CODEWITHSUNDEEP
carraysdynamic
Tuntuni
Tuntuni

Reputation: 491

Allocating an array of an unknown size

Context: I'm trying to do is to make a program which would take text as input and store it in a character array. Then I would print each element of the array as a decimal. E.g. "Hello World" would be converted to 72, 101, etc.. I would use this as a quick ASCII2DEC converter. I know there are online converters but I'm trying to make this one on my own.

Problem: how can I allocate an array whose size is unknown at compile-time and make it the exact same size as the text I enter? So when I enter "Hello World" it would dynamically make an array with the exact size required to store just "Hello World". I have searched the web but couldn't find anything that I could make use of.

Upvotes: 3

Views: 931

Answers (4)

MattJ
MattJ

Reputation: 7924

I'm going to guess you mean C, as that's one of the commonest compiled languages where you would have this problem.

Variables that you declare in a function are stored on the stack. This is nice and efficient, gets cleaned up when your function exits, etc. The only problem is that the size of the stack slot for each function is fixed and cannot change while the function is running.

The second place you can allocate memory is the heap. This is a free-for-all that you can allocate and deallocate memory from at runtime. You allocate with malloc(), and when finished, you call free() on it (this is important to avoid memory leaks).

With heap allocations you must know the size at allocation time, but it's better than having it stored in fixed stack space that you cannot grow if needed.

This is a simple and stupid function to decode a string to its ASCII codes using a dynamically-allocated buffer:

char* str_to_ascii_codes(char* str)
{
    size_t i;
    size_t str_length = strlen(str);
    char* ascii_codes = malloc(str_length*4+1);
    for(i = 0; i<str_length; i++)
        snprintf(ascii_codes+i*4, 5, "%03d ", str[i]);
    return ascii_codes;
}

Edit: You mentioned in a comment wanting to get the buffer just right. I cut corners with the above example by making each entry in the string a known length, and not trimming the result's extra space character. This is a smarter version that fixes both of those issues:

char* str_to_ascii_codes(char* str)
{
    size_t i;
    int written;
    size_t str_length = strlen(str), ascii_codes_length = 0;
    char* ascii_codes = malloc(str_length*4+1);
    for(i = 0; i<str_length; i++)
    {
        snprintf(ascii_codes+ascii_codes_length, 5, "%d %n", str[i], &written);
        ascii_codes_length = ascii_codes_length + written;
    }
    /* This is intentionally one byte short, to trim the trailing space char */
    ascii_codes = realloc(ascii_codes, ascii_codes_length);
    /* Add new end-of-string marker */
    ascii_codes[ascii_codes_length-1] = '\0';
    return ascii_codes;
}

Upvotes: 0

LeleDumbo
LeleDumbo

Reputation: 9340

I see that you're using C. You could do something like this:

 #define INC_SIZE 10

 char *buf = (char*) malloc(INC_SIZE),*temp;
 int size = INC_SIZE,len = 0;
 char c;

 while ((c = getchar()) != '\n') { // I assume you want to read a line of input
   if (len == size) {
     size += INC_SIZE;
     temp = (char*) realloc(buf,size);
     if (temp == NULL) {
       // not enough memory probably, handle it yourself
     }
     buf = temp;
   }
   buf[len++] = c;
 }
 // done, note that the character array has no '\0' terminator and the length is represented by `len` variable

Upvotes: 2

yaronli
yaronli

Reputation: 705

if you use cpp language, you can use the string to store the input characters,and access the character by operator[] , like the following codes:

std::string input;
cin >> input;

Upvotes: -1

Martin James
Martin James

Reputation: 24847

Typically, on environments like a PC where there are no great memory constraints, I would just dynamically allocate, (language-dependent) an array/string/whatever of, say, 64K and keep an index/pointer/whatever to the current end point plus one - ie. the next index/location to place any new data.

Upvotes: 0

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