Thread: Intended false statement is seen as true

Code:

if (x<=10 && y==1 || 2 || 3)

As you've noticed, that's not the proper way to do this. You need to specify "y==" in each of the cases, for example:

Code:

if (x<=10 && (y==1 || y == 2 || y == 3))

Notice how I've added parentheses around the whole y-testing expression. If I didn't do this, since && has higher precedence than ||, the compiler would interpret it as

Code:

if ((x<=10 && y==1) || y == 2 || y == 3)

which is probably not what you want.

I didn't learn by asking questions on this board -- I learned on my own from a textbook. (Well, multiple textbooks.) I hardly used the internet at all really. So I'm not the best person to answer that question. [Strange as it may seem, I think one of the ways I learned programming was *answering* questions on CBoard. You'd be surprised how much you can learn that way. ] But I have asked questions on this board and it's a wonderful resource. I think that you need to do a lot of work on your own (reading tutorials perhaps, or textbooks, and certainly writing lots of programs), but that when you're in a bind asking a question here is one of the best ways to get an answer.

The best way to learn is to practice over and over, write as much code as you possibly can. Textbooks are only useful for a little while, but if you're just starting you may certainly find it useful.

[edit] In regards to your second question:

A few ways I can think of for testing whether an integer is in a set of integers:

• Find a formula to describe the integers. If you're testing for oddness, for example, you can use "if(i % 2 == 1)".
• Just put the expression into another function. It's amazing how much more readable code can be if it says "if(is_day_of_month(day, month))", no matter how simplistic the function might seem.
• Create a std::set<>, put the integers you want to test for into the set, and then check if your number is contained in the set. This is quite efficient.

[/edit]

An example of the last case, since it seems the most interesting:

Code:

#include <iostream>
#include <set>

int main() {
    std::set<int> special;
    
    special.insert(1);
    special.insert(5);
    special.insert(7);
    special.insert(11);
    special.insert(20);
    
    std::cout << "Enter a number: ";
    int n;
    std::cin >> n;
    
    if(special.find(n) != special.end()) {
        std::cout << "Congrats, you guessed a special number!\n";
    }
    else {
        std::cout << "That's a boring number.\n";
    }
    
    return 0;
}

(Highlighted with codeform.)

A few example runs:

Code:

$ ./isinset 
Enter a number: 4
That's a boring number.
$ ./isinset 
Enter a number: 5
Congrats, you guessed a special number!
$

For more details, read up on documentation about std::set -- for example: set - C++ Reference

I probably shouldn't have thrown std::set at you then. Sorry. By is_day_of_month() I meant you could put the expression into a separate function, but if you're not familiar with functions then that's not a very helpful suggestion. For now I'd just write your code right inside the if statement, because that's the simplest and it works.

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