Newbie having problems w/code in O'Reilly book

Sqrlgrl

Hello All -

I am a total newbie - web designer looking to take the next step and have been wanting to learn PHP for awhile. I recently picked up O'Reilly Learning PHP5 - have been going through the chapters and doing the exercises at the end.

In Chapter 4 there is an exercise to create an array for city populations and have it total at the end. I copied the code from the official O'Reilly site and it has errors in it. In the end it prints everything out correctly, however, for each state listed it prints out an undefined index error.

Here is the code:

<?php
// Separate the city and state name in the array so we can total by state
$population = array('New York' => array('state' => 'NY', 'pop' => 8008278),
'Los Angeles' => array('state' => 'CA', 'pop' => 3694820),
'Chicago' => array('state' => 'IL', 'pop' => 2896016),
'Houston' => array('state' => 'TX', 'pop' => 1953631),
'Philadelphia' => array('state' => 'PA', 'pop' => 1517550),
'Phoenix' => array('state' => 'AZ', 'pop' => 1321045),
'San Diego' => array('state' => 'CA', 'pop' => 1223400),
'Dallas' => array('state' => 'TX', 'pop' => 1188580),
'San Antonio' => array('state' => 'TX', 'pop' => 1144646),
'Detroit' => array('state' => 'MI', 'pop' => 951270));

// Use the $state_totals array to keep track of per-state totals
$state_totals = array();
$total_population = 0;

print "<table><tr><th>City</th><th>Population</th></tr>\n";
foreach ($population as $city => $info) {
// $info is an array with two elements: pop (city population)
// and state (state name)
$total_population += $info['pop'];
// increment the $info['state'] element in $state_totals by $info['pop']
// to keep track of the total population of state $info['state']
$state_totals[$info['state']] += $info['pop'];
print "<tr><td>$city, {$info['state']}</td><td>{$info['pop']}</td></tr>\n";

}

// Iterate through the $state_totals array to print the per-state totals
foreach ($state_totals as $state => $pop) {
print "<tr><td>$state</td><td>$pop</td>\n";
}
print "<tr><td>Total</td><td>$total_population</td></tr>\n";
print "</table>\n";
?>

I'm sure this is an easy fix - just not for a newbie 🙂

Thanks All!

drew010

if you add
$state_totals[$info['state']] = "";
before line 28 it will initialize that array index and work without errors.

Sqrlgrl

Thanks -

I knew it was something simple. Now what does that line mean in relation to the rest of the code? I see that $state_totals was created as an empty array above that so how is this line being used?

Sorry for the questions but I am really trying to understand this and not get confused. I feel that I am really starting to understand and then something like this, where I had a feeling it was probably something simple, just really gets me frustrated.

Thanks for your help!

drew010

the nice thing about php, is that variables dont need to be declared before you use them. in languages like c/c++ you must explicitly create a variable with its name and type before it can be used anywhere. as a courtesy, php can warn you if you are using an undefined variable, which is what was happening in your case.

<?php
error_reporting(E_ALL); //output all errors and warnings

$arr['test'] = $arr['test'] + 5; //php will warn us that $arr is undefined, and test is an undefined index.
?>

<?php
error_reporting(E_ERROR | E_FATAL); //only fatal errors should be shown

$arr['test'] = $arr['test'] + 5; //no errors are output here.  its assumed the programmer knows what they are doing
?>

In testing environments its best to have all errors and warnings displayed so you can create code that should run error free and warning free on most system setups.
In live environments, errors should be turned off so they dont show up on your site.

But the line I added
$state_totals[$info['state']] = "";
was basically declaring the variable before it was being used so php wouldnt warn you about it.

When you run through the loop where $info['state'] = "NY",
$state_totals[$info['state']] will translate to
$state_totals['NY']

Sqrlgrl

I think I actually understand!

A statement (if that is the correct term) was created that is populated with each state when run through the loop. This way the Undefined error won't show up.

Remember - total newbie here 😉

Thanks again!

drew010

yep you pretty much got it right. have fun reading the rest of the book.