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and actually run it before your own command.
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2.22
So that if you don't understand some error
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message from a program you're compiling or a command you're running,
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2.833
you can actually see some helpful output from CS50 staff.
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eprintf, which, by contrast, is something you can put into your code,
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4.36
like printf, and print a message to yourself.
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2.23
And nicely enough, it's going to actually include the line number
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and the file name from which it came.
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So you can identify where some output is coming.
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And then lastly, debug 50 which is the most sophisticated of these programs.
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And debug 50 when you run it at the command line,
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followed by the name of your program in the usual way,
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it's actually going to open up what's called a debugger.
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A graphical user interface that CS50 staff put together
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that will actually let you step through your code,
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see variables, and generally understand it better.
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So we also looked at this in the context of Ovaltine
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in particular, and Little Ralphy.
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1.39
And we looked at cryptography, the art of concealing information.
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4.28
And indeed, among the challenges in CS50,
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ultimately if you choose to accept them, are
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things like implementing Caesar cipher and Vigenere
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cipher, cracking passwords.
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2.04
So indeed, those problems lie ahead and draw inspiration from this domain.
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4.1
And then, we focused more technically on what a string is.
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3.31
So Zamyla, a name we keep taking for granted, that a computer can express.
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Z-A-M-Y-L-A, as a string or a phrase in a program.
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5.16
But it turns out that underneath the hood,
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strings are a little more mundane.
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They really are just this grid of information.
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And we might draw Zamyla's characters boxed in like this.
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But if you think of what's inside of your computer itself,
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you have a whole bunch of memory, or RAM.
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2.24
And inside of that RAM, you have Z-A-M-Y-L-A,
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but then this special character.
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1.57
What is this /0 used for.
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Why is there this seemingly strange character at the end of Zamyla's name?
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5.1
Anyone?
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Yeah.
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AUDIENCE: [INAUDIBLE].
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SPEAKER 1: Yeah, to denote the end of the string.
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Because after all, if we were to have someone else's name here,
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if the next string I got is Maria, for instance.
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M-A-R-I-A, we need to be able to know where Zamyla's name ends
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and Maria's name starts in memory.
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Because at the end of the day, C, and programming in particular,
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isn't really all that sophisticated.
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At the end of the day, the only place you have to store things
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is your computer's memory or RAM.
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And each of these boxes represents a byte of memory inside of your computer.
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And so if all this is-- so far as the computer cares--
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is a sequence of six alphabetical letters,
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2.44
the computer needs to be told or given a hint as with this /0,
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which just represents 8 zero bits, as to where Zamyla's name ends and something
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else begins.
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1.64
And we use this as an opportunity to introduce ultimately
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command line arguments.
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1.22
So in particular, we've been writing programs thus far
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with just int main void.
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And void, we realized, signifies that a program takes no command line inputs.
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It can still call functions, like get string and get int and get [? flow. ?]
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But when you run the program, you can't put any additional words at the prompt
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after it if you've declared main as taking void as input.
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Nothing as input.
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But we introduced instead this, int main int arg
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c string arg v open bracket close bracket, which
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means main can alternatively take two inputs from the so-called command line
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when you run it.
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It can take arg c and arg v. And what was arg c?
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What was arg c?
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Arg c.
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Yeah, argument.
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Yeah.
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AUDIENCE: [INAUDIBLE].
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SPEAKER 1: Yeah, the number of command line arguments.
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Arg c for argument count.
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So how many words did you type at the prompt?
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And arg v means argument vector, which is
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another way of saying it's an array, a contiguous block of memory
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inside of which are all of the words themselves
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2.32
that you might have typed at the prompt.
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So this time, we're going to build on that.
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We're going to focus today mostly on ideas.
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Mostly on algorithms.
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Mostly on problem-solving and less on the lower-level implementation
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details of code.
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0.85
Though, we'll look at a couple of examples.
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So this more generally is what's called an array.
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If you think of your computer's memory as just a block of memory,
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block of bytes.
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And maybe it's byte 0, then byte 1.
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Then, byte 2, just because you can number them, certainly.
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If you think of them that way, this is kind of an arbitrary generic data
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structure, if you will.
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A generic way of just storing things underneath the hood.
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Now, what do I mean by that?
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And why is this useful?
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Well, consider something like this.
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Consider having 8 doors, behind which are some numbers.
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And the goal at hand is to solve a problem.
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