Hey there! As a fellow C learner who's been in your shoes, let's work through this problem together. The goal is to read lines from input.txt (each with 2 strings and 2 numbers) into a dynamically allocated array—perfect use case for malloc and structs since we don't know the number of lines upfront.

Step 1: Create a Struct to Store Each Line's Data #

Since each line has mixed data types (strings and numbers), using a struct is the cleanest way to group them together. This makes our dynamic array an array of structs, which is far easier to manage than a messy multi-type 2D array.

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

// Define a struct to hold one line's data
typedef struct {
    char str1[50];  // Adjust size based on your string needs
    char str2[50];
    int num1;
    int num2;
} LineData;

Note: I set the string buffers to 50 characters—you can increase this if your input strings are longer. If you need truly dynamic strings, we could add malloc for the strings too, but let's start simple for now.

Step 2: Dynamically Allocate the Array #

We'll start with an initial size for our array, then use realloc to expand it if we need more space as we read lines. This handles cases where we don't know the exact number of lines in advance.

int main() {
    FILE *file = fopen("input.txt", "r");
    if (file == NULL) {
        perror("Failed to open file");
        return 1;
    }

    // Initial array size (we can expand this later)
    int capacity = 10;
    LineData *data_array = malloc(capacity * sizeof(LineData));
    if (data_array == NULL) {
        perror("Failed to allocate memory");
        fclose(file);
        return 1;
    }

    int line_count = 0;
    LineData current_line;

Step 3: Read the File Line by Line #

We'll use fscanf to read each line's data, since the format is fixed (2 strings followed by 2 integers). Every time we fill up our current array capacity, we'll use realloc to double the size (a common strategy for dynamic arrays).

// Read until end of file
    while (fscanf(file, "%s %s %d %d", current_line.str1, current_line.str2, &current_line.num1, &current_line.num2) == 4) {
        // Check if we need to expand the array
        if (line_count >= capacity) {
            capacity *= 2;
            LineData *temp = realloc(data_array, capacity * sizeof(LineData));
            if (temp == NULL) {
                perror("Failed to reallocate memory");
                free(data_array);
                fclose(file);
                return 1;
            }
            data_array = temp;
        }

        // Copy the current line into the array
        data_array[line_count] = current_line;
        line_count++;
    }

Step 4: Use the Data (Example) #

Let's print out what we read to verify it works:

// Print the stored data
    printf("Read %d lines:\n", line_count);
    for (int i = 0; i < line_count; i++) {
        printf("Line %d: %s %s %d %d\n", i+1, data_array[i].str1, data_array[i].str2, data_array[i].num1, data_array[i].num2);
    }

Step 5: Clean Up Memory #

Always free the memory you allocated and close the file to avoid leaks!

// Cleanup
    free(data_array);
    fclose(file);

    return 0;
}

Key Notes for Beginners #

• Error Checking: I added checks for fopen, malloc, and realloc failures—this is crucial in C, ignoring these can lead to crashes or weird behavior.
• Structs: Using a struct keeps related data together, making your code easier to read and maintain.
• Realloc Strategy: Doubling the capacity each time is efficient because it minimizes how often we need to reallocate memory.
• String Sizes: If your input strings are longer than 50 characters, increase the buffer size in the struct, or hit me up if you want to learn dynamic string allocation!

Example input.txt #

If you use this test file:

apple banana 10 20
cat dog 30 40
elephant fox 50 60

The program will output:

Read 3 lines:
Line 1: apple banana 10 20
Line 2: cat dog 30 40
Line 3: elephant fox 50 60

内容的提问来源于stack exchange，提问作者gokhan_ozeloglu