Question

I have a question related to strings and pointers.
Please explain only with C/C++ programs….

There is a file, which contains 1 word in each line.
I knew the no. of words that are there in the file. Please explain with the help of small code, how I can store those words in RAM, efficiently.

Is fscanf(fp,"%s",word) & strcpy , the only way to store the words in RAM… no other efficient algo or logic available..

Thanks.

Answer 1

Probably the most efficient way to do this is to read the whole file into memory in one block (use fread). Then allocate an array of pointers, one for each word. Then walk through the file in memory, changing the \n characters to \0 and storing a pointer the the character after each \0 in your array.

It is efficient because it only performs one I/O operation, two memory allocations, and loops over the characters in the file twice (once to copy them to the buffer, and once again to break them up into separate strings). The algorithm you describe (fscanf and strcpy) will perform many I/O operations, allocate memory for each word, and loops over the characters at least three times (once to read into the buffer, once to find the length to allocate memory for, and once to copy from the buffer into the allocated memory).

Here’s a simple version with no error checking:

char* buffer; // pointer to memory that will store the file
char** words; // pointer to memory that will store the word pointers

// pass in FILE, length of file, and number of words
void readfile(FILE *file, int len, int wordcnt)
{
    // allocate memory for the whole file
    buffer = (char*) malloc(sizeof(char) * len);
    // read in the file as a single block
    fread(buffer, 1, size, file);

    // allocate memory for the word list
    words = (char**) malloc(sizeof(char*) * wordcnt);
    int found = 1, // flag indicating if we found a word
                   // (starts at 1 because the file begins with a word)
        curword = 0; // index of current word in the word list

    // create a pointer to the beginning of the buffer
    // and advance it until we hit the end of the buffer
    for (char* ptr = buffer; ptr < buffer + len; ptr++)
    {
        // if ptr points to the beginning of a word, add it to our list
        if (found)
            words[curword++] = ptr;
        // see if the current char in the buffer is a newline
        found = *ptr == '\n';
        // if we just found a newline, convert it to a NUL
        if (found)
            *ptr = '\0';
    }
}

Here’s a slightly simpler version using strtok:

char* buffer;
char** words;

void readfile(FILE *file, int len, int wordcnt)
{
    buffer = (char*) malloc(sizeof(char) * len);
    fread(buffer, 1, size, file);
    buffer[len] = '\0';

    words = (char**) malloc(sizeof(char*) * wordcnt);
    int curword = 0;
    char* ptr = strtok(buffer, "\n");
    while (ptr != NULL)
    {
        words[curword++] = ptr;
        ptr = strtok(NULL, "\n");
    }
}

Note that the above two examples assume that the last word in the file ends with a newline!