How do I do this with the ternary operator on the same line?
Consider this const declaration for int num
:
int main() {
bool a = true, b = false;
// ...
const int num = a ? (b? 2 : 4) : 4;
std::cout << num;
}
I want to const int num
follow this truth table (from which I am sorry, was edited from my original question to reflect the pattern in my program):
b
a true false
true 2 4
false 4 2
How do I modify the above declaration const int num
to achieve this using the ternary operator? I know how to declare such num
as const using a lambda function and nested if statements or switch statenents inside a lambda function, but I just wanted to know how to do it using a ternary operator. As a bonus, what if you need to use 3 or more of these bool values (no specific pattern in the truth table)?
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You can write a declaration like
const int num = ( a ^ b ) ? 4 : 2;
At least it's clearer than
const int num = a ? (b? 2 : 4) : 4;
Here is a demo program
#include <iostream>
int main()
{
std::cout << ( false ^ false ? 4 : 2 ) << std::endl;
std::cout << ( true ^ false ? 4 : 2 ) << std::endl;
std::cout << ( false ^ true ? 4 : 2 ) << std::endl;
std::cout << ( true ^ true ? 4 : 2 ) << std::endl;
}
Program output
2 4 4 2
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const int num = a ? (b? 2 : 3) : (b? 4: 5);
EDIT:
As a bonus, what if you need to use 3 or more of these bool values?
I wouldn't use this syntax at all. I would declare the table as
int truthTable[2][2]..[2] = {{..},...{..}}
and now it's simple:
int res = truthTable[!true][!false]..[!true]
!true
becomes 0
, !false
becomes 1
, then the correct value will be pulled from the array.
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David Haim's answer is great for a lot of other values in the truth table. However, in this particular case, there is an easily identifiable mathematical relationship that we can use:
const int num = (a ? 2 : 4) + (b ? 0 : 1);
We may not always have such an easily identifiable relationship in a truth table.
The above expression was based on the original truth table of the question, which looked like this:
b a true false true 2 3 false 4 5
Given this truth table:
b true false true 2 4 false 4 2
Another way to return the correct values for this:
const int num (a == b) ? 2 : 4
It's not really some kind of math. It simply means that when a
and are the b
same, we want 2
, but when a
and b
are different, we want 4
. This truth table is exactly the same as the operator truth table ==
, we just return 2
/ 4
instead of true
/ false
.
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