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Reading from Files in C

Reading data from files allows your program to process input stored in files. C provides several functions for reading from files.

1. Functions to Read from Files

Function Description
fscanf(FILE *fp, const char *format, ...)Reads formatted input from a file.
fgets(char *str, int n, FILE *fp)Reads a string from a file.
fgetc(FILE *fp)Reads a character from a file.
fread(void *ptr, size_t size, size_t nmemb, FILE *fp)Reads data from a file into an array.

2. Example: Reading Text from a File

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

int main() {
    FILE *fptr;
    char buffer[100];
    fptr = fopen("output.txt", "r");
    if (fptr == NULL) {
        printf("Error opening file!\n");
        exit(1);
    }
    while (fgets(buffer, 100, fptr) != NULL) {
        printf("%s", buffer);
    }
    fclose(fptr);
    return 0;
}

3. Example: Reading Binary Data from a File

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

int main() {
    FILE *fptr;
    int numbers[5];
    fptr = fopen("data.bin", "rb");
    if (fptr == NULL) {
        printf("Error opening file!\n");
        exit(1);
    }
    fread(numbers, sizeof(int), 5, fptr);
    fclose(fptr);
    printf("Data read from file:\n");
    for (int i = 0; i < 5; i++) {
        printf("%d ", numbers[i]);
    }
    printf("\n");
    return 0;
}

Best Practices

  • Use fgets() to read strings and avoid buffer overflows.
  • Check for end-of-file using feof() or the return values of reading functions.
  • Handle possible read errors, such as corrupted files or incorrect formats.

Don'ts

  • Don't ignore the possibility of partial reads; always check the number of items read.
  • Don't assume the file contains the expected data; validate inputs.
  • Don't forget to close the file after reading.

Key Takeaways

  • Reading from files allows your program to utilize external data sources.
  • Various functions are available for reading text and binary data.
  • Proper error handling ensures robust file reading operations.
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