如何檢查一行是否是 c++ 中的所有空格？

Question

我的任務是通讀用戶輸入的文件，然后檢查每一行以查看該行是否有有效的命令。 例如，如果文件包含以下內容：

run        prog1
%
delete
% execute next command
delete myfile


copy file1 file2


print myfile

% terminate script

我的代碼的預期 output 是這樣的：

Enter the name of a file to read from:

Total lines: 13
Commented lines: 3
Valid Command lines: 5
Invalid Command lines: 0
Run commands: 1
Print commands: 1
Copy commands: 1
Delete commands: 2

我的代碼是這樣的：

#include <iostream>
#include <string>
#include <fstream>
using namespace std;

int main(){
    ifstream inClientFile;
    string filename;

    cout << "Enter the name of a file to read from:" << endl;
    cin >> filename;

    inClientFile.open(filename.c_str());
    if( !inClientFile)
    {
        cerr << endl;
        cerr << "File cannot be opened" << " " << filename << endl;
        exit (1);
    }

    string line;
    string commands;
    int total = 0, comment = 0, valid_lines = 0, invalid_lines = 0, run_lines = 0, print_lines = 0, copy_lines = 0, delete_lines = 0;

    while (getline(inClientFile,line)){
        total++;
        if(line.size()==0){
            continue;
        }
        else if(line[0]=='%'){
            comment++;
            continue;
        }
        else{
            int i=0;
            while (i<line.size() and line[i]!=' '){
                i++;
            }

            commands = line.substr(0,i);
            if(commands == "run"){
                run_lines++;
                valid_lines++;
            }
            else if(commands == "print"){
                print_lines++;
                valid_lines++;
            }
            else if(commands == "copy"){
                copy_lines++;
                valid_lines++;

            }
            else if(commands == "delete"){
                delete_lines++;
                valid_lines++;

            }
            else if(line.size()==0){
                continue;
            }
            else{
                cout<<"\nError: Unrecognizable command in line " <<total<<endl;
                invalid_lines++;
            }
        }
    }
    cout<<endl;
    cout << "Total lines: " << total << endl;
    cout << "Commented lines: " << comment << endl;
    cout << "Valid Command lines: " << valid_lines<< endl;
    cout << "Invalid Command lines: " << invalid_lines << endl;
    cout << "Run commands: " << run_lines << endl;
    cout << "Print commands: " << print_lines << endl;
    cout << "Copy commands: " << copy_lines << endl;
    cout << "Delete commands: " << delete_lines << endl;
    return 0;
}

問題是我的代碼正在讀取所有空格作為無效命令的行。 所以 output 最終是：

Enter the name of a file to read from:                                                                                                                                                                                                   
                                                                                                                                                                                                                                         
Error: Unrecognizable command in line 7                                                                                                                                                                                                  
                                                                                                                                                                                                                                         
Error: Unrecognizable command in line 10                                                                                                                                                                                                 
                                                                                                                                                                                                                                         
Error: Unrecognizable command in line 12                                                                                                                                                                                                 
                                                                                                                                                                                                                                         
Total lines: 13                                                                                                                                                                                                                          
Commented lines: 3                                                                                                                                                                                                                       
Valid Command lines: 5                                                                                                                                                                                                                   
Invalid Command lines: 3                                                                                                                                                                                                                 
Run commands: 1                                                                                                                                                                                                                          
Print commands: 1                                                                                                                                                                                                                        
Copy commands: 1                                                                                                                                                                                                                         
Delete commands: 2

請教我如何閱讀包含所有空格的行，然后忽略該行或繼續。 我想到了這條線：

if(line.size()==0){
            continue;
        }

可以解決這個問題，但它不適用於兩個連續的空行，因為包含所有空格的行的值大於空行的值，即 0。請幫助我！

Answer 1

最簡單的做法是使用std::istringstream和>>來拉開這條線。 如果該行只是空格， >>將無法找到令牌，並且 stream 將被放入失敗的 state。 測試 stream state，您會知道該行是否除了空白之外什么都沒有。

不利的是，這將讀取

            run

作為run並接受它作為有效令牌。 如果您不接受以空格開頭的行

std::isspace(static_cast<unsigned char>(line[0]));

可以用來檢測這種情況。 請參閱std::isspace文檔中的注釋，以了解看起來很傻的演員表的解釋。

為了簡單起見，我仍然會使用istringstream來標記該行。 它將完全標記一個命令，如

copy file1 file2

容易死。

更新代碼

#include <iostream>
#include <string>
#include <fstream>
#include <sstream> // for istringstream
using namespace std;

int main(){
    ifstream inClientFile;
    string filename;

    cout << "Enter the name of a file to read from:" << endl;
    cin >> filename;

    inClientFile.open(filename.c_str());
    if( !inClientFile)
    {
        cerr << endl;
        cerr << "File cannot be opened" << " " << filename << endl;
        exit (1);
    }

    string line;
    string commands;
    int total = 0, 
        comment = 0, 
        valid_lines = 0, 
        invalid_lines = 0, 
        run_lines = 0, 
        print_lines = 0, 
        copy_lines = 0, 
        delete_lines = 0;

    while (getline(inClientFile,line)){
        total++;
        if(line.size()==0){
            continue;
        }
        else if(line[0]=='%'){
            comment++;
            continue;
        }
        else{
            // test std::isspace(static_cast<unsigned char>(line[0])); here if
            // necessary
            std::istringstream strm(line); //use stream to split up the line 
            if (strm >>commands) // skip all whitespace leading to first 
                                 // whitepace-delimited token. If the line is 
                                 // nothing but whitespace, the read will fail.
                                 // downside: 
                                 //         run 
                                 // is now a valid command. May need extra checks
            {
                if(commands == "run"){
                    run_lines++;
                    valid_lines++;
                }
                else if(commands == "print"){
                    print_lines++;
                    valid_lines++;
                }
                else if(commands == "copy"){
                    copy_lines++;
                    valid_lines++;

                }
                else if(commands == "delete"){
                    delete_lines++;
                    valid_lines++;

                }
                else if(line.size()==0){
                    continue;
                }
                else{
                    cout<<"\nError: Unrecognizable command in line " <<total<<endl;
                    invalid_lines++;
                }
            }
            else // no token at all
            {
                continue; // bit of a waste now, but if someone adds more cases 
                          // after this one... One less possible bug.
            }
        }
    }
    cout<<endl;
    cout << "Total lines: " << total << endl;
    cout << "Commented lines: " << comment << endl;
    cout << "Valid Command lines: " << valid_lines<< endl;
    cout << "Invalid Command lines: " << invalid_lines << endl;
    cout << "Run commands: " << run_lines << endl;
    cout << "Print commands: " << print_lines << endl;
    cout << "Copy commands: " << copy_lines << endl;
    cout << "Delete commands: " << delete_lines << endl;
    return 0;
}