PHP Undefined offset: -1 in comparison loop

Question

I am receiving this error Undefined offset: -1.

I am pretty sure it is because I am comparing a value of an array index with the value of the previous index, so on the first iteration its failing because there is no -1 index.

for ($i=0; $i<$rows_n; $i++) 
{
    formatTR($i, $rows, 'YEAR');
}


// Function
function formatTR($i, $rows, $year) 
{

    if ($rows[$year][$i] != $rows[$year][$i-1] ) {
        print "<tr class='header-year-tr'><td colspan='2'>{$rows[$year][$i]}</td></tr>";
    }

    print "<tr>";
}

The function checks to see if the previous value of $rows[year][i] is the same as the previous $rows[year][i-1]. If its different, it prints out a formatted < TR >.

Seems like a way to avoid this error would be to first check if the value of $i is zero, but I'm wondering if there is another way. Possibly in my if conditional.

if ($rows[$year][$i] != $rows[$year][$i-1] ) { should be if (!isset($rows[$year][$i-1]) || $rows[$year][$i] != $rows[$year][$i-1]) { — DaveRandom
– DaveRandom, Commented Aug 22, 2012 at 14:10

CoursesWeb · Accepted Answer · 2012-08-22 14:13:06Z

1

I think the easest way is to check if $i>0

if ($i > 0 && $rows[$year][$i] != $rows[$year][$i-1] ) {
  // ...
}

answered Aug 22, 2012 at 14:13

CoursesWeb

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Comments

Emil Vikström · Accepted Answer · 2012-08-22 14:10:49Z

0

"Undefined offset" is almost always an off-by-one error, and here is the culprit:

if ($rows[$year][$i] != $rows[$year][$i-1] ) {
-------------------------------------^^^^

Make sure you never call this row if $i == 0

answered Aug 22, 2012 at 14:10

Emil Vikström

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Comments

Zac · Accepted Answer · 2012-08-22 14:14:20Z

0

function formatTR($i, $rows, $year) 
{
    if($i > 0){
        if ($rows[$year][$i] != $rows[$year][$i-1] ) {
        print "<tr class='header-year-tr'><td colspan='2'>{$rows[$year][$i]}</td></tr>";
        }
    }
    print "<tr>"; //not sure why you have this here.

}

If you want $rows[$year][0] to print a <tr>:

function formatTR($i, $rows, $year) 
{
    if($i > 0){
        if ($rows[$year][$i] != $rows[$year][$i-1] ) {
          print "<tr class='header-year-tr'><td colspan='2'>{$rows[$year][$i]}</td></tr>";
        }
    } else {
        print "<tr class='header-year-tr'><td colspan='2'>{$rows[$year][$i]}</td></tr>";
    }
    print "<tr>"; //not sure why you have this here.

}

answered Aug 22, 2012 at 14:14

Zac

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1 Comment

Jeremy A Over a year ago

Thanks, I ended up using an isset check that DaveRandom Suggested, but this would have also worked well.

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