File Pointers - CS50 Shorts

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CS50 SHOP
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David J. Malan
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[@JoeHartshorn]

fopen: 3:04 fclose 5:17 fgetc: 6:05 fputc: 9:35 fread: 11:24 fwrite: 15:20 @CS50 timestamps per topic on each of the shorts and walk-throughs, like with the lectures, would be great :)

[@AlqantDBlank]

hero

[@danivanon]

e·phem·er·al - lasting for a very short time 0:08

[@Balidor]

Yes please, it would be very usefull as we always come back to them :D

[@asmgutinho]

Thanks a lot man!!!!

[@labradoryshockdart9734]

Doug Lloyd is the Superman of CS50. He explained things very well that I don't understand from the lectures!

[@sefonefinpsefnpefn4191]

he is a legend! his explanation is so clear!

[@TobiasR-ig3pb]

They are all legends

[@el_kraken]

Love these CS50 shorts. Doug has an incredible talent for teaching.

[@jjhj_]

I'm really grateful for these free, incredibly high-quality online courses. Harvard's my hero for 2020

[@merry1195]

2023 too

[@user-ex5pg3oy9c]

2025 here

[@shaanssonnets430]

@@user-ex5pg3oy9c 😂

[@krys6544]

I cant tell you how much these videos have helped me 😭😭😭😭 THANK YOU SO MUCH

[@jayant9151]

Krystal Danielle why crying

[@wol2231]

@@jayant9151 It's called tears of joy.

[@seal0118]

bruh these videos actually help

[@wade3696]

This is super informative. You summed up so much in such a short time.

[@Firnila]

Doug, you wonderful human being.

[@dinoman6481]

This is really helpful for understanding the recover problem in pset4

[@lunarskiy]

Thanks, Doug! Amazing explanation!😎

[@colonizatorgg5960]

Nice tutorial man, very straightforward

[@florentjung5799]

Doug you are my hero !

[@coffeeisthepathtovictory1290]

Definition of Ephemeral: lasting for a very short time via google

[@Mrkichanpark]

finals is 2 days, this video is a life saver

[@TornikeBatavani]

It is possible to use r+, w+ or a+ modes of fopen as well. They open a file for reading and writing, or reading and appending.

[@OK-ri8eu]

Thank you so much, I was in need for that little bit of information regarding fread, fwrite which you've helped me acquire.

[@terra_h4429]

I legendarily love Doug.

[@calintatar]

You gotta love Doug's T-shirts though

[@joncolumbino744]

this guy is a god

[@conraduouou]

it's my 5th time to watch this video and I can't seem to wrap my head around recover

[@morpy3077]

bruh same

[@matnreyes]

Same, I've no idea how these work

[@bencooper2753]

@@matnreyes Same

[@ashish2099]

At first I too was confused about what those ptr2 thing was, but turned out it was just a pointer to an address. fread reads info from that pointer block (the file handle we opened as "r") to buffer (simply said the variable we have declared) and fwrite takes info from buffer to pointer.

[@liriocoentro]

Thankfully just problem set 7 was changed, so I'll be able to finish this course this year. I started in june 2020. Good luck to you all, fellow coders.

[@mehdiahmed7836]

Amazing explanation , thanks doug ❤️

[@fidalgoverde]

Excellent details and explanation

[@annamaniuk6265]

The following code is incorrect: char ch; while ((ch = fgetc(file)) != EOF) printf("%c", ch); If we assume EOF

[@ryukthegodofdeath8063]

You are right it needs to be int instead of char.

[@huanghowie4570]

totally right thanks bro

[@kjpg2005]

Indeed, char did not work correctly. It only copies part or none of the source image.

[@xrafter]

Yes man you are right thanks for the knowledge

[@Drifting_Turtle]

According to the authors of the C Programing Language (Brian W. Kernighan, Dennis M. Ritchie): "We use int for a subtle but important reason. The problem is distinguishing the end of the input from valid data. The solution is that getchar returns a distinctive value when there is no more input, a value that cannot be confused with any real character. This value is called EOF, for "end of file". We must declare c to be a type big enough to hold any value that getchar returns. We can't use char since c must be big enough to hold EOF in addition to any possible char. Therefore we use int" (The C Programming Language, 2nd edition, p. 16).

[@ybleon7131]

great explanation! Thank you!

[@ejif2]

Мне очень нравится, как доносит информацию Doug Lloyd. Интересно, понятно, плюс примеры Хорошего кода. Спасибо!!!

[@sashabachynskyi8931]

Закончили ли вы курс cs50?

[@ejif2]

@@sashabachynskyi8931 Здравствуйте) Стыдно признаться, завис на JavaScript. Уже два месяца собираюсь добить)) Есть оправдание - я закончил оффлайн курсы по JavaScript и работаю fullstack developer'ом на React + Node.js ))

[@aquere]

@@ejif2Можете дать телеграм? Интересно узнать как у вас так вышло устроиться фуллстаком за такое короткое время с начала прохождения курса cs50)

[@ejif2]

@@aquere Я не показатель. Я начинал в 1993году, потом бросал на 20 лет. Случились в жизни изменения, посмотрел, что не сделал за прошлую часть жизни, решил "добить" начатое. Начал с CS50, потом поступил на оффлайн курсы по JavaScript. Сейчас работаю JavaScript программистом.

