File Input Output

Write

C

src/c/file-io/01-write-file.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
// compile: gcc file.c -lm

int main()
{
    int ii;
    double theta, result1, result2;

    FILE *fp;
    fp = fopen("results.dat", "w");
    if (fp == NULL)
    {
        printf("File could not be created.\n");
        exit(1);
    }

    for (ii = 0; ii < 100; ii++)
    {

        theta = 4 * 3.14 * ii / 100;

        if (theta == 0) // To avoid the 0/0 situation
        {
            theta = 0.000001;
        }

        result1 = sin(theta) / theta;
        result2 = sin(2 * theta) / theta;

        // printf("%f\t%f\n", theta, result);
        fprintf(fp, "%f\t%f\t%f\n", theta, result1, result2);
    }
    fclose(fp);
    return 0;
}

C++

src/cpp/file-io/01-write-file.cpp

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    // ofstream is used to write to file
    ofstream fp_out{"data.txt"};
    // ofstream fp_out{"data.txt", ios::out};

    if (!fp_out)
    {
        cerr << "File could not be created!\n";
        return 1;
    }

    for (int i = 1; i <= 10; i++)
    {
        fp_out << i << " " << i * i << endl;
    }
    
    fp_out.close();
    return 0;
}

Read

C

src/c/file-io/02-read-file.c

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

#define MAX_SIZE 10

void read_file(FILE *fp, int data[], int *size)
{
    *size = 0;
    while (fscanf(fp, "%d", &data[*size]) == 1) 
    // fscanf returns 1 if success, 0 if fails
    {
        (*size)++;
    }
}

void print_data(int data[], int size)
{
    int i;
    for (i = 0; i < size; i++)
    {
        printf("%d\n", data[i]);
    }
}

double average(int data[], int size)
{
    int i;
    double average = 0.0;

    for (i = 0; i < size; i++)
    {
        average += data[i];
    }

    return (average / size);
}

int main()
{
    int i;
    int size = MAX_SIZE;
    FILE *fp;
    int data[MAX_SIZE] = {0};

    fp = fopen("file.dat", "r");
    if (fp == NULL)
    {
        printf("File could not be opened.\n");
        exit(1);
    }

    read_file(fp, data, &size);

    printf("My %d scores are:\n", size);
    print_data(data, size);

    printf("Average score: %.2f\n", average(data, size));
    fclose(fp);
    return 0;
}

C++

src/cpp/file-io/02-read-file.cpp

#include <iostream>
#include <fstream>
#include <string>
using namespace std;

int main()
{
    // ifstream is used to read from file
    ifstream fp_in{"data.txt"};
    // ifstream fp_in{"data.txt", ios::in};
    if (!fp_in)
    {
        cerr << "File could not be opened for reading!\n";
        return 1;
    }

    while (fp_in)
    {
        string input_string;
        getline(fp_in, input_string);
        cout << input_string << endl;
    }

    fp_in.close();
    return 0;
}

Append

src/cpp/file-io/03-append-file.cpp

#include <iostream>
#include <fstream>
using namespace std;

int main()
{
    // ofstream is used to write to file
    ofstream fp_out{"data.txt"};

    if (!fp_out)
    {
        cerr << "File could not be created!\n";
        return 1;
    }

    for (int i = 1; i <= 10; i++)
    {
        fp_out << i << " " << i * i << endl;
    }
    
    fp_out.close();

    // now we will open the file again to append more content
    ofstream fp_app{"data.txt", ios::app};
    // it is also possible to open the same fp_out
    // fp_out.open("data.txt", ios::app};

    if (!fp_app)
    {
        cerr << "File could not be created!\n";
        return 1;
    }

    for (int i = 11; i <= 15; i++)
    {
        fp_app << i << " " << i * i << endl;
    }

    fp_app.close();
    return 0;
}

Save buffer

Saving and reading buffer as binary is faster than IO using formatted data.

src/c/file-io/04-save-buffer.c

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

int main()
{
    int *base, x;
    FILE *fp;

    // allocate memory
    base = (int *)malloc(sizeof(int) * 10);
    if (base == NULL)
    {
        fprintf(stderr, "Failed to allocate memory.\n");
        exit(1);
    }

    // fill with values
    for (x = 0; x < 10; x++)
    {
        *(base + x) = x * 10;
    }

    // open file
    fp = fopen("numbers.bin", "w");
    if (fp == NULL)
    {
        printf("File could not be created.\n");
        exit(1);
    }

    printf("%d\n", *(base + 1));
    // save buffer to file
    int size;
    size = fwrite(base, sizeof(int), 10, fp); 
    // this returns number of items
    // the data is written in the binary format as it exit in the buffer
    printf("%d\n", size);
    fclose(fp);
    return 0;
}

Read buffer

src/c/file-io/05-read-buffer.c

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

int main()
{
    int *base, x, r;
    FILE *fp;

    // allocate buffer
    base = (int *)malloc(sizeof(int) * 10);
    if (base == NULL)
    {
        fprintf(stderr, "Error while allocating memory.\n");
        exit(1);
    }

    // open file
    fp = fopen("numbers.bin", "r");
    if (fp == NULL)
    {
        fprintf(stderr, "Error reading file.\n");
        exit(1);
    }

    // fill the buffer
    r = fread(base, sizeof(int), 10, fp);
    if (r != 10)
    {
        fprintf(stderr, "Could not read 10 numbers.\n");
        exit(1);
    }

    // print values from buffer
    for (int i = 0; i < 10; i++)
    {
        printf("%d\n", *(base + i));
    }

    return 0;
}