CS50x 2024 - Lecture 4 - Memory

[@MasterOfYourself]

I love how David is really trying to hear people's questions, and he encourages them with cookies, or jokes.

[@kent1k359]

Best educator imho

[@Aprokind]

If you are reading this, I hope you finish cs50 and go on to do well in whatever endeavor you choose.

[@thebrightchannel6898]

I will and i hope you will to, good luck!

[@BerkGoknil]

Even if you have been writing programmes for 20 years, it is an excellent series that must be completed to the end. Thank you David and Harvard.

[@meegel]

Theres a lot of "A-HA" moments for me this week, it is incredibly motivational for me to keep on learning CS. Thank you Professor Malan and the CS50 team!

[@worrystone]

Feeling honored to experience buffer overflow during the overflow section.

[@kurrizzle]

that little disappearing magic trick at 1:28:20 was subtle but nice

[@randomforest_dev]

CS50 2024 lectures are really awesome!

[@codewithsajid]

for those scratching your head at 49:20, perhaps this will help. what is the relationship between arrays and pointers/addresses? --- In C, arrays and pointers are closely related. Here's a breakdown of their relationship: 1. Array Name as a Pointer: When you define an array in C, the array name acts as a pointer to the first element of the array. For instance, if you have int arr[10];, arr can be thought of as a pointer to arr[0]. 2. Pointer Arithmetic: You can perform arithmetic on pointers just as you can with arrays. When you increment a pointer that points to an array, it points to the next element of the array. This is because the size of the type the pointer points to is considered in the arithmetic. 3. Indexing: You can access elements of an array using the subscript notation []. Under the hood, this is converted to pointer arithmetic. The expression arr[i] is equivalent to *(arr + i). This means "get the value at the address arr plus i times the size of the array's element type". --- Thanks chatgpt

[@dangodono6096]

very helpful

[@DRNOval]

finally got thru tideman after a week and a half of struggle and am so happy to be watching this finally 😭😭

[@krish-ut9de]

lol me too, gl for this week's assignment

[@emmanuelprudente6463]

Lmao exactly same for me over here. See you around folks!

[@DRNOval]

@@emmanuelprudente6463 im on week 7 now, as long as u can push past week 5 it gets easier after it seems

[@db2360]

I have cried as well. A lot. 😂 (And ddb helped a LOT)

[@amirabdelli2295]

Starting 2024x here as this is where I was in the 2023x series (and I could definitely benefit from rewatching this one)

[@yannicbrose]

same... :)

[@FigueMonk]

I am here after trying to understand lecture 5. Probably is good idea to refresh these lecture's concepts. Keep learning folks, one day at time, step by step

[@Hermeticoh11]

Thank you, David, for your amazing teaching skills! I can't believe I'm being able to understand aaaaall this information having started learning computer science this year

[@naboulsikhalid7763]

I am overwhelmed by the info introduced and very glad to know it and move with my C computing and programming. Thank you David

[@jiayiweng8985]

22:58 pointers - getting the address(&n) of the integer stored in memory 24:11 pointers - getting the integer of that address

[@realJaeyongLim]

I started CS50 since 2021 but did not make it to the endpoint. Now I have decided to resume it while serving at the military. I really love watching Professor Malan's lectures:)

[@Senko1800]

YES , keep it up champ🏆

[@brandoncarris9015]

thank you for your service !!!

[@michaelphilps]

Don't kill innocent people.

[@davidestadilla8271]

Just want to add something on pointer arithmetic. If you had a pointer to an integer (which consists of 4 bytes) and allocate memory for two integers and for some reason, you want to put values in it using the dereference (*) symbol, you should not do the following: int *n = malloc(sizeof(int) * 2) *n = 1 *(n + 4) = 2 or *(n + sizeof(int)) = 2 Instead, you should do this: int *n = malloc(sizeof(int) * 2) *n = 1 *(n + 1) = 2 No need to type in the quantity of bytes for the specific data type (in this case, it's 4 since it's an integer). C already knows that.

[@a23oj28]

Is this the same as an array of ints but just dynamically allocated? Does C know because of the type (int) that n points to or does it infer from the argument of malloc()?

[@hmmmzaDev]

I was just watching the lecture 4 of CS50-2023x and during the break, I have stumbled onto this video. Not sure now if I should continue watching that one or start from here again.

