I am looping through several thousands of rows from a mysql result and adding them to different arrays.
I get a reference to the last element in the array to operate on (for reasons outside the scope of this question).
I.e. (just an example of the scenario)
$myarray = array();
$result = &$myarray;
$row = &$result[count($result)-1]
Of course this works, but as this thread ( Why is calling a function (such as strlen, count etc) on a referenced value so slow? ) explains , calling a function which doesnt expect a variable by reference , With a variable by reference, causes the function to make a copy of the variable to operate on.
As my $result array grows to thousands of elements , continuously calling count on it , causing copies of it being made , makes the code unusable slow.
How can I get around this - Without suggesting to not use variables by reference.
Also, I cant keep separate rowcounters.
Basically I need to know how to dereference a by-reference variable (which I understand cant be done) , or some other way to get a array length ? , or a way to get a reference to the last row in an array (without popping it)
Bearing in mind the array could also be empty.
Already took me ages to find out that This is the problem , so I would really appreciate any help with solving the problem.
EDIT
To everyone that says that count cannot possibly be slower than end/key:
Just run this simple test to confirm
//setup array of 10 000 elements
$ar = array();
for ($i = 0 ; $i < 10000 ; $i++)
{
$ar[] = $i;
}
//get a reference to it
$ref = &$ar;
error_log (date("Y/m/d H:i:s")." : speed test 1 \r\n",3,"debug.log");
//do 10 000 counts on the referenced array
for ($i = 0 ; $i < 10000 ; $i++)
{
$size = count($ref);
}
error_log (date("Y/m/d H:i:s")." : speed test 2 \r\n",3,"debug.log");
//do 10 000 end/key on the referenced array
for ($i = 0 ; $i < 10000 ; $i++)
{
reset($ref);
end($ref);
$size = key($ref);
}
error_log (date("Y/m/d H:i:s")." : end \r\n",3,"debug.log");
Output: 15 seconds to do the counts ... less than a second to do the end/key
2012/07/10 17:25:38 : speed test 1
2012/07/10 17:25:53 : speed test 2
2012/07/10 17:25:53 : end
I'm still not sure I understand why this is necessary. However, you've indexed the array as a numerical sequential array, so you can do this "trick" to get the size:
// Make sure the array pointer is at the end of the array
end($result); // Not sure if this is necessary based on your description
$size = key($result) + 1; // Get the numeric key of the last array element
Note that both end() and key() take their parameters by reference. However, I wouldn't be surprised if they're now operating on references of references, but this is something you can investigate.
depending on your other needs, that you mentioned but didn't descripe, maybe a few of the realy oldschool (deep level) functions coud help:
end($result); // set pointer to (current) last element
$pos = key($result); //index of the last element
$row = &$result($pos);
edit: I type too slow :)
You should have a look to the SPL data structures. Maybe a simple SplHeap or SplDoublyLinkedList can fit to your requirements. Moreover, performances are pretty good!
Related
Hey everyone I have a question about arrays and loops in PHP.
For a game I'm making, I need to write a function that generates a stack of crystal_id's based on a given size and ratio.
The ratio is the ratio between black crystals and different colored crystal (so a ratio of 0,25 (1:4) and a stack of 50 would yield a stack with 40 black crystals and 10 colored crystals).
I have all the math to calculate the amount of crystals per color and stuff figured out, but I can't figure out how to create an array with the right amount of colored crystals where each color is represented equally.
For reference, the array the code gets to choose from is a variable called $crystals_array, which is an array filled with integers where each integer represents a different colored crystal (e.g. [2,3,4,5,6]).
In the above case we have 5 different colored crystals and we needed a total of 10 colored crystals where each crystal is represented equally. So I need to create an array that looks a little something like this:
[2,2,3,3,4,4,5,5,6,6].
The code I have so far is this:
for($i = 0; $i <= count($amount_crystals_color) - 1; $i++)
{
$array = array_fill(0, $amount_crystals_per_color_stack, $crystals_array[$i]);
$i++;
}
Using the above example $amount_crystals_per_color_stack is equal to 2 and amount_crystals_color is equal to 5.
When executing this code it outputs an array: [2,2] which is how many 2's we need, but I can't figure out how to add the remaining crystals to this array.
Can anyone help me out?
Your code has several problems and i will address each of them individually.
Using the for loop to iterate over an array
You are using a for loop in your code, that has the following loop head:
for($i = 0; $i <= count($crystals_array) - 1; $i++)
The loop had consists of three parts, each separated by a semicolon (;).
