Does an IF-Statement checks every OR operator? - c++

At the moment I´m testing around with particles and have one important question.
if (condition a || condition b || condition c)
or
if(condition a)
if(condition b)
if(condition c){
}
Which is faster?

C++ uses what is known as short-circuit expression evaluation, which means that as soon as it encounters a term which determines the final result of the expression, (regardless of what the remaining terms may evaluate to,) it will stop evaluating terms.
Since TRUE OR X is TRUE regardless of the value of X, C++ will not bother evaluating X.
However, your cascaded if statement is not equivalent to the first expression. It is equivalent to an expression with multiple ANDs not multiple ORs.

This has likely been answered somewhere else before, but C++ uses the short circuit method, that is, if any condition passes, the rest are ignored (in the case of logical or: |).
The reverse is true for logical and: & - the first condition that fails short circuits the if statement and it exits early.
Here's an example:
if (condition a || condition b || condition c) {
// This code will execute if condition a is true, condition a or b is true, or if all three are true
}
if (condition a && condition b && condition c) {
// This code will only execute if all three are true, but if a is false, it will exit early, the same can be said for b
}

Related

Else keyword in non void function in C++ [duplicate]

I am always in the habit of using if, else-if statement instead of multiple if statements.
Example:
int val = -1;
if (a == b1) {
return c1;
} else if (a == b2) {
return c2;
} ...
...
} else {
return c11;
}
How does it compare to example 2:
if (a == b1) {
return c1;
}
if (a == b2) {
return c2;
}
....
if (a == b11) {
return c11;
}
I know functionality wise they are the same. But is it best practice to do if else-if, or not? It's raised by one of my friends when I pointed out he could structure the code base differently to make it cleaner. It's already a habit for me for long but I have never asked why.
if-elseif-else statements stop doing comparisons as soon as it finds one that's true. if-if-if does every comparison. The first is more efficient.
Edit: It's been pointed out in comments that you do a return within each if block. In these cases, or in cases where control will leave the method (exceptions), there is no difference between doing multiple if statements and doing if-elseif-else statements.
However, it's best practice to use if-elseif-else anyhow. Suppose you change your code such that you don't do a return in every if block. Then, to remain efficient, you'd also have to change to an if-elseif-else idiom. Having it be if-elseif-else from the beginning saves you edits in the future, and is clearer to people reading your code (witness the misinterpretation I just gave you by doing a skim-over of your code!).
What about the case where b1 == b2? (And if a == b1 and a == b2?)
When that happens, generally speaking, the following two chunks of code will very likely have different behavior:
if (a == b1) {
/* do stuff here, and break out of the test */
}
else if (a == b2) {
/* this block is never reached */
}
and:
if (a == b1) {
/* do stuff here */
}
if (a == b2) {
/* do this stuff, as well */
}
If you want to clearly delineate functionality for the different cases, use if-else or switch-case to make one test.
If you want different functionality for multiple cases, then use multiple if blocks as separate tests.
It's not a question of "best practices" so much as defining whether you have one test or multiple tests.
The are NOT functionally equivalent.
The only way it would be functionally equivalent is if you did an "if" statement for every single possible value of a (ie: every possibly int value, as defined in limits.h in C; using INT_MIN and INT_MAX, or equivalent in Java).
The else statement allows you to cover every possible remaining value without having to write millions of "if" statements.
Also, it's better coding practice to use if...else if...else, just like how in a switch/case statement, your compiler will nag you with a warning if you don't provide a "default" case statement. This prevents you from overlooking invalid values in your program. eg:
double square_root(double x) {
if(x > 0.0f) {
return sqrt(x);
} else if(x == 0.0f) {
return x;
} else {
printf("INVALID VALUE: x must be greater than zero");
return 0.0f;
}
}
Do you want to type millions of if statements for each possible value of x in this case? Doubt it :)
Cheers!
This totally depends on the condition you're testing. In your example it will make no difference eventually but as best practice, if you want ONE of the conditions to be eventually executed then you better use if else
if (x > 1) {
System.out.println("Hello!");
}else if (x < 1) {
System.out.println("Bye!");
}
Also note that if the first condition is TRUE the second will NOT be checked at all but if you use
if (x > 1) {
System.out.println("Hello!");
}
if (x < 1) {
System.out.println("Bye!");
}
The second condition will be checked even if the first condition is TRUE. This might be resolved by the optimizer eventually but as far as I know it behaves that way. Also the first one is the one is meant to be written and behaves like this so it is always the best choice for me unless the logic requires otherwise.
if and else if is different to two consecutive if statements. In the first, when the CPU takes the first if branch the else if won't be checked. In the two consecutive if statements, even if the the first if is checked and taken, the next if will also be check and take if the the condition is true.
