Using an associative array to calculate an average grade - php

Question

If I have an average score of:
$average = 95.00000000
And I have an array:
$grades = array("91-100"=>"A+","80-89"=>"A","70-79"=>"B","60-69"=>"C","50-59"=>"D","0-49"=>"F");
When I try to get an average grade by doing:
$grade = $grades[$average];
I get an error:
Notice: Undefined index: 95.00000000
I think the issue comes from the the key's of the array, but is there a way to do what i'm trying to achieve?

Answer 1

You have to iterate over the keys, and check if your value is between them :
$grades = array("91-100"=>"A+","80-89"=>"A","70-79"=>"B","60-69"=>"C","50-59"=>"D","0-49"=>"F");
$average = 95.00000000 ;
$grade = '' ;
foreach ($grades as $val => $cur_grade) {
list($min, $max) = explode('-', $val); // split key into min and max
if ($average >= $min && $average <= $max) { // compare
$grade = $cur_grade ; // get the value
break ; // stop the loop
}
}
echo $grade ;
Will outputs :
A+
Note that if your $average is not in the range (ex. 69.9), it will match will no case. So you could use "90-100", "80-90", ...
$grades = array("90-100"=>"A+","80-90"=>"A","70-80"=>"B","60-70"=>"C","50-60"=>"D","0-50"=>"F");
$average = 69.9 ;
// ..code above..
echo $grade ; // Outputs "C"
And
$average = 70.0 ;
// ..code above..
echo $grade ; // Outputs "B"

Answer 2

I would suggest changing the grade array to a simpler structure, thus you would get a simpler and more predictable code
<?php
$average = 95.00000000;
$grades = array(
array(
'grade' => 'A+',
'max' => 100,
'min' => 90
),
array(
'grade' => 'A',
'max' => 89,
'min' => 80
),
array(
'grade' => 'B',
'max' => 79,
'min' => 70
),
array(
'grade' => 'C',
'max' => 69,
'min' => 60
),
array(
'grade' => 'D',
'max' => 59,
'min' => 50
),
array(
'grade' => 'F',
'max' => 49,
'min' => 0
),
);
$result = null;
$averageScore = (int) floor($average); // it's better to compare int to int instead of float to int
foreach($grades as $grade) {
if ($average < $grade['max'] && $average >= $grade['min']) {
$result = $grade['grade'];
break;
}
}
if ($result !== null) {
echo 'Your grade is: ' . $result;
} else {
echo 'Grading error, please ask your professor for details!';
}