PHP - Function to assign a string to a variable with switch

Question

I have to make a function to assign a string to a variable according to the value of other variable using OOP. I created this function (calculaIMC) inside a constructor class, and to achieve that I used an array ($imc_arr). I realize my code probably looks cumbersome and even inappropriate perhaps but it's an uni exercise designed to teach specific things. That's what I came up with so far:

<?php
class CalculoIndice{
    ...
    public $imc_arr = array(
        'Magreza grave',
        'Magreza moderada',
        'Magreza leve',
        'Saud&aacute;vel',
        'Sobrepeso',
        'Obesidade Grau I',
        'Obesidade Grau II (severa)',
        'Obesidade Grau III (m&oacute;rbida)');

    function CalculoIndice(){
        $this->preparaCalculo();
        $this->calculaIMC();
    }

    function preparaCalculo(){
        ...
    }

    function calculaIMC(){
        switch ($this->imc) {
            case ($this->imc < 16):
                $this->imc_cat = $this->imc_arr[0];
                break;
            case ($this->imc < 17):
                $this->imc_cat = $this->imc_arr[1];
                break;
            case ($this->imc < 18.5):
                $this->imc_cat = $this->imc_arr[2];
                break;
            case ($this->imc < 25):
                $this->imc_cat = $this->imc_arr[3];
                break;
            case ($this->imc < 30):
                $this->imc_cat = $this->imc_arr[4];
                break;
            case ($this->imc < 35):
                $this->imc_cat = $this->imc_arr[5];
                break;
            case ($this->imc < 40):
                $this->imc_cat = $this->imc_arr[6];
                break;
            default:
                $this->imc_cat = $this->imc_arr[7];
        }
    }
}
?>

It is not working. I couldn't quite figure what is going on and what the problem is but it doesn't echo the variable (imc_cat) as I intended. I'm sure it's something simple that I am missing but I've spent a few hours looking for an answer with no success. I appreciate any insight as to what may be wrong.

First, your switch statement looks incorrect because all will equate to true. You should use rage i.e $this->imc >0 && $this->imc < 16 , $this->imc >16 && $this->imc < 18.5, $this->imc >18.5 && $this->imc < 25 — Samuel James
– Samuel James, Commented Oct 15, 2017 at 2:36

Barmar · Accepted Answer · 2017-10-15 02:39:27Z

2

You don't put conditions inside case expressions. case performs an equality test with the expression in the switch() statement, so

case ($this->imc < 16):

means

if ($this->imc == ($this->imc < 16))

You should be using if/elseif instead of switch/case.

if ($this->imc < 16) {
    $this->imc_cat = $this->imc_arr[0];
} elseif ($this->imc < 17) {
    $this->imc_cat = $this->imc_arr[1];
} ...
} else {
    $this->imc_cat = $this->imc_arr[7];
}

Actually, there's a method that some programmers use (but I don't personally endorse):

switch(true) {
    case ($this->imc < 16):
        $this->imc_cat = $this->imc_arr[0];
        break;
    case ($this->imc < 17):
        $this->imc_cat = $this->imc_arr[1];
        break;
    ...
}

answered Oct 15, 2017 at 2:39

Barmar

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1 Comment

pharaohlxvi Over a year ago

Thank you very much! That solved my problem. As I thought my mistake was a simple one, in this case not understanding properly how 'switch' works. I had seen the "true" method during my researches but it wasn't working, probably because of something else, then I completely forgot about it. It's now working. The if/else solution may be a better one, but considering it's just an exercise for university I will leave as it is. Thank you very much again!

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