The first part $i = 0 is the initialization part. Here you initialize a variable, that is later used as an iterator, which shall change in each loop iteration. You could name this the start point as well.
The second part $i <= count($crystals_array) - 1 is the condition part. A condition is formed, that shall express for how long the loop shall iterate. As long as the expression evaluates as true, the loop will run again. Therefore this condition is evaluated at the start of each iteration. As soon as the condition evaluates as false the loop will end. Therefore this part can be named endpoint as well.
The third part $i++ is the step size. This is executed at the end of each iteration and determines how the iterator (the variable you defined in the first part) shall change. $i++ is in this context equal to $i = $i + 1 and represents a step size of 1. Therefore the variable $i gets increased by 1 for each run of the loop.
This said, you can improve and fix your code regarding the for loop with two changes:
Save functions, that are executed in your condition part into an variable, if they return a constant result for each iteration. You use the count() function, which will then count your array again for each iteration of the for loop. By saving it in a variable $count = count($crystals_array); before the for loop and changing the condition to $i < $count, the function is only called once and your code gets faster.
Do not change the iterator variable $i outside of your loop header. This is really bad code style. You added the line $i++; to the end of your loop, but that is already done in the step size part of the for header. Because that is executed at the and of each iteration as well you increased the step size to two, meaning that you only run the for loop with $i = 0, $i = 2 and $i = 4 instead of for each element.
For your code the usage of the $i iterator is only to address the elements of the initial array. Even though you should understand the for loop for the future, you should use a foreach loop for this case instead. The following code would be equivalent to your for loop.
//This code still contains another major bug and is jsut a partial improvvement
foreach($crystals_array as $crystal) {
$array = array_fill(0, $amount_crystals_per_color_stack, $crystal);
}
As you can see, you neither need to worry about counting the initial array, nor in which index the current value is. Instead the variable $crystal will automatically contain the next element for each iteration.
Appending elements to an array
You used the following line to save the newly generated elements in your array:
$array = array_fill(0, $amount_crystals_per_color_stack, $crystal);
If you look closely, you use a standard assignment with $array = at the beginning of your line. This means, that (like with each variable assignment) the previous value of the variable gets overwritten by the new value provided from the right side of the assignment. What you do not want yet is to overwrite the array, but to append something to it.
This can be done by adding two squared brackets to the end of the variable name: $array[] = .... Now, if the variable $array is really an array, what ever value is on the right side of the assignment will be appended to the array, instead of overwriting it.
Managing result types the right way
The following line still contains a major problem:
$array[] = array_fill(0, $amount_crystals_per_color_stack, $crystal);
The result type of array_fill() is an array itself. By appending it to the previous array, you would get the following structure:
$array = [
[2, 2],
[3, 3],
[4, 4],
[5, 5],
[6, 6],
];
As you can see, the code did exactly what it should but not what you wanted. Each result (array) was appended to the array. The result is therefore an array or arrays (or a multidimensional array). What you want instead, is that the values of the result are appended to the existing array.
PHP offers a function for that, named array_merge(). This function takes all elements for one (or more) array(s) and appends them to the end of the first array, that was given to the function. You can use it as followed:
$newCrystals = array_fill(0, $amount_crystals_per_color_stack, $crystal);
$array = array_merge($array, $newCrystals);
As you can see the latter line contains a normal assignment again. ($array =) This is because array_merge() does not modify the first array given to it, but creates a new array with the merged fields. Therefore the new array contains all values from the old one and it is safe to overwrite the old one with it.
The complete code would therefore be:
$array = [];
foreach($crystals_array as $crystal) {
$newCrystals = array_fill(0, $amount_crystals_per_color_stack, $crystal);
$array = array_merge($array, $newCrystals);
}
As I understood the problem
$crystals_array = [2,3,4,5,6];
$amount_crystals_per_color_stack = 2;
$res = [];
foreach($crystals_array as $v) {
// repeat each item from array $amount_crystals_per_color_stack times
$res = array_merge($res, array_fill(0, $amount_crystals_per_color_stack, $v));
}
print_r($res); // [2,2,3,3,4,4,5,5,6,6]
You need to merge your array at each iteration of the loop (repl online) or you lose the result each time.
Like:
$array = array();
for($i = 0; $i < count($amount_crystals_color); $i++)
{
$array = array_merge($array, array_fill(0, $amount_crystals_per_color_stack, $crystals_array[$i]);
}
Also you don't need the $i++ in the loop, because it iterate twice otherwise, and you don't need count(..)-1 if the condition is < instead of <=.
