 |
 |
| View previous topic :: View next topic |
| Author |
Message |
yadobaka
Joined: 25 Feb 2007
Posts: 4
|
| Filling an array without loops |
 Posted: Mon Mar 12, 2007 5:09 pm |
 |
|
Hello. I was wondering if anyone knows a way I can fill an array multiple times without looping through all of the elements.
Basically, what I would like to do is this:
| Code: | int8 mode_data[8] = { 0, 2, 2, 1,5 , 2, 2, 5 };
int8 mode_data[8] = { 2, 3, 1, 4, 2, 6, 4, 7 }; |
But you only declare an array like that once.
Is there any way I can change all of the data in an array at the same time with one command, rather than building a special function fill_array() or something?
Thanks! |
|
 |
Guest
|
|
| Posted: Mon Mar 12, 2007 6:19 pm |
|
|
Well, you could create several arrays and use a pointer to switch among them. But you said you don't want to declare more arrays. It would be the fastest, though.
Look up memcpy() in the compiler manual. This lets you copy a block of data. Thing is, even if you don't have to use a loop, the compiler will use a library routine uses a loop anyway.
Depends what optimization or tradeoff you're really trying to get with this. |
|
|
PCM programmer
Joined: 06 Sep 2003
Posts: 21708
|
|
|
yadobaka
Joined: 25 Feb 2007
Posts: 4
|
|
| Posted: Mon Mar 12, 2007 11:54 pm |
|
|
Thanks guys. memcpy worked great. What I ended up doing was declaring the all the data arrays, and then i declared one more empty array the same size as the data arrays. I then used memcpy as you guys suggested. The reason I was looking to do this:
I have a for loop that adds up different data depending on certain cases. I wanted to use a switch, but I didn't want each case to have its own for loop.
all systems go! |
|
|
barryg
Joined: 04 Dec 2006
Posts: 41
|
|
| Posted: Wed Mar 14, 2007 1:15 pm |
|
|
This code will switch arrays using pointers.
| Code: | /* Pointer-to-arrays demo */
#include <16f877.h>
int test_odd[] = { 1, 3, 5, 7, 9 } ; // Define 3 similar arrays
int test_even[] = { 0, 2, 4, 6, 8 } ;
int test_pwr2[] = { 1, 2, 4, 8, 16} ;
void verify(int pp[]) // A function that uses a 5 element array
{
int i;
int a;
for(i=0; i<5; i++) {
a = pp[i];
a += pp[i]; // Put a breakpoint here to look at value of a
a=0; // BTW this code is nonsense
}
}
int main(void)
{
int i;
int * params[];
params = &test_odd[0]; // proper way to specify address of array start
verify(params);
params = test_even; // in C, array name is passed as its address,
verify(params); // so this works, too.
params = test_pwr2;
verify(params); // runs same test on each array
} |
You would only need to do this if you had to eliminate the time spent copying the array. The reason I post this now is that I find the best time to learn about something is when you are right there in a concrete example where you could use it. It may help you or someone else. |
|
|
barryg
Joined: 04 Dec 2006
Posts: 41
|
|
| Posted: Wed Mar 14, 2007 1:23 pm |
|
|
And here is a cut-down version of the same thing. I think it's clearer if you start from the first example, and then see that this works, too. This example eliminates the extra pointer variable which isn't necessary, but helps understanding the mechanism.
| Code: | /* Pointer-to-arrays demo */
#include <16f877.h>
int test_odd[] = { 1, 3, 5, 7, 9 } ; // Define 3 similar arrays
int test_even[] = { 0, 2, 4, 6, 8 } ;
int test_pwr2[] = { 1, 2, 4, 8, 16} ;
void verify(int params[]) // A function that uses a 5 element array
{
int i;
int a;
for(i=0; i<5; i++) {
a = params[i];
a += params[i]; // Put a breakpoint here to look at value of a
a=0;
}
}
int main(void)
{
int i;
verify(test_odd);
verify(test_even); // run test again
verify(test_pwr2);
} |
I ran these using MPLAB-SIM to verify that they compile and work. In order to get a look at the value of "a" in the function, you have to take out the "int a" declaration and put it outside the function to make it global. Maybe the CCS debugger is smarter 
The mechanism is not limited to fixed length arrays, but as you get more clever you also risk stomping your memory, so you have to be careful.
Barry |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