I tend to think that using else if is easier more robust in the face of code changes. If someone were to adjust the control flow of the function and replaces a return with side-effect or a function call with a try-catch the else-if would fail hard if all conditions are truly exclusive. It really depends to much on the exact code you are working with to make a general judgment and you need to consider the possible trade-offs with brevity.
With return statements in each if branch.
In your code, you have return statements in each of the if conditions. When you have a situation like this, there are two ways to write this. The first is how you've written it in Example 1:
if (a == b1) {
return c1;
} else if (a == b2) {
return c2;
} else {
return c11;
}
The other is as follows:
if (a == b1) {
return c1;
}
if (a == b2) {
return c2;
}
return c11; // no if or else around this return statement
These two ways of writing your code are identical.
The way you wrote your code in example 2 wouldn't compile in C++ or Java (and would be undefined behavior in C), because the compiler doesn't know that you've covered all possible values of a so it thinks there's a code path through the function that can get you to the end of the function without returning a return value.
if (a == b1) {
return c1;
}
if (a == b2) {
return c2;
}
...
if (a == b11) {
return c11;
}
// what if you set a to some value c12?
Without return statements in each if branch.
Without return statements in each if branch, your code would be functionally identical only if the following statements are true:
You don't mutate the value of a in any of the if branches.
== is an equivalence relation (in the mathematical sense) and none of the b1 thru b11 are in the same equivalence class.
== doesn't have any side effects.
To clarify further about point #2 (and also point #3):
== is always an equivalence relation in C or Java and never has side effects.
In languages that let you override the == operator, such as C++, Ruby, or Scala, the overridden == operator may not be an equivalence relation, and it may have side effects. We certainly hope that whoever overrides the == operator was sane enough to write an equivalence relation that doesn't have side effects, but there's no guarantee.
In JavaScript and certain other programming languages with loose type conversion rules, there are cases built into the language where == is not transitive, or not symmetric. (In Javascript, === is an equivalence relation.)
In terms of performance, example #1 is guaranteed not to perform any comparisons after the one that matches. It may be possible for the compiler to optimize #2 to skip the extra comparisons, but it's unlikely. In the following example, it probably can't, and if the strings are long, the extra comparisons aren't cheap.
if (strcmp(str, "b1") == 0) {
...
}
if (strcmp(str, "b2") == 0) {
...
}
if (strcmp(str, "b3") == 0) {
...
}
I prefer if/else structures, because it's much easier to evaluate all possible states of your problem in every variation together with switches. It's more robust I find and quicker to debug especially when you do multiple Boolean evaluations in a weak-typed environment such as PHP, example why elseif is bad (exaggerated for demonstration):
if(a && (c == d))
{
} elseif ( b && (!d || a))
{
} elseif ( d == a && ( b^2 > c))
{
} else {
}
This problem has beyond 4^2=16 boolean states, which is simply to demonstrate the weak-typing effects that makes things even worse. It isn't so hard to imagine a three state variable, three variable problem involved in a if ab elseif bc type of way.
Leave optimization to the compiler.
In most cases, using if-elseif-else and switch statements over if-if-if statements is more efficient (since it makes it easier for the compiler to create jump/lookup tables) and better practice since it makes your code more readable, plus the compiler makes sure you include a default case in the switch. This answer, along with this table comparing the three different statements was synthesized using other answer posts on this page as well as those of a similar SO question.
I think these code snippets are equivalent for the simple reason that you have many return statements. If you had a single return statements, you would be using else constructs that here are unnecessary.
Your comparison relies on the fact that the body of the if statements return control from the method. Otherwise, the functionality would be different.
In this case, they perform the same functionality. The latter is much easier to read and understand in my opinion and would be my choice as which to use.
They potentially do different things.
If a is equal to b1 and b2, you enter two if blocks. In the first example, you only ever enter one. I imagine the first example is faster as the compiler probably does have to check each condition sequentially as certain comparison rules may apply to the object. It may be able to optimise them out... but if you only want one to be entered, the first approach is more obvious, less likely to lead to developer mistake or inefficient code, so I'd definitely recommend that.
CanSpice's answer is correct. An additional consideration for performance is to find out which conditional occurs most often. For example, if a==b1 only occurs 1% of the time, then you get better performance by checking the other case first.
Gir Loves Tacos answer is also good. Best practice is to ensure you have all cases covered.