[@harppsricprince]

Same thing with me. That’s exactly what I am thinking

[@user-yk1cw8im4h]

u r just into lecture 4… of course continue with this

[@Kampouse]

its same content only a news "version"

[@andoryu14]

the 2024 versions are almost exactly the same and the course itself now asks you to do the 2024 version, and carries over progress from 2023

[@jonnysegway7866]

1:09:55 could also write 'for (int i = 0, n = strlen(s) + 1; i < n; i++)' as per for malloc

[@ProudyShankalpa]

can't wait to solve the assignment with these new tools. thank you david and whole cs50 team

[@hawamigil4291]

What about tideman

[@TheIonizator]

@@hawamigil4291 pls no

[@kaunglin8447]

I love you David. You are such a wonderful teacher.

[@rida_brahim]

there is something guys about changing the string s to char *s, if you do this without the cs50.h you can't change the value of a char in a string, i was going back and forth with chatgpt until i tested it myself, even if professor david changed string s to char *s the actual string which is "Hi!" cannot be changed by trying to change char H like this : s[0] = 'h'; , you're probably asking yourself why and it's because the get_string function is returning an actual array that we store it's address in char *s and not the same thing when creating a string on the fly with the pointer, two different things, if you're creating a string on the fly without using [ ] to tell C it's an array you won't be able to change the characters in the string, I'm telling this to anyone got stuck with this asking himself this question, professor david probably didn't reveal this so things doesn't escalate quickly and I'm only 55min in so probably he's gonna reveal it later.

[@ethylmusic]

string literal vs string in array is the reason for this. the former is immutable

[@rida_brahim]

@@ethylmusic yeah, at the time it took me a while to get this while the compiler kept giving me errors then i found out it's read only and can't change it until i copy the string to an actual allocated memory and it seems that the get_string function actually does that.

[@user-sb5vt8iy5q]

In the file I/O section at the end, you're probably wondering, how did he call fwrite(...) in a loop, without it overwriting the first byte every time, shouldn't he have some sort of byte counter? I think they should consider mentioning this in cs50 2025 and onwards, essentially, the type "FILE* ", which is a file pointer, secretly holds a byte counter, so whenever you use fwrite, or fread, or similar, C will remember how many bytes you read or wrote, and it will keep track of it for you, just to clear up the confusion. TLDR: C keeps the number of bytes written/read to files for you, so you don't have to count, and that's why it works in loops, it automatically increments itself.

[@zeshenzheng]

Thanks！You answer my question.

[@user-sb5vt8iy5q]

@@zeshenzheng np!

[@thezaytunpeach]

That was a PHENOMENAL lecture but brother did it escalate...

[@xavier2586]

Thank you so much Professor Malan.

[@jonnysegway7866]

2:00:00 can also limit the scanf input, but means you have to statically declare the char array first, which may be limiting/wasteful of memory i.e. *_char c[100];_* then *_scanf("%99s", s);_*

[@oanhtuan2408]

Love it again ❤ Thank you, David! This lesson teaches me how we can efficiently open a file by using just its first address.

[@CườngNguyễnPhú-n2i]

what an energetic teacher. Thanks David

[@spicywe1ner]

Memory in C, Always the fun part.. Fantastic job as always.

[@fardinkhandaker3436]

I hate that I wasn't able to be there in person but thank you to those of you who were there and asked those great questions (some which were what I wanted to know about)!

[@csanders429]

In the Tideman Algorithm swapping is by reference instead of value. When you’re working array indices, you’re essentially working with the actual memory locations - the entire structure no need for pointers; not basic variables `int`… The array elements hold the data. Know the difference between [Passing by Value] and [Passing by Reference].

[@jonathanconnelly6993]

Must finish before the end of summer…. Must finish before the end of summer….. keep going …

[@victorbadia6709]

same bro we can do it!

[@illillillill]

how wonderful teaching skill

[@harizhakim5983]

the toughest lecture and problems for me so far!

[@The.Oh1183]

Please tell me I'm not the only one whose brain collapsed...

[@mohammadsuhaanhaq3590]

you are not my brain also got fried

[@MeowMuscleTv]

you not alone it diffucult for me too but don't give up !!

[@DizzyBusy]

No, I had to distract myself with the Euro Cup for a bit before coming back to this week's PSET

[@AdityaWilson]

Welcome to the gang

[@berkaycirak]

56:12, I have a question about comparison. While we comparing two string, we compare its first char's addresses. Also, while we comparing two integer, why didn't we compare its addresses? I thought, while comparing two equal integer, their addresses will be different. For example, int n = 5; int x=5; . There will be two different addresses for n and x, and if we compare addresses they must be different

[@sagarpanwar2845]

because in int we compare their values not their address, because in string your variable contain a address of the first char and in int your variable contain a integer not their address.

