无法检查文件中的字符串

Question

我有我的这个学校项目，需要一些帮助来进行故障排除；我已经调试了它是否进入while（prod_file.good（））循环，但确实如此，它只是无法在文件中找到现有的字符串。

#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string>
#include <conio.h>
#include <stdlib.h>
#include <string.h>

using namespace std;

#define PROD_SIZE 22

class PRODUCT {
public:

    string NAME;
    string QUANTITY;

} product;


int main() {
    int diff;
    string length;
    char answer = 0;
    string line;
    bool foundAndReplaced = false;

    cout << product.NAME.length() << endl;
    cout << product.NAME;

    fstream prod_file("data/available.dat", ios::out | ios::app);
    do {
        if(prod_file.is_open()) {

            cout << "Molq vavedete imeto na produkta: \n";
            getline(cin, product.NAME);


            if(product.NAME.length() < 30) {
                diff = 30 - product.NAME.length();

                for(int i = 1; i < diff; i++){
                    product.NAME.append(".");
                }
            }
            else if(product.NAME.length() > 30) {
                cout << "Product name cannot exceed 30 characters.";
                return 0;
            }

            //

            cout << "Molq vavedete kolichestvoto na produkta: \n";

            getline(cin, product.QUANTITY);



            size_t pos;

            while(prod_file.good()) {
                cout << "asd\n";
                getline(prod_file, line);
                pos = line.find(product.NAME);
                cout << "stignah\n";
                if(pos != string::npos) {
                    prod_file << product.NAME;
                    prod_file << "-" << product.QUANTITY << "\n";
                }
            }

        }

        else {
            cout << "Error: File inaccessible. (0x1)";
            return 0;
        }

        cout << "Do you want to add more products? Y/N \n\n";
        answer = 0;
        while (answer != 'y' && answer != 'Y' && answer != 'n' && answer != 'N') {
        answer = _getch();
        }
    }

    while(answer == 'Y' || answer == 'y');
    prod_file.close();

    cout << "Entries added." << "\n\n";

    return 0;
}

的想法

size_t pos;

            while(prod_file.good()) {
                cout << "asd\n";
                getline(prod_file, line);
                pos = line.find(product.NAME);
                cout << "stignah\n";
                if(pos != string::npos) {
                    prod_file << product.NAME;
                    prod_file << "-" << product.QUANTITY << "\n";
                }
            }

是要检查文件中的某个字符串，如果没有找到要添加到文件中的字符串，如果找到了，则不会添加它。知道为什么它不能通过if吗？

Answer 1

您打开prod_file仅用于输出，而不是输入，因此std::getline()不能读取任何内容。

您不应尝试使用单个流来读取和写入同一文件。打开原始文件以供一个流读取，然后创建一个新文件供一个独立的流写入。使用第一个流扫描原始文件中的现有条目，并使用第二个流将条目写入新文件。完成后，根据需要用新文件替换原始文件。

尝试这样的事情：

#include <iostream>
#include <string>
#include <fstream>
#include <limits>
#include <cstdio>

using namespace std;

struct Product {
    string Name;
    int Quantity;
};

int main() {
    string line;
    char answer;
    bool found, added = false, copyLines = true;
    Product product;

    ifstream prod_file("data/available.dat");
    ofstream prod_file_new("data/available_new.dat", ios_base:::trunc);

    do {
        if (!prod_file || !prod_file_new) {
            cerr << "Error: File inaccessible." << endl;
            return 0;
        }

        do {
            cout << "Molq vavedete imeto na produkta: " << endl;
            if (!getline(cin, product.Name)) {
                if (!cin.eof()) {
                    cerr << "Error: user input failure." << endl;
                    return 0;
                }
                break;
            }

            if (product.Name.length() <= 30) {
                break;
            }

            cerr << "Product name cannot exceed 30 characters." << endl;
        }
        while (true);

        if (cin.eof()) {
            break;
        }

        if (product.Name.length() < 30) {
            product.Name.append(30 - product.Name.length(), '.');
        }

