C File Handling Techniques: Data Storage and Retrieval Examples

C Example 1: Writing Structured Data to a File

This program demonstrates appending structured date data (assuming struct fecha is defined elsewhere) to a file named Dades.txt. It includes basic file opening and error checking.

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

int main() 
{
    int temp = 1;
    int auxiliar = 0;
    int i = 0;
    FILE *v1;
    
    v1 = fopen("Dades.txt", "a");
    
    // Note: Variables 'origen' and 'destino' are undefined in this snippet.
    if (origen == NULL || destino == NULL) {
        printf("Problemas con los ficheros.\n");
        exit(1);
    }
    
    printf("Inserte el día, mes y año. El programa finalizará marcando 0 en el día:\n\n");
    
    while (temp != 0) {
        printf("Día:\n\n");
        scanf("%d", &datos[i].dia);
        temp = datos[i].dia;
        
        if (temp == 0) break; // Exit condition check
        
        printf("Mes:\n\n");
        scanf("%d", &datos[i].mes);
        
        printf("Año:\n\n");
        scanf("%d", &datos[i].any);
        
        // Writing the structure to the file
        fwrite(&datos, sizeof(struct fecha), 1, v1);
        i++;
    }
    
    fclose(v1);
    return 0;
}

C Example 2: Searching for a Date in a Binary File

This program reads records from prova2.txt and checks if a specific day entered by the user exists in the stored data. It also counts the total number of records processed.

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

int main() 
{
    // Inicializo el contador en 1 ya que antes del bucle while ya se realiza un fread
    int i = 0, dia = 0, trobat = 0, cont = 1;
    FILE *pFile;
    
    pFile = fopen("prova2.txt", "r");
    
    printf("Introduce el día que quieres comprobar si existe dentro del fichero:\n");
    scanf("%d", &dia);
    
    // First read operation
    fread(&historial, sizeof(struct fecha), 1, pFile);
    
    while (!feof(pFile) && trobat == 0) {
        if (historial.dia == dia)
            trobat = 1;
        
        // Subsequent read operation
        fread(&historial, sizeof(struct fecha), 1, pFile);
        cont++;
    }
    
    if (trobat == 1)
        printf("¡Existe!\n");
    else
        printf("No existe esta fecha.\n");
    
    printf("Hay %d registros.\n", cont);
    
    system("PAUSE");
    fclose(pFile);
    return 0;
}

C Example 3: Capturing User Input and Writing to Text File

This program reads characters from standard input until a period (.) is detected, stores them in an array, and then writes the entire content to textentrat.txt using putc.

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

int main ()
{
    FILE *pFile;
    char c;
    char text[100];
    int i = 0, llarg = 0;
    
    printf("Introduce el texto, el punto marca el final:\n");
    
    // Reading input until '.'
    while (c != '.') {
        scanf("%c", &c); // guarda datos
        text[i] = c;
        i++;
    }
    llarg = i;
    
    pFile = fopen("textentrat.txt", "wt");
    
    // Writing stored characters to the file
    for (i = 0; i < llarg; i++) {
        putc(text[i], pFile);
    }
    
    fclose(pFile);
    return 0;
}

C Example 4: Case Swapping Characters in a File

This program reads the entire content of textminus.txt into memory, swaps the case of letters using ASCII values, and writes the modified content to textcanv.txt.

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

int main ()
{
    FILE *pFile;
    FILE *pFile2;
    char c;
    int i = 0, llarg = 0;
    char text[100];
    
    pFile = fopen("textminus.txt", "r");
    pFile2 = fopen("textcanv.txt", "w");
    
    // Reading file content into array (Note: feof check is often unreliable)
    while (!feof(pFile)) {
        fread(&c, sizeof(char), 1, pFile);
        text[i] = c;
        i++;
    }
    llarg = i - 1; // Adjusting length due to potential extra read caused by feof loop
    
    // Processing: Case swapping using ASCII values
    // ASCII: A=65, Z=90 (Uppercase); a=97, z=122 (Lowercase). Difference is 32.
    for (i = 0; i < llarg; i++) {
        // Check for Uppercase (65-90)
        if (text[i] >= 65 && text[i] <= 90) {
            text[i] = text[i] + 32; // Convert to Lowercase
        } 
        // Check for Lowercase (97-122)
        else if (text[i] >= 97 && text[i] <= 122) {
            text[i] = text[i] - 32; // Convert to Uppercase
        } 
    }
    
    // Writing modified content to the output file
    for (i = 0; i < llarg; i++) {
        fwrite(&text[i], sizeof(char), 1, pFile2);
    }
    
    fclose(pFile);
    fclose(pFile2);
    return 0;
}