Read these codes, my answer is 1234, but it's wrong. Can anyone point out my mistakes?

Question

What does the following code display?

$numbers = array(1,2,3,4); $total = count($numbers); $sum = 0; $output = ""; $loop = 0;

foreach($numbers as $number) { $loop = $loop + 1; if ($loop < $total) { $output = $number . $output; } }

echo $output;

?>

Answer 1 · 2018-12-01T17:01:22Z

December 1, 2018 5:01pm

The actual output is "321" not "1234"

You have 2 mistakes that pop out.

The loop checks for less than 4, not less than or equal to 4, so it never processes the 4th array entry
The concat is putting the current value of $number before the previous values - $output = $number . $output; .

If you run the following code with some extra echo statements, hopefully you can see the inner details:

<?php
$numbers = array(1,2,3,4); 
$total = count($numbers); 
$sum = 0; 
$output = ""; 
$loop = 0;

foreach($numbers as $number) { 
    echo "<br>In For Each, number is $number";
    $loop = $loop + 1; 
    if ($loop < $total) { 
        echo "<br>In If loop is $loop";
        echo "<br>In If total is $total";
        echo "<br>In If, output before is $output";
        $output = $number . $output;
        echo "<br>In If, output after is $output";
    } 
}
echo "<br> <br>";
echo $output;

?>

Which produces the following output - notice the final output is "321" not "1234":

In For Each, number is 1
In If loop is 1
In If total is 4
In If, output before is 
In If, output after is 1
In For Each, number is 2
In If loop is 2
In If total is 4
In If, output before is 1
In If, output after is 21
In For Each, number is 3
In If loop is 3
In If total is 4
In If, output before is 21
In If, output after is 321
In For Each, number is 4

321

Answer 2 · 2018-12-01T16:51:01Z

December 1, 2018 4:51pm

Hi Amy,

I'll try to break down the stages of the loop, so you can understand what is happening.

First time around the loop:

<?php

$output = ""
$total = 4;
$loop = 0 + 1; // 1


// is $loop < $total (is 1 less than 4)? Yes, so add $number to the beginning of $output (known as prepending)
// $output = "1"
// Go around the loop again.

?>

Second time around the loop:

<?php

$output = "1"
$total = 4;
$loop = 1 + 1; // 2
// Is $loop < $total (is 2 less than 4)? Yes, so prepend $number to $output, 
//$output = "21"
// Go around the loop again.

?>

3rd Time around the loop

<?php

$output = "21"
$total = 4;
$loop = 2 + 1 // 3

// Is $loop < $total (is 3 less than 4)? Yes, so prepend $number to $output
// $output = "321"
// Go around the loop again.

?>

4th Time around the loop

<?php

$output = "321"
$total = 4;
$loop = 3 + 1 // 4
// Is $loop < $total (is 4 less then 4)? No, so exit the loop. 
// Note: We don't output anything this time, because we have exited the loop before reaching the echo statement.

?>

Finally we echo out $output, which contains the string "321"

The tricky thing here, besides the loop itself, is the way we prepend (rather than append) $number to the $output string

If you were to change the line

$output = $number . $output;

to

$output = $output . $number

You would get "123" instead. As the number will be added to the end of the string, rather than the beginning.

I added some echo statements to the code, so you can see what is happening each time around the loop:

<?php

$numbers = array(1,2,3,4);
$total = count($numbers);
$sum = 0;
$output = "";
$loop = 0;

foreach($numbers as $number) {
    $loop = $loop + 1;
    if ($loop < $total) {
        $output = $number . $output;
       echo "\$loop = " . $loop . ", ";
       echo "\$number = " . $number . ", ";
       echo "\$total = " . $total . ", ";
       echo "\$output = " . $output . ", ";
       echo "<br/>";
    }
}

echo $output;

?>

Hope this helps :)

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Amy Hsieh

Amy Hsieh

Read these codes, my answer is 1234, but it's wrong. Can anyone point out my mistakes?

2 Answers

Dave StSomeWhere

Dave StSomeWhere

James Anwyl

James Anwyl