使用命令行界面减少程序的代码？

Question

I want my program to have an interface that looks like this:

gen_data [start] [stop] [step]

The [start] , [stop] and [step] are optional and are by default set to -3*PI/2 , 3*PI/2 and 0.01 . I have the following code:

#include <stdio.h>
#include <math.h>
#include <stdlib.h>

#define PI 3.14569

int main (int argc, char **argv)
{
    float i, start, stop, step;
    printf ("# gnuplot data\n" 
            "# x sin(x) cos(x)\n");
    switch (argc) {
        case 1:
            start = -(3*PI)/2;
            stop = (3*PI)/2;
            step = 0.01;
            break;

        case 2:
            start = atof (argv[1]);
            stop = (3*PI)/2;
            step = 0.01;
            break;

        case 3:
            start = atof (argv[1]);
            stop = atof (argv[2]);
            step = 0.01;
            break;

        case 4:
            start = atof (argv[1]);
            stop = atof (argv[2]);
            step = atof (argv[3]);
            break;
    }
    for (i = start; i <= stop; i += step)
    printf ("%6f\t%6f\t%6f\n", i, sin (i), cos (i));

    return 0; 
}

As you can see all three variables start , stop and step are being assigned every time - isnt this redundant? I was roughly thinking of something like this:

• if argc = 1: set all 3 to their default values
• if argc = 2: only set start to command line arg
• if argc = 3: only set start and stop to command line args
• if argc = 4: set start , stop and step to command line args

The reason I used a switch - case is to be able to exploit fall through - but could not get it to work. Any thoughts? Is the code fine as it is?

Answer 1

That is really quite easy using a ternary . You can simply do:

    if (argc < 2) {
        fputs ("error: insufficient arguments\n"
               "usage: ./program start [stop] [step]\n", stderr);
        return 1;
    }
    
    char *endptr;
    float start = strtof (argv[1], &endptr),                         /* validation omitted */ 
          stop = argc > 2 ? strtof (argv[2], &endptr) : -3*PI/2.,
          step = argc > 3 ? strtof (argv[3], &endptr) :  3*PI/2.;
    
    /* rest of code */

( note: suggest using double and strtod() instead of float unless on a microcontroller)

That way you will optionally set stop and step if sufficient arguments are given, and if not, you will use your default values.

Avoid using atoi() , atof() in practice, they provide zero error detection and provide no indication if a failure occurs. atof() will happily take atof("my cow"); and silently fail returning 0 without you ever knowing.

Look things over and let me know if you have further questions.

Answer 2

You can set default values for all the variables first. Then set them not according to equality of argc , but by > for example

#include <stdlib.h>
#include <stdio.h>

int main (int argc, char **argv)
{
    float start = -(3*PI)/2;
    float stop (3*PI)/2;
    float step = 0.01;
    
    printf ("# gnuplot data\n"  
            "# x sin(x) cos(x)\n");
            
    if (argc > 1)
        start = atof (argv[1]);
    if (argc > 2)
        stop = atof (argv[2]);
    if (argc > 3)
        step = atof (argv[3]);

    for (float i = start; i <= stop; i += step)
        printf ("%6f\t%6f\t%6f\n", i, sin (i), cos (i));
    return 0; 
}

As mentioned, the use of float and atof leaves much to be desired, but that is another question.