CODEWITHSUNDEEP
phpmath
user947462
user947462

Reputation: 949

bad value in calculation output

The following code outputs "3". I was expecting "1".

echo $resultado."\n"; // show 2
$valor = $resultado * ($resultado - 1 / 2);

echo $valor."\n"; // show 3, and should be 1

Why does this happen?

Upvotes: 0

Views: 52

Answers (7)

UnhandledExcepSean
UnhandledExcepSean

Reputation: 12804

2*(2-1/2)

The dividing operator has higher order operator precedence than the minus sign, so the computer will calculate it like this: 2*(2-(1/2)) = 2 * 1.5 = 3

Use parentheses liberally.

Upvotes: 0

Draiken
Draiken

Reputation: 3815

The / takes precedence over + or - To get 1 as a result you need to use

$resultado * (($resultado - 1) / 2)

Upvotes: 1

DaveRandom
DaveRandom

Reputation: 88647

Change it to:

echo $resultado."\n";
$valor = $resultado * (($resultado - 1) / 2);

echo $valor."\n";

You were effectively doing 2 * (2 - (1 / 2) = 2 * 1.5 = 3

Upvotes: 0

Dan W
Dan W

Reputation: 5782

Because the division 1 / 2 takes precedence in the order of operations. So you have really have this expression:

$resultado * ($resaltudo - (1 / 2))

You should add parenthesis to be:

$resultado * (($resaltudo - 1) / 2)

to get the answer you want.

Upvotes: 3

Andre
Andre

Reputation: 3181

That's because the division operator (/) has a higher precedence than the subtraction operator (-).

Your expression becomes, in order:

1 / 2 = 0.5 // Executed first since it's the highest precedence operation inside ()
$resultado - 0.5 = 1.5 // Still in the ()
$resultado * 1.5 = 3 // Final result

To correct your expression, insert parethesis around the subtraction, like this:

$resultado * (($resultado - 1) / 2);

Upvotes: 1

Niet the Dark Absol
Niet the Dark Absol

Reputation: 324620

Replacing $resultado in the expression, you get:

$valor = 2 * (2 - 1 / 2);

2 - 1 / 2 = 1.5
2 * 1.5 = 3

My suggestion is review basic math ;)

Upvotes: 0

Aurelio De Rosa
Aurelio De Rosa

Reputation: 22152

No, you're wrong. The / has priority on - and so your line is like:

$valor = $resultado * ($resultado - (1 / 2));

and that is:

$valor = 2 * (2 - 0.5); // and so $valor = 3

Upvotes: 1

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