php float comparison not working - php

Question

I want to change the students marks to GP so I have code like this
$mark=80;
if($mark>=84.50 && $mark<=100){$GP = 4;}
if($mark>=79.50 && $mark<=84.49){$GP = 3.67;}
if($mark>=74.50 && $mark<=79.49){$GP = 3.33;}
if($mark>=69.50 && $mark<=74.49){$GP = 3;}
if($mark>=64.50 && $mark<=69.49){$GP = 2.50;}
if($mark>=59.50 && $mark<=64.49){$GP = 2;}
if($mark>=54.50 && $mark<=59.49){$GP = 1.50;}
if($mark>=49.50 && $mark<=54.49){$GP = 1;}
else { $GP = 0; }
echo $GP;
Expected output here is 3.67 but here I am getting 0. Can anyone explain why I am getting 0?

Answer 1

Each if is evaluated independently. Regardless of what happens with the first couple of ifs, you reach the last one, and since $mark isn't between 49.50 and 54.49, the code enters the else block, and sets $GP to 0. If you replace all the ifs except the first one with else ifs you'll get the intended logic:
$mark=80;
if($mark>=84.50 && $mark<=100){$GP = 4;}
else if($mark>=79.50 && $mark<=84.49){$GP = 3.67;}
else if($mark>=74.50 && $mark<=79.49){$GP = 3.33;}
else if($mark>=69.50 && $mark<=74.49){$GP = 3;}
else if($mark>=64.50 && $mark<=69.49){$GP = 2.50;}
else if($mark>=59.50 && $mark<=64.49){$GP = 2;}
else if($mark>=54.50 && $mark<=59.49){$GP = 1.50;}
else if($mark>=49.50 && $mark<=54.49){$GP = 1;}
else { $GP = 0; }
echo $GP;

Answer 2

Your code fails because your else only applies to the last condition. That means all the previous if statements have come to nothing.
You should use elseif to make sure that the conditions are truly distinct:
$mark=80;
if($mark>=84.50 $GP = 4;
elseif($mark>=79.50) $GP = 3.67;
elseif($mark>=74.50) $GP = 3.33;
elseif($mark>=69.50) $GP = 3;
elseif($mark>=64.50) $GP = 2.50;
elseif($mark>=59.50) $GP = 2;
elseif($mark>=54.50) $GP = 1.50;
elseif($mark>=49.50) $GP = 1;
else $GP = 0;
echo $GP;
As you see, you can also dispense with the && additional condition. Finally, you don’t need braces ({}) around a simple statement.

Answer 3

<?php
$mark=80;
if($mark>=84.50 && $mark<=100){$GP = 4;}
elseif(abs($mark>=79.50 && $mark<=84.49)){$GP = 3.67;}
elseif($mark>=74.50 && $mark<=79.49){$GP = 3.33;}
elseif($mark>=69.50 && $mark<=74.49){$GP = 3;}
elseif($mark>=64.50 && $mark<=69.49){$GP = 2.50;}
elseif($mark>=59.50 && $mark<=64.49){$GP = 2;}
elseif($mark>=54.50 && $mark<=59.49){$GP = 1.50;}
elseif($mark>=49.50 && $mark<=54.49){$GP = 1;}
else { $GP = 0; }
echo $GP;
?>
Your comparision not in proper way check above code.