How to get the previous value from incrementing variable in for loop?

Question

In My last post I asked : How to create dynamic incrementing variable using "for" loop in php? like wise: $track_1,$track_2,$track_3,$track_4..... so on....

whose answer I selected as

for($i = 0; $i < 10; $i++) {
  $name = "track_$i";
  $$name = 'hello';
}

and

for($i = 0; $i < 10; $i++) {
    ${'track_' . $i} = 'val'
}

Now, What If I need the Value of variable previous than the current variable?

Answer 1

if ($i != 0)
{
  $prev = ${'track_' . ($i-1)} ;
}

?

Answer 2

I guess the simples way would be to use two variables.

$name2 = "track_0";
for($i = 1; $i < 10; $i++) {
  $name1 = $name2;
  $name2 = "track_$i";
  $$name1 = 'hello_previous';
  $$name2 = 'hello_this';
}

Or if you explicitly use i = [0...10] to generate a variable name, you could simply write $name2 = "track_". $i; $name1 = "track_" . ($i - 1);

Answer 3

for($i = 0; $i < 10; $i++) {
    ${'track_' . $i} = 'val'
    if($i != 0){
        $prev_val = ${'track_' . ($i - 1)}
    }
}

But it's much better to use arrays for this, which are meant for this application.

$tracks = array();
for($i = 0; $i < 10; $i++){
    $tracks[$i] = 'val';
    if($i != 0){
        $prev_val = $tracks[$i-1];
    }
}

Answer 4

I know the others are saying just subtract by 1, but what if your list goes 1, 2, 4, 5, 8, 9? The previous of 8 is not 7, but 5, this following method (with a bit of modification to work as you require) will provide a way of getting the true previous value, and not a guessed one.

for($i = 0; $i < 10; $i++) {
    ${'track_' . $i} = 'val'
    if(!empty($last_val))
         // do what you want here

    // set a var to store the last value
    $last_val=$i;
}

Answer 5

${'track_' . ($i-1)}; won't suffice?