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24.9
The asterisk that actually has special meaning in certain contexts, not just
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multiplication.
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But in this case, it specifies that the type of a variable
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is not a char literally, but a char star, the address
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of a char, a pointer to a char.
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Now, why char?
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I thought we were talking about strings.
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But again, recall that a string is just a sequence of characters
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back to back to back, and therefore, you can define a string
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by the address of its first character.
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Ergo, what we really need underneath the hood
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is a data type that lets us store the address of a character.
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There is no string.
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And so what does this allow us to do?
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Let me go into CS50 IDE, and let me declare then, on this line
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here, that s, this time, will not be a "string" quote unquote.
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That was from the CS50 library.
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But rather it's going to be a char star.
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It turns out that all this time get string, again, does not
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return a string, it returns the address of a string, AKA
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the address of the first character in a string.
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And so the type of value it's returning is not just a number.
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It's not just an int.
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It's a special type of an int.
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It's used for a different purpose.
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It's simply an integer that represents the address of a char.
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And the way you type out address of a char is literally char star.
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So this, then, is identical to my previously in weeks past having typed
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string s.
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Now I'm going to start typing it as char star.
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Meanwhile, t is going to be the same.
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So when I prompt the user for another string there,
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I'm going to store that return value inside of t.
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And now, notice, just for good measure, I'm making sure
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that both s and t are not null.
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I'm using a bit of conditional logic there, saying if s is not null
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and t is not null, it's safe to proceed.
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Because recall that get string can accidentally return null sometimes
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if your computer is out of memory, or something else goes wrong.
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Or not so much accidentally, but by design.
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But notice this new chunk of code.
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It turns out-- and we know now from a moment
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ago-- you can't just compare s against t.
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They're not going to equal the same thing
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if you type in two independent strings.
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We need a special function that actually compares strings in a conceptual way.
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I mean that a string is equal to another string if every one of its characters
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equals every one of the other string's characters.
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Thankfully, there exists in C a function that does exactly that called strcomp,
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string compare, and it takes two arguments.
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The first is a string.
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The second is a string.
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Or more properly, the first is the address of a string.
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Or even more properly, the first is the address of a character.
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The second is the address of a character.
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And str compare is just going to hope that both of those strings
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eventually end in a backslash zero, so that they
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don't loop forever through memory.
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They eventually hit that special null terminating byte.
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And if so, and those characters are all entirely equal, you print same.
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Else, as before, we print out different.
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So now, when I compile this program, make compare 1,
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and then I do ./compare1.
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Now I'll type in david in all lowercase, david in all lowercase.
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They're indeed the same.
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Let's do it again.
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Zamyla with a capital Z. Zamyla with a capital Z. There are indeed the same.
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Let's do Zamyla with a capital Z. zamyla with a lowercase z.
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Different.
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And then FUBAR, clearly different.
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Now we're actually comparing these things properly.
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Because now we're appreciating what it actually means to be a string,
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and we are underneath the hood comparing what we should be doing.
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Now, underneath the hood, what is str compare doing?
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Honestly, it's probably just a while loop or a for loop
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that is iterating over the string and their lengths
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and looking at the i-th character in each string,
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and making sure they're all in fact equal.
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Let's go ahead and fix copy with version 1 here.
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Copy 1.c that I've written in advance now looks like this.
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I still prompt the user for a string s with these lines here.
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I then, just to be safe, say hey, wait a minute, if s equals
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equals null, return 1.
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And again, 1 is just arbitrary.
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I just want to get out, lest I break something later.
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Now, down here, this is a new line of code.
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And this is perhaps one of the most powerful ingredients
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we'll see this week, is this new function called malloc,
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memory allocate.
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This is a special function via which you can ask the operating system, Linux
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in the case here, or Mac OS, or Windows, if you're running the code locally,
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hey operating system, please give me a bunch of bytes of memory.
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Now, why do I want this?
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This program is copy 1.c.
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The goal at hand is to create a program that copies a string s
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and stores the copy in t, so to speak.
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Last time, it was not sufficient just to say t equals s,
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because that copied the addresses.
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That didn't give me a copy of Z-A-M-Y-- Z-A-M-Y-L-A backlash 0.
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It instead just gave me a copy of the address.
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