You could use simple foreach() to achieve this-
$amount_crystals_per_color_stack = 2;
$array = [2,3,4,5,6];
$result = array();
foreach($array as $a){
for($i=1;$i<=$amount_crystals_per_color_stack;$i++){
array_push($result, $a);
}
}
print_r($result);
I need to calculate the sum of the values in array where key index is lower than... something.
I have done that, by this:
$temp_sum = 0;
for($temp_start = 0; $temp_start < 10; $temp_start++)
{
$temp_sum += $array[$temp_start];
}
But my question is... Another way to do this in more fashion way?
With usage of array functions? Maybe one specific array function for this task?
This for loop (or using even foreach loop) doesn't look nice and proper - but maybe it's only way to use standard looping.
for loops allow multiple statements inside its expressions, separated by commas. So, you could instantiate $temp_sum inside the for construct, to make it a little more concise:
for($temp_sum = 0, $temp_start = 0; $temp_start < 10; $temp_start++)
{
$temp_sum += $array[$temp_start];
}
I don't know if it necessarily helps readability though. Also, you might want to make sure there's at least 10 elements in $array to begin with, if you are not already verifying this.
Another alternative could be:
$sum = array_sum( array_slice( $array, 0, 10 ) );
but this requires the array keys to start at 0 and be adjacent and sequentially sorted. In other words, this array:
$array = [
13 => 12,
31 => 23,
1 => 24,
0 => 21
/* ... */
];
will create an undesirable result.
Not sure if it's more efficient either, but it's a nice one-liner and it has the possibly added benefit that it won't complain if there's less than 10 elements inside the array. You'll have to decide if that is desirable or not.
All in all, I think your initial use of the for loop is a pretty good example of a typical use-case. foreach loops, for example, are typically used in cases where you want to iterate all elements, not just a limited set.
Please tell me which is the best way to unset middle element of associative array in PHP?
Suppose I have an array of 10,000 elements and I want to remove middle element of that array, which is efficient way to remove middle element?
$temp = array('name1'=>'value1','name2'=>'value2',...,'name10000'=>'value10000');
$middleElem = ceil(count($temp) / 2);
$i = 0;
foreach ($temp as $key=>$val) {
if ($i == $middleElem) {
unset($temp[$key]);
break;
}
$i++;
}
Is above code efficient way?
Considering $array is your array, this code remove the middle element if it has odd number of elements. If its event it'll remove the first of 2 middle elements.
$i = round(count($array)/2) - 1;
$keys = array_keys($array);
unset ($array[$keys[$i]]);
Test Result: http://ideone.com/wFEM2
The thing you have to figure out is what you want to do when you have an array with an even number of elements. What element do you want to get then?
The above code picks the 'lower' element, the code could easily be edited to make it pick the 'higher' element. The only thing you have to check is (what all others answers failed to do) what happens if you have three elements. It doesn;t pick the middle element, but the last. So you would have to add a check for that then.
$temp = Array("name1"=>"value1","name2"=>"value2",...,"name10000"=>"value10000");
$middleElem = ceil(count($temp)/2);
$keys = array_keys($temp);
$middleKey = $keys[$middleElem];
unset($temp[$middleKey]);
There ^_^
I think it's a proper way to do it. Try this:
array_remove_at( $temp, ceil(count($temp) / 2) - 1);
function array_remove_at(&$array, $index){
if (array_key_exists($index, $array)) {
array_splice($array, $index, 1);
}
}
You can find the size of the array, divide that number by two and then proceed to remove the element. Not sure about the performance isssues about that though
Firstly, I wouldn't worry too much about what is the most efficient way at this point. You're much better off coding for how easy the code is to read, debug and change. Micro-optimisations like this rarely produce great results (as they're often not the biggest bottlenecks).
Having said that, if you want a solution that is easy to read, then how about using array_splice.
$temp = array('name1'=>'value1','name2'=>'value2',...,'name10000'=>'value10000');
$middleElem = ceil(count($temp) / 2);
array_splice( $temp, $middleElem, 1 );
I would take it that the following code is more efficient, because you don't have to execute it in a loop. I generally follow the same pattern as Kolink, but my version checks if there actually is a "middle element". Works on all types of arrays, I think.
<?php
for( $i = 0; $i <= 9; $i ++ ) {
$temp['name'.$i] = 'value'.$i;
}
if( ( $count = count( $temp ) ) % 2 === 0 ) {
/** Only on uneven arrays. */
array_splice( $temp, ( ceil( $count ) / 2 ), 1 );
}
var_dump( $temp );
EDIT: Thavarith seems to be right; array_splice is much faster than simply unsetting the value. Plus, you get the added benefit of not having to use array_keys, as you already now at what $offset the middle is.