Using nested if statements vs logical operator && in c++

Let us say I have an integer array representing the chess pieces on a board;
int board[8][8];
In my chess game, I am currently coding a generator function that will return an integer vector of all legal moves.
Naturally, I will be using if statements
I am at a point where I need to check a certain element in the board relative to a piece on the board
For example, If I have a pawn piece;
board[row][col] == 'p';
I need to generate [row+1][col], [row+2][col] and in some cases if it can attack a piece, a change in column too.
But if a piece is on any edge of the board, board[row+1][col] will return be index out of range
For that reason I need an extra if statement.
My question is, shall i use:
if (pieceisnotonedge && board[row+1][col] == 0)
or
if (pieceisnotonedge)
{
if (board[row+1][col] == 0)
}
For the first example, if pieceisnotonedge returns false, will it also check the next condition? Because if it does, then I am in trouble.
For the first example, if pieceisnotonedge returns false, will it also
check the next condition?
No, it will not. Because the build-in logical operators do short-circuiting. From cppreference.com:
Builtin operators && and || perform short-circuit evaluation (do not
evaluate the second operand if the result is known after evaluating
the first), but overloaded operators behave like a regular function
calls and always evaluate both operands
Therefore, in
if (pieceisnotonedge && board[row+1][col] == 0)
if the pieceisnotonedge is false, the second will not be evaluated. Therefore, having a nested ifs is redundant, and you can go with the first version.
For the first example, if pieceisnotonedge returns false, will it also check the next condition?
No. It will "short-circuit" because if the first condition is false, checking the conditions after it is unnecessary. Read more here and here.
This is guranteed by the C++ standard:
7.6.14
... && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.
Note that, for || this is opposite, that is, if the first condition is "true", then checking the conditions afterwards is unnecessary
Shall i use; or...
Both are same, if you have a short if statement (with only two conditions), I would suggest using the first approach. In terms of efficiency there is no difference and you can verify this by looking at the generated assembly for both cases on godbolt
For the shown code there is no difference, both board[row+1][col] == 0 are only evaluated if pieceisnotonedge is true.
If you should use one or the other, cannot be said be in general, as it depends on other factors (like semantic, readability, …) too.
If you e.g. want to have an action that it done if either pieceisnotonedge or board[row+1][col] == 0 is false, then you probably would use &&, because then you can write:
if (pieceisnotonedge && board[row+1][col] == 0) {
} else {
// do something else
}
with your other style you would need to write:
if (pieceisnotonedge){
if (board[row+1][col] == 0) {
// do something else
}
} else {
// do something else
}
In general, your first aim should be to have readable code. While pieceisnotonedge && board[row+1][col] == 0 easy to read, more complex expressions might not be. So you would decide that on a case by case basis, if you want to use one expression or multiple if.
For the first example, if pieceisnotonedge returns false, will it also check the next condition?
No. It will stop immediately after pieceisnotonedge is evaluated to false. There is no subsequent check for the remainder condition board[row+1][col] == 0
You can use nested if as the second code as well - no difference. It's just a matter of what code would look clearer.

