setting variables to command line arguments

This is a discussion on setting variables to command line arguments within the C++ Programming forums, part of the General Programming Boards category; Hi, I am trying to make my program alter the number of times it requests info by using command line ...

  1. #1
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    setting variables to command line arguments

    Hi,

    I am trying to make my program alter the number of times it requests info by using command line arguments. However I can't assign a variable to the value of the argument in the array i.e. int var = argv[2];

    Code:
    #include<iostream>
    #include<string>
    #include<cmath>
    #include <iomanip>
    
    using namespace std;
    
    void transferTime ( int hddSpeeds[], float sizeOfDir, float transferRate[] );
    
    int main(int argc, char *argv[])
    {
    	char control = argv[2];
    	string hddNames[control];
    	float transferRate[control];
    	int hddSpeeds[control];
    	float sizeOfDir;
    	float counter = 0;
    	int fastest;
    	for ( int loop = 0; loop < control; loop++ )
    	{//open loop
    		cout<<"Please enter a harddrive name to be compared: ";
    		cin>> hddNames[loop];
    		cout<<"Please enter the transfer speed of that drive in megabits per second: ";
    		cin>> hddSpeeds[loop];
    	}//end loop
    	cout<<"Please enter the size of the directory being transferred in Gigabytes: ";
    	cin>> sizeOfDir;
    	transferTime( hddSpeeds, sizeOfDir, transferRate, control );
    	for ( int loop = 0; loop < control; loop++ )
    	{//open loop
    		if ( counter < transferRate[loop] )
    		{//open if
    			 fastest = loop;
    		}//close if
    	}//close loop
    	float topspeed;
    	topspeed = transferRate[fastest];
    	cout<<"Hard Drive\tTransfer rate (Mbps)\tDirectory Size(GB)\tTransfer time(s)\n";
    	for (int loop = 0; loop < control; loop++ )
    	{//open loop
    		cout<< hddNames[loop] << "\t\t";
    		cout<< hddSpeeds[loop] << "\t\t\t";
    		cout<< sizeOfDir << "\t\t\t";
    		cout<< std::fixed << setprecision(1) << topspeed << "\n";
    	}//close loop
    	cout<<"\n";
    	cout<<"The fastest of the given Hard Drives was: \n";
    	cout<<"Hard Drive\tTransfer rate (Mbps)\tDirectory Size(Gb)\tTransfer time(s)\n";
    	cout<< hddNames[fastest] << "\t\t";
    	cout<< hddSpeeds[fastest] << "\t\t\t"; 
    	cout<< sizeOfDir << "\t\t\t"; 
    	cout<< std::fixed << setprecision(1) << topspeed << "\n";
    }
    
    void transferTime ( int hddSpeeds[], float sizeOfDir, float transferRate[], int control )
    {
    	for (int loop = 0; loop < control; loop++ )
    	{//open loop
    		int dummy = hddSpeeds[loop];
    		transferRate[loop] = sizeOfDir * 1024 * 8/dummy;
    	}//close loop
    	return;
    }
    Here are the errors I am getting:

    Code:
    hdd.cpp: In function ‘int main(int, char**)’:
    hdd.cpp:12: error: invalid conversion from ‘char*’ to ‘char’
    hdd.cpp:8: error: too many arguments to function ‘void transferTime(int*, float, float*)’
    hdd.cpp:28: error: at this point in file
    Thanks in advance.

  2. #2
    and the hat of int overfl Salem's Avatar
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    Because all args are strings, so even if you do

    myprog 123

    You end up with "123"
    So perhaps
    myint = atoi( argv[2] );

    Also, you should check argc before looking at argv

    > string hddNames[control];
    Variable length arrays are not standard C++
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  3. #3
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    Also to add at the very beginning
    void transferTime ( int hddSpeeds[], float sizeOfDir, float transferRate[] );

    does not match the function at the bottom.

    for var = argv ....
    a sollution is:
    create a char* and use strcpy to copy it to the pointer. (obviously properly allocated)

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    Thanks for the replies. Sorry I am very new to c++ so could you maybe give me an example of how to do the strcat method please?

    Also so if I can't use variable length arrays how do I solve my problem? or does it still work but just isn't official?

  5. #5
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    You are using the C++ string class, so don't use strcpy or strcat. It's not necessary here anyway, since the point is that you need an int version of the command line argument.

    For the variable length arrays, it is allowed on some compilers because it is legal in C. In C++ you would generally use a vector instead of arrays for this (another option is dynamic arrays with new[]/delete[]). Whether you switch to the other alternatives or not is up to you, and it depends on whether this code is only for a one-time project like a homework assignment, or if you are trying to write code that will work in other places.

