the first line and swap.
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And notice now on the right hand side, what's going on?
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a is 1 and b is 2.
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And that's as expected, because I passed an x and y.
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Swap calls it's two inputs a and b respectively, so a is 1, b is 2.
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Temp though, very curiously is 32,767.
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And where is that value coming?
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Well, it turns out when you declare a variable,
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but haven't yet said it's of some value it's going to by definition have
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an undefined value.
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1.18
A so-called garbage value.
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1.51
Remnants of other aspects of your program
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having used that chunk of memory before.
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2.17
And we'll see why this is in just a bit.
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So this is anomalous.
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It's not a bug in the program.
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It's to be expected, because I haven't yet assigned
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temp a value until-- let me zoom out.
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Until I step over line 21 and actually let temp equal a.
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And notice, if you watch the right hand side of my screen now,
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temp is currently 32,767.
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And when I step over it, OK.
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It becomes 1.
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So that's as expected.
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Now, a is about to become b.
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And indeed, we now see that a is 2 b is 2 and temp is still 1.
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4.2
So now we have to put temp into b, so let's step over that line.
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3.4
And indeed, we've done a switcheroo of a and b.
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a is 2, b is 1.
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OK.
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0.5
Let's go ahead and play this out.
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Let's continue here.
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I claim that I've swapped.
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I'm printing x-- and somehow x and y are still 1 and 2.
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6.68
Even though the debunker told me it would
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seem that my swap function was correct.
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2.33
So what is happening after all of these weeks of CS50 and all of these problems
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solved thus far?
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0.792
It would seem that all of our understanding of things
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is kind of unraveling.
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And that's because we've been very careful over the past few weeks,
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at least in class.
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And in sections to try to avoid tripping over some of these lower-level
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implementation details.
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1.26
And there's not that many of them.
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1.56
But today, is now a time to peel back this layer
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and understand exactly what it is that's going on.
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Indeed, all of this time when you run a program on your computer,
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double-click an icon or run the program's name with dot slash
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something or other at your terminal window-- what happens
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is that your computer gives that program the illusion of a really big chunk
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of memory all of its own.
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Maybe two gigabytes of memory, even though it doesn't necessarily
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use all of that memory.
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And that memory-- if you just think of your computer's memory,
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as we've done before, is like a rectangular region.
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And we could number of the bytes in my memory.
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1.922
But it doesn't really matter what the addresses are, what the numbers are,
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just that it exists.
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It turns out that when you run a program,
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