        //

        found = false;

        do {
            if (!getline(prod_file, line)) {
                if (!prod_file.eof()) {
                    cerr << "Error: input file failure." << endl;
                    return 0;
                }
                break;
            }

            if (copyLines) {
                if (!(prod_file_new << line << "\n")) {
                    cerr << "Error: output file failure." << endl;
                    return 0;
                }
            }

            found = (line.find(product.Name) != string::npos);
        }
        while (!found);

        if (!found) {
            cout << "Molq vavedete kolichestvoto na produkta: " << endl;

            if (!(cin >> product.Quantity)) {
                cerr << "Error: user input failure." << endl;
                return 0;
            }
            cin.ignore(numeric_limits<streamsize>::max(), '\n');

            if (!(prod_file_new << product.Name << "-" << product.Quantity << "\n")) {
                cerr << "Error: output file failure." << endl;
                return 0;
            }

            added = true;
        }

        cout << "Do you want to add another product? Y/N" << endl << endl;

        cin >> answer;
        while (answer != 'y' && answer != 'Y' && answer != 'n' && answer != 'N') {
            cin >> answer;
        }

        if (answer != 'Y' && answer != 'y')
            break;

        if (!prod_file.seekg(0)) {
            cerr << "Error: input file failure." << endl;
            return 0;
        }

        copyLines = false;
    }
    while (true);

    prod_file.close();
    prod_file_new.close();

    if (added) {
        if (remove("data/available.dat") == 0) {
            if (rename("data/available_new.dat", "data/available.dat") == 0) {
                cout << "Entries added." << endl << endl;
            }
            else {
                cerr << "Error: renaming available_new.dat to available.dat" << endl << endl;
            }
        }
        else {
            cerr << "Error: removing available.dat" << endl << endl;
        }
    }
    else {
        remove("data/available_new.dat");
        cout << "No Entries added." << endl << endl;
    }

    return 0;
}

或者，将现有文件读入内存，然后根据需要搜索并追加到文件中，然后仅在添加了条目的情况下写出新文件，例如：

#include <iostream>
#include <string>
#include <fstream>
#include <sstream>
#include <vector>
#include <algorithm>
#include <limits>
#include <cstdio>

using namespace std;

struct Product {
    string Name;
    int Quantity;
};

int main() {
    string line;
    char answer;
    bool found, added = false;
    Product product;
    vector<Product> products;

    ifstream prod_file("data/available.dat");
    if (!prod_file) {
        cerr << "Error: input file inaccessible." << endl;
        return 0;
    }

    while (getline(prod_file, line)) {
        istringstream iss(line);
        if (!(getline(iss, product.Name, '-') && (iss >> product.Quantity))) {
            cerr << "Error: input file malformed." << endl;
            return 0;
        }
        products.push_back(product);
    }

    if (!prod_file) {
        cerr << "Error: input file failure." << endl;
        return 0;
    }

    prod_file.close();

    do {
        do {
            cout << "Molq vavedete imeto na produkta: " << endl;
            if (!getline(cin, product.Name)) {
                if (!cin.eof()) {
                    cerr << "Error: user input failure." << endl;
                    return 0;
                }
                break;
            }

            if (product.Name.length() <= 30) {
                break;

            cerr << "Product name cannot exceed 30 characters." << endl;
        }
        while (true);

        if (cin.eof()) {
            break;
        }

        if (product.Name.length() < 30) {
            product.Name.append(30 - product.Name.length(), '.');
        }

        //

        found = std::find_if(products.begin(), products.end(),
            [&](const Product &p){ return (p.Name == product.Name); }
            ) != products.end();

        if (!found) {
            cout << "Molq vavedete kolichestvoto na produkta: " << endl;

            if (!(cin >> product.Quantity)) {
                cerr << "Error: user input failure." << endl;
                return 0;
            }
            cin.ignore(numeric_limits<streamsize>::max(), '\n');

            products.push_back(product);
            added = true;
        }

        cout << "Do you want to add another product? Y/N" << endl << endl;

        cin >> answer;
        while (answer != 'y' && answer != 'Y' && answer != 'n' && answer != 'N') {
            cin >> answer;
        }

        if (answer != 'Y' && answer != 'y')
            break;
    }
    while (true);

    if (added) {
        ofstream prod_file_new("data/available_new.dat", ios_base:::trunc);

        if (!prod_file_new) {
            cerr << "Error: output file inaccessible." << endl;
            return 0;
        }

        for (auto &p : products) {
            if (!(prod_file_new << p.Name << "-" << p.Quantity << "\n")) {
                cerr << "Error: output file failure." << endl;
                return 0;
            }
        }

        prod_file_new.close();

        if (remove("data/available.dat") == 0) {
            if (rename("data/available_new.dat", "data/available.dat") == 0) {
                cout << "Entries added." << endl << endl;
            }
            else {
                cerr << "Error: renaming available_new.dat to available.dat" << endl << endl;
            }
        }
        else {
            cerr << "Error: removing available.dat" << endl << endl;
        }
    }
    else {
        cout << "No Entries added." << endl << endl;
    }

    return 0;
}