Why doesnt this work?

[deadluck]

Everything works however the when I put a capital letter it gets an error in the first 3 letters aswell but i dont understand why because the statement that messes it up is about the last 3 digits ánd has nothing to do with the first 3.

  #include <iostream>
#include <exception>
using namespace std;
int main()
{
    string ID;
    int i;
    try
    {   
        cout << "enter your id (3 letters into 3 numbers: ";
        cin >> ID; 
            // if (ID.at(0&&1&&2) >= 'A' && ID.at(0&&1&&2) <= 'Z'); (not important but i tried to fix this way.
            if (isdigit (ID.at(0&&1&&2))) throw std::runtime_error ("error: letter expected");
            if (ID.at(3&&4&&5) >= 'A' && ID.at(3&&4&&5) <= 'Z' || ID.at(3&&4&&5) >= 'a' && ID.at(3&&4&&5) <= 'z') 
            throw std::runtime_error ("error: letter expected");
    
        cout << ID << endl;
    } 
    catch(std::runtime_error & e)
    {
        std::cerr << e.what() << endl;
    }
    return 0;
}

if (ID.at(3&&4&&5) >= 'A' && ID.at(3&&4&&5) <= 'Z'

thats the part that messes with the first 3 digits for some reason not allowing for a capital letter.

[mbozzi]

The expression 0&&1&&2 always evaluates to false. Similarly, the expression 3&&4&&5 always evaluates to true.

This isn't what you intended. To phrase it another way, do_something(a && b) does not mean do_something(a) && do_something(b).

[deadluck]

thankyou mbozzi that made it work but i dont fully understand why the true and false.

[jonnin]

in c++ zero is false, and everything else is not false.
so the first one is
false and true and true = false.
the second one is
true and true and true = true.

the collapsed values (false for the first, true for the second) are then passed forward.

[keanedawg]

mbozzi is correct as well as jonnin. I'll chime in my two cents as well:

You're assuming that AND statements can be used to collect elements.

Take a look at this code.

 string myString = "abc";
cout << myString.at(0&&1&&2));

You might assume that this is going to return "abc". But behind the scenes,
0&&1&&2 is actually a function which returns

true/false

This may seem unintuitive, but in C++,

true

and

1

are actually the same thing. Likewise,

false

is the same thing as

0

So what happens instead? 0&&1&&2 will be evaluated to false. You're code is going to get calculated down to the following statement.

 string myString = "abc";
cout << myString.at(0));

which means, only get the first letter in my string. So actually you're only going to print "A".

I hope that helps!

[mbozzi]

0&&1&&2 is actually a function which returns true/false

This (the built-in operator &&) is not a function (and neither is the expression 0&&1&&2). This is an important distinction because the built-in operator && evaluates its arguments in order such that short-circuiting evaluation (See: https://en.wikipedia.org/wiki/Short-circuit_evaluation ) is possible. Function arguments, however, are evaluated in unspecified order.

Also important is to note that && is left-associative, so 0&&1&&2 is equivalent to ((0 && 1) && 2).