CODEWITHSUNDEEP
c++command-lineuser-input
Robert Ryder
Robert Ryder

Reputation: 39

Processing command line arguments, allowing for any order

Ok I am creating a program that allows user input in any order. The term -1 or -2 is used to indicate an operating mode. while the argument -c always precedes a filename. Here is the loop I have written to allow these to be entered in any order the user wishes.

int main(int argc, char *argv[])
{
    string qcolors_name;
    string inputFileName;
    pmode_t mode = run_process2;
    // perform command line argument checking
    if(argc < 2){
        cerr << "Not enough arguments" << endl;
        return 0;
    }

    for(int i = 1; i < argc - 1; i++){
        if(argv[i] == "-1"){
            mode = run_process1;
            cout << "Mode 1" << endl;
        }else if(argv[i] == "-2"){
            mode = run_process2;
        }else if(argv[i] == "-c"){
            qcolors_name = argv[i+1];
            cout << qcolors_name << endl;
            i++;
        }else if(argc-1 == i){
            break;
        }else{
            cout << argv[i] <<endl;
            cerr << "arguments error"<<endl;;
            return 0;
        }

    }

However the program seems to always dump out to the arguments error message even though it outputs that argv[i] is = -1. This is one of the things I have tested for though and should never have reached this case. Im sure its something syntax wise that I am doing wrong.

EDIT

Here is an imput with a sample error output.

USER> ./colorfun -1 -c qc_bw.txt mountains.ppm      <-  9:46PM
-1
arguments error

Upvotes: 0

Views: 2403

Answers (2)

Andres
Andres

Reputation: 1146

In C++ you have to convert the arguments to std::string first, then you can compare them using the == operator, like this:

int main(int argc, char* argv[]) {
    for (int i = 1; i < argc; i++) {
        if (std::string(argv[i]) == "-a") {
            std::cout << "You passed parameter -a" << std::endl;
        } else if (std::string(argv[i]) == "-b") {
            std::cout << "You passed parameter -b" << std::endl;
        }
    }
    return 0;
}

Upvotes: 0

user3159253
user3159253

Reputation: 17455

It's C, you can't compare strings with just str1 == "smth". Use strcmp() instead.

More detailed explanations: operator == in C compares integral entities: numbers, pointers, etc. "smth" is essentially a pointer to memory area in constant segment of your program. That's why code like

const char* a = "abc";
const char* b = "abc";
assert(a == b);

may give true, both a & b may point to a single memory location. But this expression never compares letters "a", "b", "c" from a and b, it compares just pointers. Likewise in Java you have to use str1.equals(str2) because Java operator == also compares integral entities (numbers, characters and "pointers to Objects"). In opposite in C++, Python and other programming languages with operator overloading, == for two strings usually leads to character by character comparison.

Upvotes: 2

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