runtime, compile time...

kai85 · #1 18-May-2005, 23:39

can someone please explain the difference between run time and compile time for me ( in an easy understandable way.)

dsmith · #2 19-May-2005, 07:59

Hello kai85. I am not sure of the official or school definition, but I can tell you the meaning that it has for me.

When you declare a variable, you have the option of allocating memory for that variable at compile time or at run time. For example, let's take an array. Suppose that you want work with 2-d matrices. So in your program, you would define:

CPP / C++ / C Code:

int matrix[100][100];

That would give you a 100x100 matrix of integers and it can not be changed unless you change your program and recompile it. This is known as compile time memory allocation. Right or wrong, I ussually refer to this as a static definition. Beyond, that it can't be changed, there is also some differences of how & where the memory is allocated and in addition it can not be "free'd" until the program is over.

Now, for the next example:

CPP / C++ / C Code:

int** matrix;

printf("How big would you like your matrix? ");
//get the number the using something like fgets & atoi
matrix = (int**) malloc( sizeof(int) * number * number);

This is run-time memory allocation. In C, you would use the malloc command and in C++ you would use the new command. Again, right or wrong, I call this dynamic memory allocation. I can make this any size I want, delete it, resize it, etc. all during program execution without recompiling. Thus it is run-time memory allocation.

There may be more to it than that, but that is my basic understanding.

HTH,
d

kai85 · #3 19-May-2005, 23:05

Quote:

Originally Posted by dsmith

Hello kai85. I am not sure of the official or school definition, but I can tell you the meaning that it has for me.

When you declare a variable, you have the option of allocating memory for that variable at compile time or at run time. For example, let's take an array. Suppose that you want work with 2-d matrices. So in your program, you would define:

CPP / C++ / C Code:

int matrix[100][100];

That would give you a 100x100 matrix of integers and it can not be changed unless you change your program and recompile it. This is known as compile time memory allocation. Right or wrong, I ussually refer to this as a static definition. Beyond, that it can't be changed, there is also some differences of how & where the memory is allocated and in addition it can not be "free'd" until the program is over.

Now, for the next example:

CPP / C++ / C Code:

int** matrix;

printf("How big would you like your matrix? ");
//get the number the using something like fgets & atoi
matrix = (int**) malloc( sizeof(int) * number * number);

This is run-time memory allocation. In C, you would use the malloc command and in C++ you would use the new command. Again, right or wrong, I call this dynamic memory allocation. I can make this any size I want, delete it, resize it, etc. all during program execution without recompiling. Thus it is run-time memory allocation.

There may be more to it than that, but that is my basic understanding.

HTH,
d

yea , thats what it thought too, just wanted to be sure.
thanks.

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