    Another solution if you really aren't supposed to be using vectors or dynamic arrays is to use a maximum size and make the arrays that size, then only use part of the arrays based on the control value.

  6. #6
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    ok I understand what you are saying. This is just for practice but I would like to learn to code properly, so I will definetly look into vector arrays. However I would like to try to get it working with dynamic arrays first, if not I'll just use vector arrays.

    I have updated the code and now it will compile but I get a segmentation fault.

    Code:
    #include<iostream>
    #include<string>
    #include<cmath>
    #include <iomanip>
    
    using namespace std;
    
    void transferTime ( int hddSpeeds[], float sizeOfDir, float transferRate[], int control );
    
    int main(int argc, char *argv[])
    {	
    	int control;
    	control = atoi( argv[2] );
    	string hddNames[control];
    	float transferRate[control];
    	int hddSpeeds[control];
    	float sizeOfDir;
    	float counter = 0;
    	int fastest;
    	for ( int loop = 0; loop < control; loop++ )
    	{//open loop
    		cout<<"Please enter a harddrive name to be compared: ";
    		cin>> hddNames[loop];
    		cout<<"Please enter the transfer speed of that drive in megabits per second: ";
    		cin>> hddSpeeds[loop];
    	}//end loop
    	cout<<"Please enter the size of the directory being transferred in Gigabytes: ";
    	cin>> sizeOfDir;
    	transferTime( hddSpeeds, sizeOfDir, transferRate, control );
    	for ( int loop = 0; loop < control; loop++ )
    	{//open loop
    		if ( counter < transferRate[loop] )
    		{//open if
    			 fastest = loop;
    		}//close if
    	}//close loop
    	float topspeed;
    	topspeed = transferRate[fastest];
    	cout<<"Hard Drive\tTransfer rate (Mbps)\tDirectory Size(GB)\tTransfer time(s)\n";
    	for (int loop = 0; loop < control; loop++ )
    	{//open loop
    		cout<< hddNames[loop] << "\t\t";
    		cout<< hddSpeeds[loop] << "\t\t\t";
    		cout<< sizeOfDir << "\t\t\t";
    		cout<< std::fixed << setprecision(1) << topspeed << "\n";
    	}//close loop
    	cout<<"\n";
    	cout<<"The fastest of the given Hard Drives was: \n";
    	cout<<"Hard Drive\tTransfer rate (Mbps)\tDirectory Size(Gb)\tTransfer time(s)\n";
    	cout<< hddNames[fastest] << "\t\t";
    	cout<< hddSpeeds[fastest] << "\t\t\t"; 
    	cout<< sizeOfDir << "\t\t\t"; 
    	cout<< std::fixed << setprecision(1) << topspeed << "\n";
    }
    
    void transferTime ( int hddSpeeds[], float sizeOfDir, float transferRate[], int control )
    {
    	for (int loop = 0; loop < control; loop++ )
    	{//open loop
    		int dummy = hddSpeeds[loop];
    		transferRate[loop] = sizeOfDir * 1024 * 8/dummy;
    	}//close loop
    	return;
    }
    here is the error, this is received when I try to execute.

    Code:
    Segmentation fault (core dumped)

  7. #7
    CSharpener vart's Avatar
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    check argc value before accessing argv[2]
    check control value before using it
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  8. #8
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    Sorry I don't understand what you mean by check? Print them out?

  9. #9
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    Make sure they exist and are valid. You can do this by printing them out or using a debugger.

    Also, you could mention what command line arguments you are giving your program. It will most likely crash on anything where the second argument does not exist or is non-numeric.

  10. #10
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    something like:
    Code:
    if(argc!=2) {
       cout << "Wrong arguments specified\n";
       exit(0);
    }

  11. #11
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    argc has to be greater than or equal to 3 for argv[2] to be valid.
    Last edited by Daved; 11-01-2006 at 11:57 PM. Reason: correction from vart

  12. #12
    CSharpener vart's Avatar
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    not less than...
    Code:
    int control = 0;
    if (argc >= 3)
       control = atoi(argv[2]);
    
    if (control >= 0)
    {
       //do your stuff
    }
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  13. #13
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    True.

    Also, I think you'd want control to be greater than (and not equal to) 0 if you are using it to size your arrays.

  14. #14
    CSharpener vart's Avatar
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    true - my fault. Probably was still half sleeping
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  15. #15
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    Thanks guys, I got it working. However, I have to do the following './speedcalc2 - 5' I must include the '-' to make argc to 3 is there no way around this? or can I make c++ use '-5' instead?

    Thanks in advance.

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