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GenericUser01 February 2016

Passing parameters to a function from the Bash command line

I am running a C++ program from the command line on Bash, which is in a Linux environment. I am curious how you pass in a parameter from the command line. Here is my program:

#include <iostream>
using namespace std;

int large_pow2( int n );

int main()
{
   int value = 15;
   int largest_power = large_pow2(value);

   cout << "The highest power of 2 in " << value << " is " << large_power << "." << endl;

   return 0;
} 

int large_pow2( int n )
{
   int i = n
   int j = i & (i - 1);
   while( j != 0)
   {
      i = j;
      j = i & (i - 1);
   }
   return j;
}

After I compile the program I want to be able to use the command line to pass in a number to use for value. For instance, to run the program you type ./"program_name" where "program_name" is the name of my program without quotes. Is there a way to set value = n or something? When I run the program let's say I want n to be 20 so on the command line I type something like ./"program_name" 20. Then the program would run with n = 20. Is there a way to do that? I am completely new to a Linux environment and Bash so don't quite know how to do things in it yet.

Answers


dlmeetei February 2016

Use argc and argv in int main(int argc, char *argv[]) and modify your code accordingly.

The argc arguments tracks the number of arguments passed to your program from CLI and is always >=1. When 1 it is it name of program. So argc[0] is program name.

argv holds the command line arguments, other than program name and is always char string. Hence we need to use appropriate converter like atoi, if you don't want string.

So your code will look like, error checking not done for simplicity

int main(int argc, char *argv[])
{
    //Now we expect argc ==2, program value, This will when argc != 2
    // We should use if in real scenario
    assert(argc == 2);
    int value = atoi(argv[1])
    int largest_power = large_pow2(value);

    cout << "The highest power of 2 in " << value << " is " << large_power << "." << endl;

    return 0;
} 


Mike Samuel February 2016

Your main method can take (int argc, char** argv) which are the count of arguments and the NUL terminated args. The program path is argv[0] so atoi(argv[1]) is probably what you want. Check argc ==2.

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Asked in February 2016
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