[@Cadeyrnyes]

The duck actually taught me a bit of this in the Substitution pset 😂

[@bpopphixo9627]

each video introduction: All right . This is CS50 LECTURE 3: ALGORITHMS

[@sulikns4152]

Yeah David you are great man! thanx a lot🙏🏻

[@CygnusX-11]

Brilliant teacher

[@GenZ_01_]

Pure gold. Thank you Cs50 team

[@brownie2006]

I LOVE CS50

[@josephlevin]

Love the lecures. Had to laugh at "mostly just an excuse to use the foam fingers."

[@jonnysegway7866]

1:00:00 my pc uses the same address for both! apparently the compiler knows the strings are the same and uses an optimisation technique known as 'string interning'!

[@virendxr]

21:17 so a pointer is simpy a varible which stores the memeory address of some other variable inside the variable called pointer . So do pointer variables* also have their own address? and can the adrees of pointer variables can be stored in another pointer variable?

[@andrewpolakaus]

Yes. Examples: // x is a char char x; // y is a pointer to a char that stores the address of x char *y = &x // z is a pointer to a pointer to a char that stores the address of y. char **z = &y

[@virendxr]

@@andrewpolakaus woh that's nice, thanks!

[@ZerubbabelT.]

@@andrewpolakaus Why double astrex on z?

[@andrewpolakaus]

​@@ZerubbabelT. that's a great question! If we declare "char x" then x is of type "char" i.e. x can store a char. Now iff we declare "char *y" then y is of type "pointer to a char" i.e. y can store the address of a char. And finally, if we declare "char **z" then z is of type "pointer to a pointer to a char" i.e. z can store the address of something that stores the address of a char. Each star add a level of indirection.

[@ZerubbabelT.]

@@andrewpolakaus Got it. Thanks

[@ilove0908]

Very useful lecture. Thanks a lot !!

[@riyajath_hmd]

I lost my streak of watching one lecture per day, but I convinced myself to resume watching from where I left off. Today, I finished watching lecture 4 and I am determined to complete CS50 by the first week of February. After that, I plan to solve the problem sets and then I am thinking of learning JavaScript next.

[@urosuros2072]

no one cares

[@davidoffonly]

​@@urosuros2072 hahahaha don't be that harsh man lol

[@riyajath_hmd]

@@urosuros2072 did I ever mentioned your name and said that, if you don't like it just leave dude

[@lovesickem3]

That's also what I'm doing! I try hard not to fall asleep lol

[@123_wowowowowo]

Doing the problem sets right after the lectures would be a better idea because your brain is still fresh with knowledge from the lecture.

[@sibusisomakema7718]

glad to see that the training wheel joke landed this year😂

[@AgrimaSingh-w9d]

29:42 opening the mailbox (pointer) and using * -- go to that location and show me what is there

[@_allegra]

1:34:17 Caleb missing the opportunity to down one of the glasses and demonstrate clearing the memory allocation

[@muhammadmx]

that would have not kept the original glasses swaped

[@KiraIsGod]

there is a hidden message on the raw file for the recover problem from the problem set :)

[@kmlime]

I miss the old intro 😢 but the course is amazing regardless

[@BaibhavDeka]

best week of C i bet

[@krish-ut9de]

i suddenly feel like a real programmer now

[@siraniwaysdamelon]

this was the final boss level for me

[@kenlow4048]

mind blowing

[@regularnick]

it's kinda sad that he didn't explain with the code how the CS50 get_string actually works. Unfortunately, for now this is the only training wheel I can't take off

[@clarerussell1]

It's explained in previous lectures. If you check out their library you will see how get_string works

[@JAVPindia0]

Did you mean playlist? When you say library or that's something different from that?​@@clarerussell1

[@DizzyBusy]

I second the previous comment. Just go to the library to see what get_string actually is

[@AuroraClair]

2:06:36 line 12 - shouldn't we have deregerenced the name and number? Or does the fact that we used %s return the dereferenced pointer already?

