PHP script that calculates powers that are < X - php

Question

I have a problem with my PHP script for calculating powers.
$A = $_POST["a"]; //Base number
$B = $_POST["b"]; //Value that POW of $A can't exceed. for example if A = 2, B = 1000, then highest solution = 512
for ($i = 1; $A <= $B; $i++) {
echo $i . ". ";
echo pow($A, $i);
echo '<br>';
}
With the example i mentioned above, i know this script would still display 1024 but the problem is it doesn't work at all. I tried doing this many other ways and nothing works for me... please any guidance is really appreciated.

Answer 1

Let's take the example of $A = 2; $B = 10; so you'd expect to get 2, 4, 8.
Your for loop has as condition, execute as long as $A <= $B.
Let us manually do some iterations:
Run 1: $A: 2, $B: 10, ($A <= $B): true, $i: 1, output: "1. 2<br>"
Run 2: $A: 2, $B: 10, ($A <= $B): true, $i: 2, output: "1. 4<br>"
Run 3: $A: 2, $B: 10, ($A <= $B): true, $i: 3, output: "1. 8<br>"
Run 4: $A: 2, $B: 10, ($A <= $B): true, $i: 4, output: "1. 16<br>"
Run 5: $A: 2, $B: 10, ($A <= $B): true, $i: 5, output: "1. 32<br>"
Run X: $A: 2, $B: 10, ($A <= $B): true, $i: X, output: "1. 2^X<br>"
As we can see, the tested condition $A <= $B is always true. How to fix this? One option is to update variable $A, like this:
$A = $_POST["a"]; //Base number
$B = $_POST["b"]; //Value that POW of $A can't exceed. for example if A = 2, B = 1000, then highest solution = 512
for ($i = 1; $A <= $B; $i++) {
$A = pow($A, $i);
if ($A <= $B) {
echo $i . ". ";
echo $A;
echo '<br>';
}
}
Another option would be to use a break condition:
$A = $_POST["a"]; //Base number
$B = $_POST["b"]; //Value that POW of $A can't exceed. for example if A = 2, B = 1000, then highest solution = 512
for ($i = 1; true; $i++) {
$A = pow($A, $i);
if ($A > $B) {
break;
}
echo $i . ". ";
echo $A;
echo '<br>';
}
This is an infinite loop, which usually gets written as
$A = $_POST["a"]; //Base number
$B = $_POST["b"]; //Value that POW of $A can't exceed. for example if A = 2, B = 1000, then highest solution = 512
$i = 1;
while (true) {
if ($A > $B) {
break;
}
echo $i . ". ";
$A = pow($A, $i);
echo $A;
echo '<br>';
$i++;
}
One last thing, since $A and $B get submitted by a client, you shouldn't assume they're numbers. You can simply cast them to numbers using:
$A = (int) $_POST['a'];
$B = (int) $_POST['b'];
How I personally would write that code:
// get variables
$base = (int) $_POST["a"];
$max = (int) $_POST["b"];
// validate input
if ($max < $base) {
echo 'error: Max number bigger than base number!<br>';
} elseif ($base < 2) {
echo 'error: Base number needs to be >= 2!<br>';
}
/** SOLUTION 1 - using `pow` */
// starting exponent
$exp = 1;
// assign $base^$exp to $result and test it against $max
while (($result = pow($base, $exp)) <= $max) {
// echo current row
echo $exp.'. '.$result.'<br>';
// increment exponent
$exp++;
}
/** SOLUTION 2 - using dynamic programming instead of calling `pow` in each iteration */
// multiply $result with the base and check against $max
for ($result = 1, $exp = 1; ($result *= $base) <= $max; $exp++) {
echo $exp.'. '.$result.'<br>';
}
/** In case you don't need the exponent:
for ($result = 1; ($result *= $base) <= $max;) {
echo $result.'<br>';
}
*/
Try to pick variables that explain what they're doing and add comments to explain what's happening.

Answer 2

$C = 1;
for ($i = 1; $C <= $B; $i++) {
$C = pow($A, $i);
if ($C <= $B) {
echo $i . ". " . $C . '<br>';
} else {
break;
}
}
Just added a variable $C that changes each time the loop runs so it can actually finish.