Thread: Merging into a number.

bah, I think I am destined to never get this. I spent some time dissecting that loop and trying to understand it but I dont get it :/

I dont understand the "dash - " and I dont really understand how it solves my problem.

Looking through what the atoi() function does, I think I need exactly that (not sure though). Where can I see the source code for it?

In the end this is just a curious question, and it its too hard for me I might as well dump it :/

Ohhh! I see, yes. Thank you! But what if I dont know the size of the inputted number? It could be 2,3,4,5 digits long (Its an int so 5 would be max). How would I determine that?

Originally Posted by +Azazel+

bah, I think I am destined to never get this. I spent some time dissecting that loop and trying to understand it but I dont get it :/

You have to realize that every base-10 number(the numbering system that we use) is composed of ones, tens, hundres, thousands, etc.

Eg: 1234 = (1 * 1000) + (2 * 100) + (3 * 10) + (4 * 1).

Just try that in the calculator, or by hand

Code:

   
   1000
  + 200
  +  30
  +   4
-----------
  =1234

Yes?

If you put that number into the function posted:

Code:

result = 0;
for(i = 0; i < n; i++)
        result = result * 10 + (arr[i] - '0');

There will be 4 steps, as there are 4 digits:

Code:

result = 0   * 10 + 1 = 0    + 1 = 1
result = 1   * 10 + 2 = 10   + 2 = 12
result = 12  * 10 + 3 = 120  + 3 = 123
result = 123 * 10 + 4 = 1230 + 4 = 1234
result = 1234

If you understand all this, but don't understand (arr[i] - '0'), read on:

In essence, all things in a computer are numbers. Your compiler and C programs 'convert' chars to integers, as integers are native to the computer(Which is why, when possible, always use ints instead of char). With this in mind, you can look at an ascii chart and find the integer value for for 0(zero), which is 48.
Now, if you look at the ASCII chart at take a random number on it, 1 for example, you will see that its integer value is 49, right after 0. If you subtract 48 from 49, you get 1, which is why the algorithm he posted works.

FYI, ASCII stands for American Standard Code (for) Information Interchange. Basically, this standard simply controls which integers correspond to which letters. The ASCII table lists the characters and what their integer, hex and octal equivalents are.

There are numerous other standards though, such as EBCDIC for example, and there are also other ASCII sets(Often called Extended ASCII sets) which define the sets for various foreign alphabets.

Any further questions? - Google, then ask.

Alright. Well Im almost done but one little problem.

char array[5] = {'3','5','0','7','0'};

If I do sizeof(array) it gives me "5". (Correct)

But if I do

Code:

int my_getnbr(char *str)
{
    int z = sizeof(*str);
    printf("%d", z);
}

It gives me 1. Why and how would I find the sizeof array through a pointer?