How to exclude arguments passed from command prompt argc argv in C? [duplicate]

问题

I need to add fractions given by the user through command prompt in the format

a/b b/c

I thought I could do it this way:

n1 = atoi(argv[1]);
d1 = atoi(argv[3]);
n2 = atoi(argv[4]);
d2 = atoi(argv[6]);

Thereby skipping the slashes, but this just crashes the program. Is there some way to skip over certain characters passed as arguments through command prompt? Thank you in advance.

回答1:

If the user writes:

add_fractions 1/2 2/3

then your program is given just 3 arguments:

argv[0] = "add_fractions"
argv[1] = "1/2"
argv[2] = "2/3"
argv[3] = 0

and argc is 3.

You would have to do something like this (after checking that you were given 2 arguments):

char *ptr;
n1 = atoi(argv[1]);
if ((ptr = strchr(argv[1], '/') != 0)
    d1 = atoi(ptr+1);
else
    d1 = 1;
n2 = atoi(argv[2]);
if ((ptr = strchr(argv[2], '/') != 0)
    d2 = atoi(ptr+1);
else
    d2 = 1;

Or you could pack the repeated sequence of statements into a function (with a slightly different twist in the logic):

void read_fraction(char const *str, int *num, int *den)
{
    char *ptr;
    *num = atoi(str);
    if ((ptr = strchr(str, '/') == 0)
        ptr = "/1";
    *den = atoi(ptr+1);
}

and in main():

read_fraction(argv[1], &n1, &d1);
read_fraction(argv[2], &n2, &d2);

You might want to think about validation; the numerator becomes 0 if there isn't a number as the first part of the string, but the denominator becomes 0 if there is a slash and there isn't a number after the slash, which might not be exactly what you want. One fix is to set the denominator to 1 if the value from atoi(ptr+1) is 0. It's a cop-out, but prevents rampant crashes. Or you could use another completely different implementation in the body of the function:

int read_fraction(char const *str, int *num, int *den)
{
    if (sscanf(str, "%d/%d", num, den) != 2)
        return -1;
    return 0;
}

And then check in main():

if (read_fraction(argv[1], &n1, &d1) == 0 &&
    read_fraction(argv[2], &n2, &d2) == 0)
    ...process valid fractions...
else
    ...make appropriate comments about how to use the program...

Using a function is 'better' from several points of view. In The Pragmatic Programmer, it is called the DRY principle: Don't Repeat Yourself.

Kernighan and Plauger summarized it neatly in their book The Elements of Programming Style:

The subroutine call permits us to summarize the irregularities in the argument list, where we can see quickly what is going on.
The subroutine itself summarizes the regularities of the code, so repeated patterns need not be used.

回答2:

a/b b/c

It just creates 2 entries in the argument list, one is a/b and another is b/c.

So, argv[0], argv[1] and argv[2] are valid and argc is 3.

Spaces are important to separate arguments (unless you wrap a entry inside "")

来源：https://stackoverflow.com/questions/19923006/how-to-exclude-arguments-passed-from-command-prompt-argc-argv-in-c

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function

parameters

argv

argc