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Pass arguments to stuff? HOW IZ DO???

Guyofdoom42 (8)
I'd like to be able to pass arguments to functions, based on user input.

For example, the user could type:
 
 
stuff arg1 arg2 arg3 arg4...


How does one do this?
keskiverto (10365)
Guyofdoom42 (8)
I meant inside the program... Such as:
[code]
myname@mycomputer$ ./myprogram
prompt> command arg1 arg2 arg3...
Some stuff influenced by the arguments
prompt>
Renthalkx97 (152)
Do you mean how to write a function? If so:
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return_type identifier(optional_arguments)
{
    return return_type
}
Guyofdoom42 (8)
Ummm, no. I'm making a program that uses it's own commands, and I'd like to be able to pass arguments with the commands.
megatron 0 (471)
Something like this:

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#include <string>
#include <vector>

std::vector<std::string> args;

std::string arg;

while((std::cin >> arg) && std::cin.peek() == ' ')
{
        args.push_back(arg);
}

if(args[0] == "Run")
{
        if(args[1] == "-b") SetBreakPoint();
        else if(args[1] == "-s") SetStepping();
        else Run(args[1]);
}
Last edited on
Guyofdoom42 (8)
Uhhh... What the hell is that!?
keskiverto (10365)
If you want to use what the user types, then you have to read input. megatron 0's while-loop reads words and stores them into vector.

The int main( int argc, char* argv[] ) is a result of similar process; the shell has taken what the user did write, split it into words, stored in array argv, and passed to the program it started.

Whether you have int argc, char* argv[] or std::vector<std::string> args you do have a list of words. Then your program has to decide what to do with them. Both the article that I did link to and megatron 0's show some examples, but it really is up to your actual needs.
MiiNiPaa (8886)
Extending megatron 0 example (compiling code):
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#include <functional>
#include <iostream>
#include <iterator>
#include <map>
#include <sstream>
#include <string>
#include <vector>

void loop(const std::vector<std::string>& args)
{
    //I am to lazy to use proper exception classes right now
    if (args.size() != 3) throw "invalid arguments number";
    int begin     = std::stoi(args[0]);
    int end       = std::stoi(args[1]);
    int increment = std::stoi(args[2]);
    if(begin > end || !(increment > 0) ) throw "invalid argument";
    for(int i = begin; i < end; i += increment)
        std::cout << i << ' ';
    std::cout << '\n';
}

void add(const std::vector<std::string>& args)
{
    if (args.size() != 2) throw "invalid arguments number";
    int lhs = std::stoi(args[0]);
    int rhs = std::stoi(args[1]);
    std::cout << (lhs + rhs) << '\n';
}

void echo(const std::vector<std::string>& args)
{
    for(const auto& s: args)
        std::cout << s << '\n';
}

std::vector<std::string> tokenize(std::string in)
{
    std::istringstream s(in);
    return {std::istream_iterator<std::string>(s), std::istream_iterator<std::string>()};
}

int main()
{
    std::map<std::string, std::function<void(const std::vector<std::string>&)>> func = {
        {"loop", loop}, {"add", add}, {"echo", echo}, 
        {"exit", [](const std::vector<std::string>&){std::exit(0);} },
    };
    std::string input;
    while(std::getline(std::cin, input)){
        auto tokens = tokenize(input);
        func.at(tokens[0])({tokens.begin() + 1, tokens.end()});
    }
}
 
echo hello, world
hello,
world
loop 0 10 2
0 2 4 6 8
add 1 1
2
exit

Process returned 0 (0x0)   execution time : 38.020 s
Press any key to continue.
Last edited on
megatron 0 (471)
Very interesting code MiiNiPaa.

Could you explain:

This

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std::vector<std::string> tokenize(std::string in)
{
    std::istringstream s(in);
    return {std::istream_iterator<std::string>(s), std::istream_iterator<std::string>()};
}


And this

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std::map<std::string, std::function<void(const std::vector<std::string>&)>> func = {
        {"loop", loop}, {"add", add}, {"echo", echo}, 
        {"exit", [](const std::vector<std::string>&){std::exit(0);} },
    };
keskiverto (10365)
The tokenize:
* The vector has a constructor that takes a range (the InputIterators): http://www.cplusplus.com/reference/vector/vector/vector/
* istream_iterator is an InputIterator: http://www.cplusplus.com/reference/iterator/istream_iterator/istream_iterator/
MiiNiPaa (8886)
This
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std::istringstream s(in);
return {std::istream_iterator<std::string>(s), std::istream_iterator<std::string>()};
is equivalent to
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std::istringstream s(in);
std::vector<std::string> result;
std::string temp;
while(s >> temp)
    result.push_back(temp);
return result;
It uses stringstream to access input string as a stream (much like std::cin) http://en.cppreference.com/w/cpp/io/basic_istringstream
Then it just fills the vector and returns it. Original code uses two iterator constructor (4)
http://en.cppreference.com/w/cpp/container/vector/vector
An it uses istream iterator to read elements: http://en.cppreference.com/w/cpp/iterator/istream_iterator


map is an associative container: it is like array, where indices can be anything (in this case std::string). This map holds std::function: a function object you can call like a function.
http://en.cppreference.com/w/cpp/concept/FunctionObject
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func.at(tokens[0])    ({tokens.begin() + 1, tokens.end()});
//        ↑                        ↑
// returns function   function call. Again uses 2 iter constructor to create parameter
I used at() instead of [] to force exception if user enters something wrong. I would throw an exception later anyway, but that way we avoid creation of empty function.
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