[SOLVED] C struct member: array initialization

atlantis43 · 02-14-2015, 11:48 AM

Trying to make a header for Huffman Coding, and running into the following problem:
I'm trying to initialize a struct member, which is an int array, and am not clear how to parse this. The following code is what I'm working with.

Code:

#include <stdio.h>
#include <stdlib.h>
#define MAGIC 0x46465548

   // Header for a Huffman-coded file.
typedef struct
{
    // magic number
    int magic;
    // symbols' frequencies
    int frequencies[SYMBOLS];
    // checksum for frequencies
    int checksum;
}Huffeader;

int main(void)
{
    int totalcount = 0;
    int chr;
        // open text file
    FILE* fp;
    fp = fopen("hth.txt","r");
    if (fp == NULL)
    {
        printf("could not open file to read\n");
        return 1;
    }
        // declare array for character count
    int charcnt[256];
            // initialize all values = 0
    for( int i = 0; i < 256; i++)   
    {
        charcnt[i] = 0;
    }

          // tally count of each character in text
    while ((chr = fgetc(fp)) != EOF)
    {
        charcnt[chr]++;
          // and get total count of characters    
        totcount++;    
    }

        // declare a Huffeader 
    Huffeader header;
        // initialize members
    header.magic = MAGIC;
    header.frequencies[255] = PROBLEM HERE
    header.checksum = totcount;
 
    free(fp);
    return 0;
}

Don't know how to go about initializing the above member "header.frequencies[255]. Do I need memcpy()? If so, how to parse?

metaschima · 02-14-2015, 12:08 PM

Try:

Code:

#include <stdio.h>
#include <stdlib.h>
#define MAGIC 0x46465548
//#define SYMBOLS 256

   // Header for a Huffman-coded file.
typedef struct
{
    // magic number
    int magic;
    // symbols' frequencies
    int frequencies[SYMBOLS];
    // checksum for frequencies
    int checksum;
}Huffeader;

int main(void)
{
    int totcount = 0;
    int chr;
        // open text file
    FILE* fp;
    fp = fopen("hth.txt","r");
    if (fp == NULL)
    {
        printf("could not open file to read\n");
        return 1;
    }
        // declare array for character count and initialize
    int charcnt[256] = {};

          // tally count of each character in text
    while ((chr = fgetc(fp)) != EOF)
    {
        charcnt[chr]++;
          // and get total count of characters
        totcount++;
    }

        // declare a Huffeader, initialize members
    Huffeader header = {MAGIC,{},totcount};

    free(fp);
    return 0;
}

SoftSprocket · 02-14-2015, 12:16 PM

It's an array of integers so if you want to set the 256 slot in the array to 0 just do so. If you want to initialize the entire array you have various options.

Code:

struct Heffeader header = {0};

Will initialize the entire struct. You can then individually set the members. You can also do:

Code:

struct Heffeader header = { MAGIC, { 0 }, totcount };

If you have a block of memory you wish to assign to frequencies you could use memcpy or iterate over them assign individual values.

Code:

for (int i = 0; i < SYMBOLS; ++i) {
   header.frequencies[i] = symbols[i];
}

atlantis43 · 02-14-2015, 12:47 PM

Quote:

Originally Posted by SoftSprocket

It's an array of integers so if you want to set the 256 slot in the array to 0 just do so. If you want to initialize the entire array you have various options.

Code:

struct Heffeader header = {0};

Will initialize the entire struct. You can then individually set the members. You can also do:

Code:

struct Heffeader header = { MAGIC, { 0 }, totcount };

If you have a block of memory you wish to assign to frequencies you could use memcpy or iterate over them assign individual values.

Code:

for (int i = 0; i < SYMBOLS; ++i) {
   header.frequencies[i] = symbols[i];
}

Thanks to both above replies, I now realize that I neglected to initialize all frequencies[] to 0. What I should have asked was "How to set the values of the frequecies array".
The "for loop" looks quite simple and adequate, but what would be the parsing for the use of memcpy()?

NevemTeve · 02-14-2015, 01:13 PM

What do you mean by 'parsing memcpy'?

atlantis43 · 02-14-2015, 01:21 PM

Quote:

Originally Posted by NevemTeve

What do you mean by 'parsing memcpy'?

What I meant was how to parse source and dest, but figured it out without a problem:
memcpy(header.frequencies,charcnt,256 );
Thanks to all.

NevemTeve · 02-14-2015, 01:53 PM

Another idea would be forget 'charcnt' and use header.frequencies for the counting.

SoftSprocket · 02-14-2015, 02:39 PM

Quote:

Originally Posted by atlantis43

What I meant was how to parse source and dest, but figured it out without a problem:
memcpy(header.frequencies,charcnt,256 );
Thanks to all.

That probably isn't what you want. It will copy 256 bytes to header.frequencies from charcnt but the size of header.frequencies is going to be
[code]
SYMBOLS * sizeof (int);
[code]

so if SYMBOLS == 256 you will have copied into the first 256 / sizeof (int) members of the array.