the size of two dimensional array in C

Question

I have the following two dimensional array:

#define ROW 100
#define LINE 50

int a[ROW][LINE];

but how to get the sizeof array for the last 45 rows, for example a[55][0] to a[99][99]?

can we do something like sizeof(&a[55])?

The 45 last rows would be a[55][0] to a[ROW - 1][LINE - 1]. — Some programmer dude
– Some programmer dude, Commented Jun 24, 2013 at 20:24
As Hunter said, but you generally want to avoid fixed sized arrays like that, because you are hardcoding the buffer size and at some point you will want to store more... — cmaster - reinstate monica
– cmaster - reinstate monica, Commented Jun 24, 2013 at 20:25
can we use pointer to get the size, this can avoid the fixed size arrays — user2131316
– user2131316, Commented Jun 24, 2013 at 20:27
Using the pointer to get the size doesn't really make sense -- sizeof(int *) is fixed (generally 4 or 8 bytes, depending on arch, but this is not guaranteed), regardless of how much memory is allocated at the place your pointer is pointing to. — CmdrMoozy
– CmdrMoozy, Commented Jun 24, 2013 at 20:30

AnT stands with Russia · Accepted Answer · 2013-06-24 23:49:05Z

7

sizeof of an array is guaranteed to be equal to the sizeof of a single element multiplied by the number of elements. So, if you want to know how much memory is occupied by a specific number of rows, just calculate the number of elements and multiply by element size. 45 rows would require

45 * LINE * sizeof a[0][0]

Or, alternatively

45 * sizeof a[0]

It doesn't really matter whether these are last 45 rows. All rows are the same.

edited Jun 24, 2013 at 23:49

answered Jun 24, 2013 at 20:29

AnT stands with Russia

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Some programmer dude · Accepted Answer · 2013-06-24 20:41:42Z

1

If you want just the last lines, from line 55 and forward, how about

(ROW - 55) * LINE * sizeof(int)

If you just want a generic N number of lines, then how about

N * LINE * sizeof(int)

edited Jun 24, 2013 at 20:41

answered Jun 24, 2013 at 20:23

Some programmer dude

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5 Comments

Some programmer dude Over a year ago

@user2131316 As soon as an array decays into a pointer, all you have is the pointer, and pointers don't have information (like size) about the data it points to.

Some programmer dude Over a year ago

@kotlomoy You're correct, I always mix the expression and type sizeof operators. That's why I usually always use the variant with parentheses.

Keith Thompson Over a year ago

@kotlomoy: The syntax is sizeof expr or sizeof ( type-name ). The expr can be a parenthesized expression. It's the same in C89/C90, C99, and C11.

kotlomoy Over a year ago

@KeithThompson My comment was for error in this answer. Error is fixed now. And I agree with Joachim - the best way is to always use parentheses. Thansk for explanation though - I wasn't sure about C99/C11 syntax. (Yes, I work in Visual Studio)

Keith Thompson Over a year ago

@kotlomoy: Personally, I usually apply sizeof to expressions (specifically object names), not to types, and I usually don't add parentheses unless it's necessary. Parentheses make it look too much like a function call, when in fact it's a unary operator. But as with most style questions, there's no One True Way.

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