Thread: 2-dimensional array & pointers

Hi guys, I'm pretty much a C newbie, but I've dabbled in other languages over the years. For my current project I'm using mikroElektronica Pro C on a PIC microcontroller.

I've got a 2-dimensional array defined like this:

Code:

const char FONTTABLE[96][5]= 
{ 
{0x00,0x00,0x00,0x00,0x00}, 
{0x00,0x00,0x5f,0x00,0x00}, 
{0x00,0x07,0x00,0x07,0x00}, 
/** [90 rows snipped] **/ 
{0x00,0x41,0x36,0x08,0x00}, 
{0x10,0x08,0x08,0x10,0x08}, 
{0x78,0x46,0x41,0x46,0x78} 
};

I want to select a given row from this using the index (0-95), then pass that element (a 5 element array) to a routine that will process each byte for the size of the row (5 in this case). I'm pretty sure I need to use a pointer, but no matter what I do I get compile errors.

Any tips?

Let's say you wanna select the 4th row
char *p = FONTTABLE[3][0];

p identifies the pointer to the first element of the 4th row
p+1 = p + sizeof(char) identifies the pointer to the second element of the 4th row
...
p+4 identifies the pointer to the fifth element of the 4th row

*p identifies the first element of the 4th row
*(p+1) identifies the second element of the 4th row
and so on

i hope it helped you

Interesting: one answer with pointers, one without

Perhaps I should have said, I'm happy about extracting the row, just not sure about getting a handle on it. I think I'm beginning to understand, but I'm not sure about:

Code:

p+1 = p + sizeof(char)

I think I will need to use sizeOf, because the same routine is going to process char arrays of different sizes - the above is a sample of one of them.

Originally Posted by nigelmercier

Interesting: one answer with pointers, one without

Perhaps I should have said, I'm happy about extracting the row, just not sure about getting a handle on it. I think I'm beginning to understand, but I'm not sure about:

Code:

p+1 = p + sizeof(char)

I think I will need to use sizeOf, because the same routine is going to process char arrays of different sizes - the above is a sample of one of them.

i was saying that the 1 of p+1 come from sizeof(char)
if we were talking about int, sizeof(int) on a 32 bit machine would give 4
and passing between two cells of memory array (using pointers) would be p+4

i hope i was more clear now

The last post by dwks makes that ample clear, so read it well especially the code section.

Code:

void getResults(char candidateNumber)
{
    ...
    getTotal(FONTTABLE[i], sizeof FONTTABLE[i]);
    ...
}

void getTotal(char *handle, int size)
{
    int i, total = 0;
    for (i = 0; i < size; i++) 
        total += *(handle + i);
}

Originally Posted by itCbitC

The last post by dwks makes that ample clear, so read it well especially the code section.

Thanks, but I didn't understand this bit:

Code:

 
void print(const char *data, size_t size) {
    size_t x;

It seems to have a type size_t - is that right?

Code:

void getResults(char candidateNumber)
{
    ...
    getTotal(FONTTABLE[i], sizeof FONTTABLE[i]);
    ...
}
 
void getTotal(char *handle, int size)
{
    int i, total = 0;
    for (i = 0; i < size; i++) 
        total += *(handle + i);
}

Now that I can follow, thank you. So I have to pass the size to the function, not find it within? This is probably where I have been going wrong.

Originally Posted by nigelmercier

Thanks, but I didn't understand this bit:

Code:

 
void print(const char *data, size_t size) {
    size_t x;

It seems to have a type size_t - is that right?

stdlib.h - Wikipedia, the free encyclopedia

quzah answered already for the rest of the question

Originally Posted by quzah

The only way you can find the end of an array is to either pass the size, and use it to control your loop, or, to set aside a specific value as a terminator. Quzah.

What I meant was, do I have to pass the size, or can I find in in the called function?

What Mr. Quzah want to tell u is that......
How would you know that, in calling function the array size..... for traversal

example

Code:

void print_elements(const int* array, const int array_size) {

 // How would u know the array size.......
  for (int i = 0; i < array_length/*array_size */; ++i)

}

// than u can iterate easily with array_length

Originally Posted by itCbitC

The last post by dwks makes that ample clear, so read it well especially the code section.

Code:

void getResults(char candidateNumber)
{
    ...
    getTotal(FONTTABLE, sizeof FONTTABLE);
  ...
}
 
void getTotal(char *handle, int size)
{
  int i, total = 0;
  for (i = 0; i < size; i++) 
      total += *(handle + i);
}

Oh dear, I thought I was winning for a moment. Here is my equivalent code:

Code:

void charLcd(unsigned char character) // Send any # of bytes to LCD
{
int tablept; // pointer in table -  i in above example
if ((character<0x20)||(character>0x7f))
{return;} // Exit function if character out of ASCII range
tablept = (character - 32); // Index to row of the character in table.
 
sendGlyphs(font5[tablept], 5); //LINE 128 (note ignore sizeof for tests and use 5)
}
 
void sendGlyphs(char *handle, int size)
{ char count;
for (count=0; count<size; count++) // Send all columns to print the character.
{
sendData((*handle + count));
}
sendData(0x00); // 1 pixel space after character.
}

I get an error Illegal pointer conversion in Line 128:

sendGlyphs(font5[tablept], 5); // which is the equivalent of
getTotal(FONTTABLE[i], sizeof FONTTABLE[i]);