Why does getw not display all of the numbers in the file?-CodePudding

I have followed this tutorial to implement a program that generates up to 100,000 random numbers and inserts them in a file to be sorted, but I have noticed that the loop with getw is outputting way less numbers than expected. In my machine, this code only prints 49 numbers:

#include <stdio.h>
#include <stdlib.h>

int gen_random_file(int n_values) {
    int index;
    int num, num_count = 0;
    FILE *r_file;
    r_file = fopen("random_numbers", "w");

    if (r_file != NULL) {
        printf("File created successfully!\n");
    }
    else {
        printf("Failed to create the file.\n");
        return -1;
    }

    for (index = 0; index < n_values; index  ) {
        putw(rand(), r_file);
    }

    fclose(r_file);

    r_file = fopen("random_numbers", "r");

    // display numbers
    printf("\nNumbers:\n");
    while ( (num = getw(r_file)) != EOF ) {
        printf("%d\n", num);
        num_count  ;
    }
    printf("\nEnd of file.\nNum Count = %d\n", num_count);

    fclose(r_file);

    return 0;
}

int main()
{
    gen_random_file(10000);

    return 0;
}

CodePudding user response：

You terminate the loop too early. rand() is likely to produce -1 once in a while.

Quoting man getw (section Bugs):

Since EOF is a valid integer value, feof(3) and ferror(3) must be used to check for failure after calling getw().

You need something like

    while(1) {
        if ((w = getw()) == EOF) {
            if (feof(stdin) || ferror(stdin)) {
                break;
            }
        printf(....);
        ....
    }
    // Deal with error if necessary

CodePudding user response：

This one of those rare cases where you actually want feof. You need a loop like

while ((num = getw(r_file)), !feof(r_rile)) {

to read a number and then test for EOF.

On some systems (such as Windows), you'll also need "wb" and "rb" for your fopen modes to get a binary file.

CodePudding user response：

I ended up using fwrite and fread as well as "wb" and "wr" as parameters for fopen and that solved the problem.

#include <stdio.h>
#include <stdlib.h>

int gen_random_file(int n_values) {
    int index;
    int rand_num, num_count = 0;
    int buffer[100000];
    FILE *rand_file;

    rand_file = fopen("random_numbers", "wb");

    if (rand_file != NULL) {
        printf("File created successfully!\n");
    }
    else {
        printf("Failed to create the file.\n");
        return -1;
    }

    for (index = 0; index < n_values; index  ) {
        rand_num = rand();
        fwrite(&rand_num, sizeof(rand_num), 1, rand_file);
    }

    fclose(rand_file);

    rand_file = fopen("random_numbers", "rb");

    // display numbers
    printf("\nNumbers:\n");
    fseek(rand_file, 0, SEEK_SET);
    fread(buffer, sizeof(rand_num), n_values, rand_file);
    for (index = 0; index < n_values; index  ) {
        rand_num = buffer[index];
        printf("%d\n", rand_num);
        num_count  ;
    }
    printf("\nEnd of file.\nNum Count = %d\n", num_count);

    fclose(rand_file);

    return 0;
}

int main()
{
    gen_random_file(10000);

    return 0;
}

CodePudding user response：

Perhaps this example is for linux, if you write code in Windows, you need to specify the wb flag in fopen