How to prepend a decremented number to a string in a loop?

Question

I am trying to make a triangular-shaped set of lines of decreasing numbers like this :

5
45
345
2345
12345

I tried this :

for($i=1;$i<=5;$i++)
{
    for($j=1;$j<=$i;$j++)
    {
        echo $j;
    }
    echo "<br>";
}

But it is printing the low number first and appending increasing numbers like this :

1
12
123
1234
12345

Answer 1

The inner loop needs to count down instead of up.

You can either subtract the outer loop's variable from the limit to get the starting point and count down:

for ($i = 0; $i < 5; $i++)
{
    for ($j = 5 - $i; $j > 0; $j--)
    {
        echo $j;
    }
    echo "<br>";
}

or change the outer loop to count down from the limit as well.

for ($i = 5; $i >= 1; $i--)
{
    for ($j = $i; $j >= 1; $j--)
    {
        echo $j;
    }
    echo "<br>";
}

Answer 2

This is pretty straightforward:

$max = 5;

echo "<pre>";

for($line=0; $line<$max; $line++) {
    $min_this_line = $max-$line;
    for($num = $min_this_line; $num <= $max; $num++) {
        echo $num;
    }
    echo "\n";
}

echo "</pre>";

Output:

5
45
345
2345
12345

Answer 3

I think I would declare the $peak value, then use a for() loop to decrement the counter down to 1, and use implode() and range() to build the respective strings inside the loop.

This isn't going to outperform two for() loops, but for relatively small $peak values, no one is going to notice any performance hit.

Code: (Demo)

$peak = 5;

for ($i = $peak; $i; --$i) {
    echo implode(range($i, $peak)) , "\n";
}

or with two loops: (Demo)

Decrement the outer loop and increment the inner loop.

$peak = 5;

for ($i = $peak; $i; --$i) {
    for ($n = $i; $n <= $peak; ++$n) {
        echo $n;
    }
    echo "\n";
}

Both output:

5
45
345
2345
12345