argv[1]++ operation and sort

sachitha · 10-13-2005, 10:32 AM

in a C program what does the argv[1]++ operation do???
as in like in the argv++ it incements the argument value..what does argv[1]++ do?
in the command line argument set below: if i give..
sachitha such indee
after the argv[1]++ operation how do i calculate the value of argv[1] [3] for example..

in perl how do u sort an array??
@num=(32,4,16,2);
@z=sort{$b<=>$a}@num; //does this sort @num in descending order or ascending order?? and i'm not clear about what the $b and the $a are???
pl help me clear my confusions...
thanks in advace!

koodoo · 10-13-2005, 03:30 PM

The function main can be called with two arguments. The first (conventionally called argc, for argument count) is the number of command-line arguments the program was invoked with; the second (argv, for argument vector) is a pointer to an array of character strings that contains arguments, one per string.
By convention argv[0] is the name by which the program was invoked, so argc is atleast one.

So argv is a pointer to an array of pointers
argv[i] or *(argv + i) is a character pointer that points to the first character of the i+1 th command line argument.
and *argv[i] is the charcter it points to

Let us say you invoke a program "program" like this :

Code:

koodoo@knapsacker:~$ ./program sachitha such indee

Code:

argv--------------->| argv[0] |-------> ./program
                    | argv[1] |-------> sachitha
                    | argv[2] |-------> such
                    | argv[3] |-------> indee
                    |    0    |

So argv[1] points to s in sachitha and printf("%s", argv[i]) would print sachitha. If you do argv[1]++, argv[1] will now point to a in sachitha and the string output would now be achitha.
Note however the difference between the prefix and postfix notations of the ++ operator.
printf("%s", ++argv[i]) : argv[1] points to a in sachitha and achitha gets printed while in,
printf("%s", argv[i]++) : argv[1] points to a in sachitha and sachitha gets printed

Now argv[1][3] is the (3 + 1)th character in the string pointed to by argv[1]. Note argv[1][3] is the actual character and not a pointer to char
So initially argv[1] points to s in sachitha and argv[1][3] is h. If we do argv[1]++ then argv[1] points to a in achitha and argv[1][3] is i.

I do not know anything about perl so I cannot answer your second querry.
Hope this helps.

sachitha · 10-14-2005, 11:16 AM

Thank you!