Thread: Interesting Question

The reliability of the computer has been tested by many other people, and made in such a way as to (almost) always be right. I would stress the fact that the computer is made as a machine that listens to instructions, and has been made to specification so that it always works when built properly, just like any other machine he uses. Once people that make computers are sure it's right, then they sell them.

Naturally, I'd simplify it a little bit more, but the concept of the computer being built to spec as a machine is important since he's smart to ask the question about reliability. If the computer isn't consistent, there is no reason to use it as we do.

> so he asked "So, how do you know that the computer is right then?"
Perhaps by showing that the computer isn't right all the time, and the programmer has a big part to play in getting the "right" answer.

Code:

#include <stdlib.h>
int main ( void ) {
    unsigned char u1, u2, u3;
    signed char   s1, s2, s3;
    u1 = u2 = 100;
    u3 = u1 + u2;
    s1 = s2 = 100;
    s3 = s1 + s2;
    printf("%d\n", u3 );
    printf("%d\n", s3 );
    return 0;
}

$ gcc foo.c
$ ./a.exe
200
-56

Tell him there are thousands of miniature Indians inside the computer, and that when the computer needs to add numbers together, the Indians add them up on their fingers. They can count really, really fast, and you know they're never wrong, because they're Indians.

At least this is what my dad told me...

I'd go with Salem's answer.

I'd go with MacGyver's answer. The computer is a machine. It does the things it does in the same way every time. People have tried out the computer and have checked that it is right when adding numbers, so you know it will always be right when adding.

Salem's answer involves integer ranges, and I think that's too complex. Besides, it doesn't answer the question.

Computers do break down. Usually when they break nothing works. Anything will eventually wear out or break. I always thought that the computer was built to catch any errors that might happen for some odd reason or another. When we write code we often handle such exceptions. It is just with programs, we can usually handle it gracefully and the computer doesn't start to smoke. With hardware your computer just beeps unless it is a fatal exception and then your computer will just sit there with a blank screen until you buy a new one or fix it. Usually protective circuits are built in to chips.

If you google for "when computer chips fail", you will get a lot of interesting articles about what happens when they do.
http://www.thefreelibrary.com/Sick+c...y...-a04030992

If you go further you might get stuff about the old TV show. Whatever happend to Poncho?

I would just tell him that the computer is right as long as he can verify the answer the computer provided is correct. He will be a math wiz trying to prove the computer wrong.

[edit]
actually that might not be the best idea since we already have built in rounding errors due to the limitations of the data representation

Originally Posted by Rashakil Fol

They can count really, really fast, and you know they're never wrong, because they're Indians.

Well... that depends, sometimes the Big Indians have fights with the smaller ones and then all hell breaks loose.

01 00 00 00 is little indian format
00 00 00 01 is big indian format

Salem's answer involves integer ranges, and I think that's too complex. Besides, it doesn't answer the question.

I was never so much for the coding part, but rather the fact that he mentions that you can't be sure that the computer adds correctly, and therefore, in essence, the programmer has to convince himself that the output is expected.

Even with an operation as trivial as addition, the computer is no replacement for the human brain. Only the programmer can know what is right and what is wrong.

Originally Posted by Happy_Reaper

you can't be sure that the computer adds correctly, and therefore, in essence, the programmer has to convince himself that the output is expected.

But that's misleading. I think the original question was really whether the computer could make basic mistakes like adding 1 and 1 and getting 3. It was a 5-year-old asking the question.

The programmer can do only so much. He can check that no overflow will occur, but for the actual calculation he has to trust the CPU. What will he do? assert() that a+b == a+b?

To be honest I'd frankly tell him to try it and see if the computer gets it wrong. And if that doesn't convince him, to tell him to keep trying until he either finds the computer making a mistake, or is convinced it won't make a mistake.

Originally Posted by Swordsman

I said that the computer takes the two variables and adds them together, so he asked "So, how do you know that the computer is right then?"

Can anyone think of a way in which I could explain to him without going in to too much detail, or giving the answer "it just is" please?

Tell him you don't know it's right. See Ken Thompson's Turing Award lecture.