Hi all, I've just started to learn C and it's going pretty slow...I wanted to write a program that takes in an integer argument and returns it's doubled value (aka take in integer, multiply by 2, and printf that value).

I purposely did not want to use the scanf function. Here's what I have so far and what is not compiling...

#include <stdio.h>

int main(int index)
{
    if (!(index)) {
        printf("No index given");
        return 1;
    }
    a = index*2;
    printf("Mult by 2 %d",a);

    return 0;

}

So basically when the program is executed I want to supply the index integer. So, in cygwin, I would write something like ./a 10 and 10 would be stored into the index variable. Also, I want to program to return "No index given" and exit if no index value was supplied...

Anyone care to help what I'm doing wrong?

EDIT:

This code returns 1 error upon compilation and is based on the help by @James:

#include <stdio.h>

int main(int 1, char index)
{
    int index, a;
    if (!(index)) {
        printf("No index given");
        return 1;
    }
    a = index*2;
    printf("Mult by 2 %d",a);

    return 0;

}

Thanks! Amit