CODEWITHSUNDEEP
arrayscfwrite
DrBwts
DrBwts

Reputation: 3657

Most efficient way to split an array to write to different files

I'm reading double data in from a DAQ that has 2 channels. Data is stored in read_buf, the first 1000 samples are channel 0 and the second 1000 are channel 1. I have no control over this concatenation of channel data.

I have set up 2 files like so,

FILE *fptr_0;
if ((fptr_0 = fopen("channel_0.bin", "wb")) == NULL)
{
    printf("Error opening file.\n");
    exit(1);
}

FILE *fptr_1;
if ((fptr_1 = fopen("channel_1.bin", "wb")) == NULL)
{
    printf("Error opening file.\n");
    exit(1);
}

I would then like to split read_buf and send the first half to fptr_0 and the second to fptr_1. I can read out the first half but am flummoxed on how to read out the second half. How do I point fptr_1 to just the second half of read_buf?

Do I have to copy each half into a new array?

What I have so far which works for the first half followed by what I'm not getting,

double read_buf[2000];
result = DAQ_func(device, &status, read_buf);
fwrite(&read_buf, sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_0);
fwrite( ??? , sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_1);

Upvotes: 0

Views: 92

Answers (2)

Madagascar
Madagascar

Reputation: 7345

How do I point fptr_1 to just the second half of read_buf?

You do not want to point the FILE * to anything but the stream associated with the file.

fwrite( ??? , sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_1);

Adding 1000 to read_buf will increment it by the sizeof it's contents.

So that gives us:

fwrite (read_buf + 1000, sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_1);

Or:

fwrite (&read_buf[1000], sizeof(double), (sizeof(read_buf) / sizeof(read_buf[0])) / 2, fptr_1);

where &read_buf[1000] will yield a pointer to the 1001th element of the array.

Is this the most efficient way? Perhaps not, but it's correct.

The return value of fwrite() should be checked too:

The fwrite() function shall return the number of elements successfully written, which may be less than nitems if a write error is encountered. If size or nitems is 0, fwrite() shall return 0 and the state of the stream remains unchanged. Otherwise, if a write error occurs, the error indicator for the stream shall be set, and errno shall be set to indicate the error.

Upvotes: 1

i486
i486

Reputation: 6573

??? is read_buf + 1000 (no &).

fwrite(read_buf + 1000, sizeof (double), (sizeof read_buf / sizeof read_buf[0]) / 2, fptr_1);

Or if you prefer &read_buf[1000] which is the same.

Upvotes: 2

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