Divide a string into smaller parts & organize a structure (C-programming)

Question

Hi again!

I am still learning C and I'm having some trouble figuring out how to handle this. Well, I have two structs:

struct myStruct {
    ...
    struct myString *text[5];
    ...
} allStructs;

struct myString {
    char part[100];
};

The objective is to have allStruct[n] point to 5 different parts of a text divided into lines of 100 chars each. So I allocate the space:

allStructs = calloc(n, sizeof(allStructs));

Then, assume that I have a filled char text[500] that I want to divide into 5 parts, and have allStructs[n].text[n].part point at a given part of the text. Can anyone help me with how I proceed?

Answer 1

Well, c strings are just an array of characters terminated by the null character ('\0'). So you would want to iterate through the large text[500] array putting 100 characters in each of the myString parts. Bare in mind that since c strings are null terminated you will only be able to store 99 characters in char part[100].

Also if you wanted to only end a part on a white space character (space (' '), new line ('\n') or tab ('\t')) you would have to find the nearest one to the 100 character limit and split the string up there.

Answer 2

Short answer: you can not with this because you have not consider the '\0' character to terminate each string.

Longer answer: Change structs like that to have more flexibility:

struct myStruct {
   struct myString *text;
}
struct myString {
   char *part;
}

The allocation should be:

struct myStruct *allStruct = calloc(n, sizeof(struct myStruct));

So you have a pointer/array on n struct myStruct.

Then initialize all members of allStruct;

for( i=0; i<n; ++i )
{
   allStruct[i].text = calloc(5, sizeof(myString));
   // Following for only needed if you want new strings by using the strncpy (see above)
   for( y=0; y<5; ++y )
   {
      allSTruct[i].text[y].part = calloc(101, sizeof(char));
   }
}

Now you have all vars initialized.

To copy your 500-chars long string into allStruct[n]:

for( i=0; i<5; i++ )
{
   allStructs[n].text[i].part = &text[i*100]; // If you want to point on the existing string
   // OR
   strncpy(allStructs[n].text[i].part, &text[i*100], 100); // If you want to have new strings

   // In all case, terminate the string with '\0'
   allStructs[n].text[i].part[100] = '\0';
}

This should work.

Answer 3

allStructs is an object of type struct myStruct. You cannot "reassign" it with calloc().

Your object allStructs has the member text which is an array of 5 pointers.
You need to set those pointers to point at specific objects of the struct myString type.

After you copy characters from char text[500] to the various struct myString objects, everything will work right.

pseudo code

struct myString part1;
struct myString part2;
struct myStruct allstructs;
allStructs.text[0] = &part1;
allStructs.text[1] = &part2;
strncpy(part1.part, text, 99);
part1.part[99] = '\0';

printf("part2: %s\n", allStructs.text[1].text);

Oh! And don't confuse allStructs (ends with 's') with allStruct (no 's')