boolean interview questions
Top boolean frequently asked interview questions
An interviewer recently asked me this question: given three boolean variables, a, b, and c, return true if at least two out of the three are true.
My solution follows:
boolean atLeastTwo(boolean a, boolean b, boolean c) {
if ((a && b) || (b && c) || (a && c)) {
return true;
}
else{
return false;
}
}
He said that this can be improved further, but how?
Source: (StackOverflow)
I have seen the following macro definitions in a coding book.
#define TRUE '/'/'/'
#define FALSE '-'-'-'
There was no explanation.
Please explain to me how these will work as TRUE and FALSE.
Source: (StackOverflow)
I have tried:
$var = false;
$var = FALSE;
$var = False;
None of these work. I get the error message
Bareword "false" not allowed while "strict subs" is in use.
Source: (StackOverflow)
Since MySQL doesn't seem to have any 'boolean' datatype, which datatype do you 'abuse' for storing true/false information in MySQL? Especially in the context of writing and reading from/to a PHP-Script.
Over time I have used and seen several approaches:
- tinyint, varchar fields containing the values 0/1,
- varchar fields containing the strings '0'/'1' or 'true'/'false'
- and finally enum Fields containing the two options 'true'/'false'.
None of the above seems optimal, I tend to prefer the tinyint 0/1 variant, since automatic type conversion in PHP gives me boolean values rather simply.
So which datatype do you use, is there a type designed for boolean values which I have overlooked? Do you see any advantages/disadvantages by using one type or another?
Source: (StackOverflow)
The following shows that "0" is false in Javascript:
>>> "0" == false
true
>>> false == "0"
true
So why does the following print "ha"?
>>> if ("0") console.log("ha")
ha
Source: (StackOverflow)
How can I obtain the value of a boolean field in an SQLite database on Android?
I usually use getString(), getInt(), etc. to get the values of my fields, but there does not seem to be a getBoolean() method.
Source: (StackOverflow)
Does the C++ specification define:
- the existence of the 'less than' operator for boolean parameters, and if so,
- the result of the 4 parameter permutations?
In other words, are the results from the following operations defined by the specification?
false < false
false < true
true < false
true < true
On my setup (Centos 7, gcc 4.8.2) , the code below spits out what I'd expect (given C's history of representing false as 0 and true as 1):
false < false = false
false < true = true
true < false = false
true < true = false
Whilst I'm pretty sure most (all?) compilers will give the same output, is this legislated by the C++ specification? Or is an obfuscating, but specification-compliant compiler allowed to decide that true is less than false?
#include <iostream>
const char * s(bool a)
{
return (a ? "true" : "false");
}
void test(bool a, bool b)
{
std::cout << s(a) << " < " << s(b) << " = " << s(a < b) << std::endl;
}
int main(int argc, char* argv[])
{
test(false, false);
test(false, true);
test(true, false);
test(true, true);
return 0;
}
Source: (StackOverflow)
Java boolean allows values of true and false while Boolean allows true, false, and null. I have started to convert my booleans to Booleans. This can cause crashes in tests such as
Boolean set = null;
...
if (set) ...
while the test
if (set != null && set) ...
seems contrived and error-prone.
When, if ever, is it useful to use Booleans with null values? If never, then what are the main advantages of the wrapped object?
UPDATE:
There has been such a lot of valuable answers that I have summarised some of it in my own answer. I am at best an intermediate in Java so I have tried to show the things that I find useful. Note that the question is "incorrectly phrased" (Boolean cannot "have a null value") but I have left it in case others have the same misconception
Source: (StackOverflow)
Simple question really; is there a difference between these values (and is there a difference between BOOL and bool)? A co-worker mentioned that they evaluate to different things in Objective-C, but when I looked at the typedefs in their respective .h files, YES/TRUE/true were all defined as 1 and NO/FALSE/false were all defined as 0. Is there really any difference?
Source: (StackOverflow)
Is it guaranteed that False == 0 and True == 1, in Python? For instance, is it in any way guaranteed that the following code will always produce the same results, whatever the version of Python (both existing and, likely, future ones)?
0 == False # True
1 == True # True
['zero', 'one'][False] # is 'zero'
Any reference to the official documentation would be much appreciated!
Edit: As noted in many answers, bool inherits from int. The question can therefore be recast as: "Does the documentation officially say that programmers can rely on booleans inheriting from integers, with the values 0 and 1?". This question is relevant for writing robust code that won't fail because of implementation details!
Source: (StackOverflow)
Why does this throw NullPointerException
public static void main(String[] args) throws Exception {
Boolean b = true ? returnsNull() : false; // NPE on this line.
System.out.println(b);
}
public static Boolean returnsNull() {
return null;
}
while this doesn't
public static void main(String[] args) throws Exception {
Boolean b = true ? null : false;
System.out.println(b); // null
}
?
The solution is by the way to replace false by Boolean.FALSE to avoid null being unboxed to boolean --which isn't possible. But that isn't the question. The question is why? Are there any references in JLS which confirms this behaviour, especially of the 2nd case?
Source: (StackOverflow)