[@jonnysegway7866]

if I may, char * array (or any array) is an exception to that rule, no need to de-reference

[@Ahoora-o2v]

Hello, I have a question. I want to know whether Zig language can be better than Rust or not, and why Rust has been getting so much hate for some time now, and what is your opinion about Zig or Rust or C for creating very bad malware?

[@Lvbzzzz]

2:13:53 I even copied source code from cp.c and still have an error "segmentation fault (core dumped)". what`s wrong?

[@aopen130]

36:18 oh 32 bit max out at 2 billion addreses. So that's why the FAT32 doesn't go very high in partition like exFAT. Makes sense.

[@SyariefAzman]

im confused about the usage of star and ampersand. for example *s is a pointer pointing to the address in the memory . but &s pointing to the memory adress too? what is that ?

[@lookalivebrett]

Is it convention or preference to say "oh" when encountering a zero?

[@isekesmiyake3891]

I am a litle bit confuse here. So we create an variable called pointers that contains an adress where a value of an other variable is stored ok? But the pointer has also an adress rigth? for me, all of this seems to lead to an infinite spiral of adresses. May someone help me

[@shrutisinghal2490]

Thank you Sir, for amazing teaching😃 will be always thankful for giving us CS50

[@ares1647]

At 38:45, why put & before s[0] when it was not necessary in a line above and it still worked ? Also, printf format specified we're looking for an address with "%p".

[@shubhamshetty3918]

Correct, Both are the same & points to the same address, the first one just points to the first address as a whole while the other points out to the address of the first char in that array. Hope it gives more clarity.

[@nbtwall7287]

27:19 Professor Malan said the memory address 'p' is storing an integer. Are memory addresses signed? does the computer track negative spaces in memory?

[@shiny5176]

addresses are unsigned integers

[@yunleung2631]

Must review notes. I barely understood the lecture

[@OkuEfiong]

Please what kind of VScode is David using, because my VScode terminal is not working?

[@gatephysicsnptellivevideoi5495]

14:19 Does that mean each pixel consume 8-bit or 1 byte of memory?

[@nicovulpus]

@42:32 David says that *s is the address, but isn`t it actually a pointer to the first character in the character array?

[@Marco-sm9bu]

yes, &s is the address

[@adnancodesweb]

I listen to this to go to sleep.. This is where I go to when I drift away and start day dreaming 😆

[@tibetje226]

1:53:08 I started sweating when he said 'string input'.

[@proudathiest]

If the computer needs a pointer (s) to go to the address of the first char, how does it find s in memory to get to the char?

[@mosstah]

It stores and accesses the pointer in the same way it accesses any other variable, such as storing a char without bothering with any pointers. However, if the pointer was replaced with a simple char variable, the computer would only return that character and wouldn't know to look for the rest of the values in the string which is why the pointer is necessary.

[@proudathiest]

@@mosstah But how does it access any variable? If I write int x = 10, and then use x later in the code, how does it find that x is 10?

[@gabrielangel1996]

​​@@proudathiest for local memories, it stores on the stack part of the memory. The CPU store the last adress of that stack. The compiler only needs to remember how much you push the stack memory. Imagine a shelf, you put one book, then another on top, and so on. And you write how many books you stacked. Well, if you want the first book(variable) you just have to go down for how many books you put on top of it. The compiler does that for you. Hope you got what im saying

[@proudathiest]

@@gabrielangel1996 Thankyou! I've just spent an hour looking at stack pointers and it finally makes sense!

[@not_amanullah]

Understood++

[@not_amanullah]

Thanks ❤

[@davidc8413]

Professor Malin is an actual robot

[@zinmyolatt1386]

I also tried to copy (int) instead of string and I found that the original and the copy don't exist in the same location.Why is that? int s = get_int("s: "); int t = s; printf("%p ", &s); printf("%p ", &t);

[@jonnysegway7866]

you are not using pointers, everything is being determined statically at compiliation - those variables will be in the stack instead of at run time/dynamically in the heap

[@beningsurya7479]

"It turns out that understanding pointers is not as terrifying as in previous years.

[@ZerubbabelT.]