c++ bool concatenation with |=

I'm wondering about the way to concatenate bools.
What I sometimes need is to get a flag whether at least one object in a list of objects has a certain value/state/is focused etc..
What I do is this:
bool bHasState( false );
for ( auto o : MyObjectList )
{
bHasState |= ( o->state() == SOME_STATE );
}
Some of my colleagues always replace this with
bHasState = bHasState || ( o->state() == SOME_STATE );
Is this just a syntactic difference, or is my way wrong, dangerous or slow?
Expanding from the comments: the main difference here is that your version always evaluates the right-hand expression, while the version with || (or the proposed if version, which is essentially the same) doesn't, as || has short-circuit evaluation.1
The performance difference boils down to balancing the cost of a branch (an easily predicted one, since it's going to be always taken until you find an element that makes bHasState true, and then always not taken) with the cost of calling state() on every item, which may be extremely cheap (if it's a straight inline accessor, so the only cost is going to be a potential cache miss) or quite costly (if state() isn't inline, performs more complex calculations or - say - has to acquire a busy mutex).2
Still, if you were to decide that branching at each iteration isn't going to be so costly, probably you should go one step further: just break out of the loop when you find the first item whose state matches what you are looking for
bool bHasState( false );
for ( auto o : MyObjectList ) {
if(o->state() == SOME_STATE) {
bHasState = true;
break;
}
}
as it's true that the || branch is going to be easily predicted, but not looping at all over irrelevant items is surely going to be faster, especially it MyObjectList contains many elements.
Incidentally, the same exact semantic can be reproduced with this horrible standard library one-liner:
bool bHasState = std::any_of(MyObjectList.begin(), MyObjectList.end(),
[](MyObject const& o) { return o->state() == SOME_STATE; });
IOW, it evaluates the right hand expression only if the left hand expression is false; somebody pointed out that, if || is overloaded, it isn't short-circuit anymore, still, here I wouldn't think this is the case - from the context we seem to be dealing with regular integers.
Notice that, if the compiler can prove that calling state() has no observable side-effects (which can be trivially done if it's just an inline getter) it may transform the && to & or the opposite as it prefers, as having or not having the branch isn't technically observable (as far as the C++ standard is concerned).
The replacement is not quite the same. Do please read on:
bHasState |= ( o->state() == SOME_STATE ); is an idiomatic way of switching on a bit if the right hand side is true, or leaving it as it is if the right hand side is false. But it does require the evaluation of o->state(), which if that's expensive, the || alternative might be better.
That said, to me the way you currently have it is perfectly clear. (Personally I'd go one stage further and remove the redundant parentheses, which make the expression appear more complicated than it really is.)
But as a rule of thumb, I also wouldn't fiddle with code that works as you can introduce really subtle bugs. Currently o->state() is always evaluated, it wouldn't be if you changed it to bHasState = bHasState || ( o->state() == SOME_STATE );, bHasState was already true, and || was not overloaded. (Note that || is not short-circuited if || is overloaded.)

Is the order of boolean expressions important in while loop?

what's the different between an expresion like:
int a, b;
bool p = false;
while (cin >> a >> b and not p) { // code }
// and:
int a, b;
bool p = false;
while (not p and cin >> a >> b) { // code }
The difference is that in C++, a shortcut is followed while evaluating boolean conditions.
In case of an and, you know that if the first statement is false, the whole output is false. In this case, the second condition won't be evaluated at all. Similarly, if the first statement in an or statement evaluates to true, then the second isn't evaluated.
So why is this useful, except the small effort saved in evaluating the second statement. One of the cases is AND is wherein you may not want the second expression to be evaluated if the first one is false. A reason for this is that the second expression may be ill defined if the first statement is false.
For example, when accessing vector elements in a while loop, you may have a situation where you are checking some condition. Along with that you'd have to take care to ensure that the index is within the limits of the vector. So you can just write
while (i < vector.size() && condition(vector[i]))
You can see that if the first part evaluates to false, then the program would crash upon accessing vector[i] in the condition. However, it doesn't reach that point. This is just one of the many uses of short-circuiting operators.
Yes the order is important because of short circuit evaluation.
while (cin >> a >> b and not p)
will take in the input and if that operation succeeds then it will evaluate p. If the input fails then we end the loop and never check p. On the other hand:
while (not p and cin >> a >> b)
Will evaluate p and if not p is false then the rest of the condition will not be executed.

Boolean rules are bugging me

I just encountered a propably simple problem with booleans
If I have an if statement like this one: !(!A && !B) && B, how does it exactly go?
I thought it was the same as A && B (inverts the conditions inside the brackets), but it obviously wasn't.. which is bugging me
So, would someone like to elaborate how it really goes because I think I'm missing something here?
!(!A && !B) && B is the sams as (A || B) && B you forgot to invert the and to or. Which in turn btw is just B.
!(!A && !B) = (A || B)
so
!(!A && !B) && B = (A || B) && B = A && B || B = B
The final result is simply B
Unfortunately, in general case you cannot use logic identities in programming as you do that at discrete mathematics class.
As it was mentioned by HashPsi and Bas van Stein, from the logical point of view your expression is equivalent to B. In the real world you can have the following code:
def A():
throw StupidException()
def B():
return True
!(!A() && !B()) && B() # throws exception
B() # returns True
So technically your expression differs significantly from just B. You can argue that this is very specific example, but in practice most of your functions have some side effects, so in general you can never assume that mathematical logic works for boolean expressions.
As a real-world example where ordering of boolean expressions matters, you can consider a following idiomatic C++ code:
if (foo && foo->bar()) {do something}
The code above would check that foo is not null before calling bar method. If you just rearrange that expression, you'll get a crush:
if (foo->bar() && foo) {do something}
If foo==nullptr, foo->bar will cause termination of program, since foo->bar() will try to be called before check of foo value.