[@juliabeseda1088]

А я наоборот смотрела и думала о том, как же скучно он рассказывает..)) Когда лекции ведет Дэвид, оторваться невозможно, а вот с Дагом я больше двух минут не могу удержать концентрацию, приходится ставить на паузу и делать перерыв)

[@Maen963]

Thank you so much this is so useful and simple 🌹

[@andreamacias1533]

we love you Doug!

[@amosp9120]

Thank you Doug for another informative short. But one thing in this video has me confused and I need a clarification from someone with a better understanding. The statement made at the beginning of the video: "File pointers and pointers, while interrelated, are not exactly the same thing." What is being referred to as a 'file pointer' here? Is it the structure, i.e. FILE or the pointer to the structure i.e. FILE*? If it is the latter, isn't it the exact same thing as a pointer? If it is the former, then why in this case alone we've deviating from the convention used in CS50 to place the * closer to the data type and not the variable name? I've read up on the topic and I don't think I have a clear understanding yet, but... as far as I have seen, the FILE structure stores the data about the file, its position in memory on the disk and information for read/write operations such as the point up to which read/write has taken place and so on. Because it points to the file [even if the pointing isn't exactly of the same nature of a regular pointer], this struct is called a 'file pointer'. Our usage of that 'file pointer' i.e. the struct FILE always includes a pointer to that struct, i.e. FILE* because we will be passing it to functions, and because we need to pass by reference and not by value it is important to use 'file pointers' with a pointer to 'file pointers'. Now, I'm not sure if I have grasped this concept. If someone could clarify, I'd be grateful.

[@omergulcu]

It seems like you understood it quite well. File pointers (for example FILE *input) are pointers, they just point to files and somehow also store stuff like what to you opened the file dor (w, r) and where you are in the file. They are pointers themselves and they are exactly what functions like fread and fwrite need, I don't think there's need to complicate this further.

[@zchforward]

This part is crazy hard

[@reebakhan2685]

Thanks, it is just great.

[@GOODBOY-vt1cf]

thank you so much

[@marekpumex]

Thank You :)

[@not_amanullah]

Thanks

[@iamabutalhakhalil]

hats off

[@BaturCihan]

thank u!

[@tranthetam]

13:51 bookmark fread() example

[@xrafter]

But that loop don't move the pointer?

[@DogginsFroggins]

Honestly how are you supposed to do lab 4 without knowing this? I feel they over-estimate our ability to read documentation, but then again this makes me nervous for documentation in general.

[@tibigraz]

This video is part of Week 4 but you may overlooked, there's a "shorts" page BEFORE problem sets every week and you need to watch all of them in order to get green arrows for that week.

[@coltonfox8689]

10:50 If ptr2 already contained data, wouldn't fputc(ch, ptr2) append ch to the end of ptr2 vs replace? I thought the Linux cp command overwrites whatever you copy something to

[@PausBastianus]

Only if ptr2 was opened using fopen(file2.txt, a). If it was opened using w, it would overwrite any existing data. Check 4:28 . There he opens file2.txt with the w operator.

[@xrafter]

@@PausBastianus Yeah i read about file pointers they point to the start of the file whatever the file is but if you add append it always will start at the end

[@kaliomar5988]

great explanation thanks,

[@sarahmedouni8844]

i made a file with fopen then wanted to print the contents into the terminal with fgetc and it didn't return anything, anyone know why?

[@sarahmedouni8844]

it was because i was using "w" for write 😅

[@nilaksh007]

9:08 this code is not working

[@Drifting_Turtle]

See Anna Maniuk comment bellow explaining this issue

[@henny1986]

Is fread() like fgetc() in that it reads the NEXT block of bytes?

[@xrafter]

No fread read n bytes from keyboard fgetc only read on byte

[@xrafter]

What is the difference between syscalls functions and this? Is it the same ?

[@joaquin8118]

so in 12:38 it says that what you read is stored in arr. my question is, in what part of the array the reading is stored? is it in arr[0]? is it in arr[9]? how do i know where it is?

[@stevefranzreb856]

I'm still learning this stuff (why I'm here...), but I think this is correct. In "fread(arr, sizeof(int), 10, ptr)", "arr" is the pointer that points to the array "arr[10]" that was declared in "int arr[10]". The pointer of an array is always the 0th element of the array. When we declared it, we said it will be an array of integers. We are fread()ing a quantity of 10 chunks of memory that are sizeof(int) bytes. That tells me that, from the file we are fread()ing, we are pulling 10 ints, thus filling the array "arr[10]". So, the first chunk of "sizeof(int)" (4) bytes, goes in arr[0], the second chunk goes in arr[1], etc.

[@xrafter]

Heh every one knows that the header file is just a declerations and macros. The definitions are in binary files

[@honestlee4532]

The cat.c file mentioned in the middle of this video has a BAD choice for a filename! If you make a file called cat.c and then make the file, you end up with an executable called 'cat.' A much better name would be something that does NOT match the name of an already existing command. So something like 'mycat' , 'ccat', or 'cats' would be much better. Also, if you look at the code... char ch; while ((ch = fgetc(ptr)) != EOF) printf("c", ch); The above code is INCOMPLETE. This is exactly why you DON'T want a command with the same name of one that already works. The code mentions 'ptr' which has NOT been assigned according to this little bit of code. Making cp.c is ALSO a BAD idea. Using the name of a Linux command on a Linux system is a POOR choice and using the name of a Windows command on a Windows computer is also a BAD idea.

[@xrafter]

Is double even valid for reading integers?

[@drenhyseni6633]

I don't think cp was a good name for a comand :p

[@Ss-zg3yj]

EOF is not a character