Thread: sizeof(<something>) --> WHAT DOES IT EXACTLY MEAN?

Hi all,
let's say that we have a program. In that program you just put characters and then push ENTER - for example "PUT character, ENTER , PUT character, ENTER, PUT character, ENTER, etc...". And after each ENTER, the program puts the pressed character into a "STRING".

So if my inputs were:

Code:

'f' ENTER 'y' ENTER 'a' ENTER 'k' ENTER

the output STRING would be:

Code:

"fyak"

AND HOW TO DO IT? --> I wanted to do it by creating a "STRING of CHAR" (or ARRAY of CHAR) and then make for it a "DYNAMIC ALLOCATION" of memory (but I don't know WHICH CODE I should use to dynamic allocation - it's something with MALLOC(), but I don't know what exactly...).

AND NOW MY QUESTION IS: --> It is possible to create an ARRAY that will be changing its size DURING the program running according to the situation (because before the starting of the program I don't know HOW BIG array I should create, because I don't know how many CHARACTERS I will add to the string)?



---> AND NOW TO MY MAIN PROBLEM: <---
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I have read that the function (for example for the type INT) "sizeof(int)" will return to me some number --> and that number should be the "SIZE OF INTEGER" --> AND THIS TERM I DON'T UNDERSTAND.
I understand that the concrete NUMBER (or character), for example the number "6369" needs in program some amount of memory (for example "20 BYTES", ...I know, it's probably a wrong number but it doesn't matter), but HOW BIG MEMORY DOES THE TYPE "INT" (or "char" or "real") NEED??? --> it's JUST ONLY a TYPE, and NOTHING CONCRETE (like '6369').
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So if someone could explain to me, what does this mean:

Code:

sizeof(<something>)

I have studied "C" just one semester at university, so I would appreciate simple answers --> something that I will be able to imagine.
But thanks very much for every answer.

Petike.

Yes, dynamic memory allocation "has something to do with malloc()". Technically, you won't have an array, but a pointer to memory; once you know how much memory you need, you ask for it and put its address in the pointer.

sizeof(int), for instance, gives you the number of bytes of memory that are reserved for you when you declare an int variable. On many systems nowadays, sizeof(int)=4, which means that when you say int i; the compiler reserves four bytes for you to put an integer in. (If the integer can't be represented in four bytes, you are plain out of luck.) All variables of the same type get the same amount of space.

thanks for the answers,

so if i have the array:
int arr[50] --> the size of it will be "4 * 50 == 200" - so I should allocate at least 200 bytes, thus:

Code:

arr = sizeof(sizeof(int) * 50)

- this I understand because I know how many elements are in the array.



But what if don't know how many elements will be in the array (just the very first example in my first post) - if it depends on the USER? --> I should allocate at least:

Code:

arr = sizeof(sizeof(char) * ???)

How should I solute this?

Originally Posted by laserlight

Read up to that size. If there is more input, realloc() (or something along those lines) for a bigger size, then read more. Keep on doing this until there is no more input.

Yes I have heard about this progress and I probably don't have any troubles with this steps.
I would just like to know it there is some other method, something other than always memory reallocating?


Thanks anyway.

Petike.

OK,
thank you very much for all your answers.
I am a bit more clever now.

Petike.

you could find out how much memory you need beforehand, and then allocate that.

I have read that the function (for example for the type INT) "sizeof(int)" will return to me some number ...

sizeof() is an operator and not a function.