21:02 Why am I not getting hexadecimal number?Instead its decimal number

[@BonnieByte]

Very interesting

[@wecode007]

wanted to study at harvard; but failed; dont worry we have david

[@stormbringer6255]

int a = 9; int b = 0; printf("%i ", a); //Printing the value of a printf("%i ", b); //Printing the value of b printf("%p ", &a); //Printing the address of a printf("%p ", &b); //Printing the address of b printf("%d ", *(&a)); //Printing the value of a using the address of a printf("%d ", *(&b)); //Printing the value of b using the address of b int *c = &a; int *d = &b; printf("%p ", c); //Printing the value of c printf("%p ", d); //Printing the value of d printf("%d ", *c); //Printing the value of the address in c as a pointer value printf("%d ", *d); //Printing the value of the address in d as a pointer value

[@TheNothing93]

DONE

[@sinajps378]

i have a doubt . Why dont we use *s instead of s to print out the string

[@DizzyBusy]

That was explained in the "Strings" chapter of the video

[@tibetje226]

1:31:34: 'ye sharing, whatever' LOL

[@codeandtech77]

I don't really get what the function "malloc ()" really does. Anyone who understands this concept please help me to understand it too.

[@hritviksoni6080]

okay ! ill try to explain it, simply put , malloc is a function defined in header file , simply like printf is defined in and get_int defined in , its job is to allocate some space from your memory for you to do something on it for example store some data. Consider these statements 1. char *s = get_string("s: "); 2. char *t = malloc(string length (s) + 1); --> 1. gets string input from user and stores it in s --> 2. allocates memory of string length+1 (+1 to store \0 which signifies the end of a string ) for you to do something in the allocated memory from your storage.

[@codeandtech77]

@@hritviksoni6080 Thanks a lot! Appreciate your kindness. I kind of didn't understand this line: " gets string input from user and stores it in s --> 2" .

[@RobertMooney-ut7cc]

Muito obrigado. / Merci beaucoup.

[@100drips]

after 3 weeks of trying to solve tidemen, i can finally watch this lecture :D

[@RameezRassel]

hey mah i spend like 16 hours on it and i gave up . i am not able to do the last 2 functions. i dont even have an idea on how to do it. You got any advise for me?

[@100drips]

@@RameezRassel 16 hours is nothing, haha 😅 Here was my idea, that finally worked: To detect a cycle, look, if the loser of the pair you are trying to lock is a winner of an already locked pair. If so, is the winner of the pair you are trying to lock the loser in the locked pair? When this is the case, you've detected a very simple cycle. For example, when A is winner over B in the first locked pair and you try to lock in B over A. This is obvious and fairly simple. But if the winner of the pair you are trying to lock is not the loser, you have to check the next pair down the line. So in this case, look if the loser of the first locked pair is the winner of another locked pair. If so, does the loser of this pair equal the winner of the pair you are initially trying to lock in? If yes, a cycle is detected. If not, look for the next locked pair. Hint: a recursive function is very useful for this. I actually did a few exercises with recursion beforehand to really wrap my head around the concept. Good luck, you can do it!

[@RameezRassel]

@@100drips hey thanks a lot mahn. Really Appreciate it. I was doing my research and it turns out there is an algorithm for traversing throughout graphs. It's called DFS I tried learning it but couldn't wrap my head around it . I think I understand ur logic now I have to code it. We'll see

[@thebrightchannel6898]

@@RameezRassel Bro were you able to code it?

[@dxdx7961]

hello I have a question 43:00 char *s and char s[] is the same ?

[@sein349]

char *s points at the first (0th) location of the string. Which means the string will be read from that location till the null (\0) to highlight the whole string. Meanwhile char s[] highlights the whole string (list of chars) right away. hope this helped

[@NKLGaming01]

I love youuu

[@gamerdenver7811]

thx

[@nicovulpus]

How is p an integer when it contains alphabetical values? Like 0x123 has 'x' in it`s value?

[@ninjamar_]

0x123 is hexadecimal, which is a type of notation for numbers

[@mosstah]

@@ninjamar_while this is true for the “123” part, the 0x isn’t a part of the value and is just standard convention for indicating that the following number is hexadecimal

[@ninjamar_]

@@mosstah yeah, it helps distinguish between decimal

[@licnex]

this entire week has just been a sequence of "huh?"s

[@murrigo]

1:43 "or a jif or a ping"... David... my stars I almost stopped watching

[@ismailhossain964]

Whenever I play this chapter I feel sleep. 😢

[@licnex]

phase 1) pointers huh phase 2) I feel dead inside

[@baker7120]

i like =10 pushups

[@PedroAugustoMarquesFontoura]

01:46:00

[@abdulmelik8337]

Questions?

[@shinigamisenpai-_-]

Did anybody get the joke at the end??? Please tell me I wanna knowwe