Reverse string without strrev

Question

Some time ago during a job interview I got the task to reverse a string in PHP without using strrev.

My first solution was something like this:

Answer 1

you can try this..

$string = "NASEEM";
$total_word = strlen($string);      
for($i=0; $i<=$total_word; $i++)
{
   echo substr($string,$total_word-$i,1);
}

Answer 2

PHP strings are kinda-sorta mutable, but due to copy-on-write it's very difficult to modify them in-place without a copy being made. Some of the above solutions work, but only because they're stand-alone; some already fail because they define a function without a pass-by-reference argument. To get the code to actually operate in-place in a larger program, you'd need to pay careful attention to assignments, function arguments, and scopes.

Example:

$string1 = 'abc';
$string2 = $string1;
$string1[0] = 'b';
print("$string1, $string2");

> "abc, bbc"

I suppose that if between initializing the variable and modifying it you only ever used by-reference assignments (&=) and reference arguments (function rev(&$string)) (or assign the string to an object property initially, and then never assign it to any other variable), you might be able to change the original value of the string without making any copies. That's a bit ridiculous, however, and I'd assume that the interviewer who came up with that question didn't know about copy-on-write.

This isn't quite the same as immutability in other languages, by the way, because it applies to arrays too:

$a = [0, 1, 2];
$b = $a;
$b[0] = 1;
print(implode($a).implode($b));

> "012112"

To sum up, all types (except for objects as of PHP5) are assigned with copy-on-write unless you specifically use the &= operator. The assignment doesn't copy them, but unlike most other languages (C, Java, Python...) that either change the original value (arrays) or don't allow write access at all (strings), PHP will silently create a copy before making any changes.

Of course, if you switched to a language with more conventional pointers and also switched to byte arrays instead of strings, you could use XOR to swap each pair of characters in place:

for i = 0 ... string.length / 2:
    string[i] ^= string[string.length-1-i] 
    string[string.length-1-i] ^= string[i]
    string[i] ^= string[string.length-1-i]

Answer 3

You can use the fact that in PHP a string can be thought of as an array of characters.

Then basically what you want to do is to replace each character $i on the left side of the middle of the string with the character $j on the right side of the middle with the same distance.

For example, in a string of seven characters the middle character is on position 3. The character on position 0 (distance 3) needs to be swapped with the character on position 6 (3 + 3), the character on position 1 (distance 2) needs to be swapped with the character on position 5 (3 + 2), etc.

This algorithm can be implemented as follows:

$s = 'abcdefg';

$length = strlen($s); 
for ($i = 0, $j = $length-1; $i < ($length / 2); $i++, $j--) {
    $t = $s[$i];
    $s[$i] = $s[$j];
    $s[$j] = $t;
}

var_dump($s);

Answer 4

You can use this code to reverse a string without using the reserved function in php.

Code:

<?php

function str_rev($y)// function for reversing a string by passing parameters
{
for ($x = strlen($y)-1; $x>=0; $x--) {
       $y .= $y[$x];
       $y[$x] = NULL;
 }

echo $y;

}
str_rev("I am a student");
?>

Output:

tneduts a ma I

In the above code, we have passed the value of the string as the parameter.We have performed the string reversal using for loop.

Answer 5

Here is my code to solve your problem

 <?php
$s = 'abcdefg';
for ($i = 0, $length = mb_strlen($s); $i < $length; $i++) {
    $s = $s{$i}.mb_substr($s,0,$i).mb_substr($s,$i+1);
}
var_dump($s);
?>

Answer 6

The following solution is very simple, but it does the job:

$string = 'Andreas';
$reversedString = '';

for($i = mb_strlen($string) - 1; $i >= 0; $i--){
    $reversedString .= $string[$i];
}

var_dump($reversedString) then results: string(7) "saerdnA"