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24.9
and conquer approach looking for Mike Smith,
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as well as last time's binary search.
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And then one, which doesn't have to be literally one step.
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Maybe it's two, maybe it's 10, but it is a fixed finite constant number
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of steps.
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And that too might be the running time of some algorithm.
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Now how do we describe the running times of algorithms?
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Well, we use some special notation, asymptotic notation,
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so to speak, which while it might look cryptic at first glance,
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really is just a handy way of succinctly expressing the fact that you know what
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the upper bound on some algorithms running time is,
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what the lower bounds on some algorithms running time is.
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And if those are one in the same formulas in theta,
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do you have a coincidence of the two of big O, so speak, and capital theta.
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So that, while Greek, literally is just a way
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of expressing a bit more succinctly what these running times are.
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And we'll continue to revisit this issue as we
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look at more algorithms and soon data structures still.
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But this time, we apologize.
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We pull back a layer here and admit that there is no such thing as a string.
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Indeed, all this time we've been saying that there's ins, and there's floats,
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and there's chars, and there's doubles and more.
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And we've also been saying there are strings,
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but there really aren't strings.
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This is sort of a figment of the imagination of our so-called CS50
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library.
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But it's a pedagogical simplification that we've
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been using for the past several weeks, so as to not get
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lost in the weeds, the lower level implementation
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details of what a string is, so that we can just get
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real work done in these first weeks.
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But now we'll begin to look underneath that hood
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and see what a string actually is and what the implications are.
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And it turns out, while more complicated in some sense,
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it really just boils down to some first principles,
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what it is the computer is doing underneath that hood.
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So let's take a look at string first by way of a couple of examples.
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Let's go in to CS50 IDE, create a new file, save it as Compare zero dot C,
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and look at the little program that actually doesn't necessarily
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do what we think it's going to do.
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In particular, let me go ahead and include our typical,
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include CS 50 dot H. And let me go ahead and include standard IO dot H as well.
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Let me go ahead and use main void, so I'm not
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going to worry about any command line arguments for now.
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And then I'm going to go ahead and just prompt the user.
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Hey user, give me a string called S for instance.
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And then I'm going to have no newline.
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I just want that all in one same line.
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Now I'm going to go ahead and do string S,
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gets gets string, open paren and close parens,
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so as to get a string from the user.
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And then, let me do this.
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Let me also print T colon and ask the user, essentially, for a string
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that I'll call T, since T comes after S quite simply.
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And now let me just compare these strings as the filename suggests.
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So I know how to compare, not with if s equals t,
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because that's the assignment operator.
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But we know that s equals equals t should compare the values on the left
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and on the right.
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So let's try this.
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So if s equals equals t, then I'm going to go ahead and print out same.
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Elts, they are presumably difference, so I'm
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going to go ahead and print out different with a newline character
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and then save it.
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So a pretty simple program, so let me go down into my terminal window and run
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make compare 0.
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Now let me go ahead and run dot slash compare 0. s,
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I'm going to go ahead and type in Zamyla.
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And I'm going to go ahead now and type in Maria.
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OK, and they're different.
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I expected as much.
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Now let's go ahead and run this again.
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Again with Zamyla.
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And let's just say Zamyla again, different.
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What did I do wrong?
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Let me try this again.
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Maybe it's the capitalization.
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So Zamyla in all lower case, different.
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Well, maybe it's just Zamyla's name.
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Let me try Rob or RLB?
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How about RLB?
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Those are different, as is Rob and Rob.
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So what is going on?
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Those strings pretty much look the same to me.
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I'm typing the same incantation of strings,
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so what is it that's going on here?
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You know what, let me do a test with something else.
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Let me go ahead and create a new file here.
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I'm going to call this copyzero.c.
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Maybe I'm just misunderstanding how comparison works.
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But surely I should be able to copy a string and make an identical copy,
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so let's do that now.
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Let me go ahead and create a file called copyzero.c.
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And let me do an include cs50.h.
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Include standard io.h into main void, so just as before.
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And now, let me go ahead and just prompt the user like before.
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Give me a string s.
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And I'll put that in a variable, s, calling get string as before.
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And now, I remember from prior classes that I'm supposed to do,
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if s equals equals null, maybe I need to do some error checking here.
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