C++如何读取文件忽略空格并提取固定格式行的指定字段

Hey there! Let's tackle your problem step by step—first extracting the exact values you need from each line, then covering how to handle whitespace when reading files in C++.

Since your lines follow a strict, predictable format: ID: HH-123AB, brand: Mercedes-Benz, 128000 miles, 12 L, we can leverage C++'s formatted input to skip the fixed parts and directly capture the data we care about.

Here's a straightforward example that reads each line and extracts the ID, brand, mileage, and fuel consumption:

#include <iostream>
#include <fstream>
#include <string>
#include <limits> // For ignore() if handling spaced brands

int main() {
    std::ifstream input_file("your_input_file.txt");
    if (!input_file.is_open()) {
        std::cerr << "Oops! Couldn't open the input file." << std::endl;
        return 1;
    }

    std::string id, brand;
    int miles;
    int liters; // Use double if you expect decimal values like 12.5

    // Formatted input skips fixed tokens automatically
    while (input_file >> std::ws >> "ID:" >> id >> ", brand:" >> brand >> miles >> "miles," >> liters >> "L") {
        // Do something with your extracted values
        std::cout << "Extracted: " << id << ", " << brand << ", " << miles << ", " << liters << "\n";
    }

    input_file.close();
    return 0;
}

Quick note for spaced brand names: #

The code above works perfectly if brands don't have spaces (like Mercedes-Benz). If you have brands with spaces (e.g., BMW X5), adjust the brand-reading logic to capture everything up to the next comma:

// Replace the brand reading part with this
std::string dummy;
input_file >> std::ws >> "ID:" >> id >> ", brand:";
std::getline(input_file >> std::ws, brand, ','); // Read until comma
input_file >> miles >> "miles," >> liters >> "L";

C++ offers several straightforward methods to handle whitespace during file reading:

• Default >> behavior: The >> operator for std::istream automatically skips all leading whitespace (spaces, tabs, newlines) before reading a value. This is why we can seamlessly read integers and strings without worrying about spaces between tokens.
• std::ws manipulator: Use this to explicitly skip whitespace, especially before using std::getline (which doesn't skip leading whitespace by default). Example:

  std::string line;
  std::getline(input_file >> std::ws, line); // Skips leading whitespace before reading the full line
  

• ignore() for targeted skipping: If you need to clear leftover characters (like a newline after using >>), use ignore() to skip until a specific character:

  // Ignore all characters up to and including the next newline
  input_file.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
  

That's all! This approach should reliably extract your desired values while handling whitespace correctly.

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