(double) for-loop with variable indexing in C - c

Question

I have a buffer with IDs which looks like this
InBuffer={ID1,ID2,ID3,...}
I need to iterate through, every time using the ID in a function that returns pointers to the Data assigned to this ID and the size of the Data. I then need to fill in another buffer with the result which is of the form
OutBuffer={ID1,SIZE1,DATA1.WORD1,...,DATA1.WORDSIZE1,
ID2,SIZE2,DATA2.WORD1,...,DATA2.WORDSIZE2,
...,
IDN,SIZEN,DATAN.WORD1,...,DATAN.WORDSIZEN
}
I am having problems with forming the whole for-loop for this and the indexing of it, mainly because each SIZE variable can be different. It should be simple but I can't seem to make it work.
Thanks in advance for any help.
// For example
// Iterate through the remaining of the Request Buffer (m=0,1 already set)
for (m = 2; m < InBuffer; m++)`
{
OutBuffer[m] = InBuffer[m];
returnPointersToDataAndSizeFunction(InBuffer[m], &SIZE, &DATA);
OutBuffer[m + 1] = SIZE; // e.g. SIZE = 2, therefore DATA has 2 fields
OutBuffer[m + 2] = DATA.1; // first field
OutBuffer[m + 3] = DATA.2; // second field
// and so on
}

Answer 1

The first thing I notice is that you're using m to index both buffers:
for (m = 2; m < InBuffer; m++)
{
OutBuffer[m]=InBuffer[m];
but then you're using offsets from m for the additional data in OutBuffer:
OutBuffer[m+1]=SIZE;
OutBuffer[m+2]=DATA;
So, what you do think is going to happen in the next iteration of the loop? Say you go through the loop the first time, so that m is 2. The next time, it's m++, i.e. 3, and you make this assignment again:
OutBuffer[m]=InBuffer[m];
But you already assigned something at m[3], and that's the SIZE value from the previous iteration. You also assigned DATA at m[4], and that's going to be overwritten by the SIZE value in this iteration. Eventually, you'll end up with OutBuffer containing exactly what's in InBuffer, plus the SIZE and DATA values for the very last ID.
You need to use a different variable to index OutBuffer, something like:
for (m = 2, n = m; m < InBuffer; m++) {
OutBuffer[n++] = InBuffer[m];
returnPointersToDataAndSizeFunction(InBuffer[m],&SIZE,&DATA);
OutBuffer[n++] = SIZE;
OutBuffer[n++] = DATA;
}
There are some other problems as well. For example, the condition in the for loop shouldn't compare m to InBuffer, but should instead compare m and the number of entries in InBuffer. But just straightening out your indexing should be a big step forward.
Update: I just noticed that the data for each ID is larger than just one field. You'll need another loop inside the first one, then, so that you end up with something like this:
for (m = 2, n = m; m < InBuffer; m++) {
OutBuffer[n++] = InBuffer[m];
returnPointersToDataAndSizeFunction(InBuffer[m],&SIZE,&DATA);
OutBuffer[n++] = SIZE;
for (i = 0; i < SIZE; i++) {
OutBuffer[n++] = DATA[i];
}
}
If DATA is a structure with fields rather than an array, then you may need a series of if statements to check whether each field should be included or not. You can't use the value of a variable like i as the name of a field, i.e. you can't say DATA.i where i is a variable. I don't think a C structure can have field names that are numbers -- identifiers generally have to start with a letter or underscore, so trying to do that won't make much sense anyway. If you have control over the type of DATA, you should make it an array instead of a structure. So your loop would look more like this:
for (m = 2, n = m; m < InBuffer; m++) {
OutBuffer[n++] = InBuffer[m];
returnPointersToDataAndSizeFunction(InBuffer[m],&SIZE,&DATA);
OutBuffer[n++] = SIZE;
i = 0;
if (i++ < SIZE) { OutBuffer[n++] = DATA.field1; }
if (i++ < SIZE) { OutBuffer[n++] = DATA.field2; }
// and so on for each field in DATA's type
}

Answer 2

As Caleb pointed out you should use one variable for each array.
If you're saying that DATA can contain more than one element then you should increase the variable for outBuffer by SIZE each iteration. Also use a loop to asign DATAs fields to OutBuffer
int n = XXX; // set n to the first element you need to assign an ID to
for (m = 2; m < ElementsInBuffer; m++)
{
OutBuffer[n] = InBuffer[m];
returnPointersToDataAndSizeFunction(InBuffer[m],&SIZE,&DATA);
OutBuffer[n + 1] = SIZE;
for (int i = 0; i < SIZE; i++)
{
OutBuffer[n + 2 + i] = DATA[i]; // works for array only see Calebs answer to see how it works for structs
}
n += SIZE + 1; // +1 to also skip the field for SIZE
}