Proper way seems to be:
unset(arr[id]);
arr = array_values(arr);
For first removing element at index id and then re-indexing the array arr properly.
unset($myArray[key])
since your array is associative, you can drop any element easily this way
I have an array that reflects rebate percentages depending on the number of items ordered:
$rebates = array(
1 => 0,
3 => 10,
5 => 25,
10 => 35)
meaning that for one or two items, you get no rebate; for 3+ items you get 10%, for 5+ items 20%, for 10+ 35% and so on.
Is there an elegant, one-line way to get the correct rebate percentage for an arbitrary number of items, say 7?
Obviously, this can be solved using a simple loop: That's not what I'm looking for. I'm interested whether there is a core array or other function that I don't know of that can do this more elegantly.
I'm going to award the accepted answer a bounty of 200, but apparently, I have to wait 24 hours until I can do that. The question is solved.
Here's another one, again not short at all.
$percent = $rebates[max(array_intersect(array_keys($rebates),range(0,$items)))];
The idea is basically to get the highest key (max) which is somewhere between 0 and $items.
I think that the above one-line solutions aren't really elegant or readable. So why not use something that can really be understood by someone on first glance?
$items = NUM_OF_ITEMS;
$rabate = 0;
foreach ($rabates as $rItems => $rRabate) {
if ($rItems > $items) break;
$rabate = $rRabate;
}
This obviously needs a sorted array, but at least in your example this is given ;)
Okay, I know, you don't want the solution with the simple loop. But what about this:
while (!isset($rabates[$items])) {
--$items;
}
$rabate = $rabates[$items];
Still quite straightforward, but a little shorter. Can we do even shorter?
for (; !isset($rabates[$items]); --$items);
$rabate = $rabates[$items];
We are already getting near one line. So let's do a little bit of cheating:
for (; !isset($rabates[$items]) || 0 > $rabate = $rabates[$items]; --$items);
This is shorter then all of the approaches in other answers. It has only one disadvantage: It changes the value of $items which you may still need later on. So we could do:
for ($i = $items; !isset($rabates[$i]) || 0 > $rabate = $rabates[$i]; --$i);
That's again one character less and we keep $items.
Though I think that the last two versions are already too hacky. Better stick with this one, as it is both short and understandable:
for ($i = $items; !isset($rabates[$i]); --$i);
$rabate = $rabates[$i];
This might work without changing the rebate array.
But the array must be constructed in another way for this to work
$rebates = array(
3 => 0, //Every number below this will get this rebate
5 => 10,
10 => 25,
1000 => 35); //Arbitrary large numer to catch all
$count = $_REQUEST["count"];
$rv = $rebates[array_shift(array_filter(array_keys($rebates), function ($v) {global $count; return $v > $count;}))];
echo $rv;
Working testcase, just change count in url
http://empirium.dnet.nu/arraytest.php?count=5
http://empirium.dnet.nu/arraytest.php?count=10
Best I can manage so far:
$testValue = 7;
array_walk( $rebates, function($value, $key, &$test) { if ($key > $test[0]) unset($test[1][$key]); } array($testValue,&$rebates) );
Uses a nasty little quirk of passing by reference, and strips off any entry in the $rebates array where the key is numerically greater than $testValue... unfortunately, it still leaves lower-keyed entries, so an array_pop() would be needed to get the correct value. Note that it actively reduces the entries in the original $rebates array.
Perhaps somebody can build on this to discard the lower entries in the array.
Don't have 5.3.3 available to hand at the moment, so not tested using an anonymous function, but works (as much as it works) when using a standard callback function.
EDIT
Building on my previous one-liner, adding a second line (so probably shouldn't count):
$testValue = 7;
array_walk( $rebates, function($value, $key, &$test) { if ($key > $test[0]) unset($test[1][$key]); } array($testValue,&$rebates) );
array_walk( array_reverse($rebates,true), function($value, $key, &$test) { if ($key < $test[0]) unset($test[1][$key]); } array(array_pop(array_keys($rebates)),&$rebates) );
Now results in the $rebates array containing only a single element, being the highest break point key from the original $rebates array that is a lower key than $testValue.
Starting with an array with 10K values. I want to randomly get 1000 values from it and put them into another array.
Right now, I am using a for loop to get the values, but I want to pick 1000 values and not have to loop 1000 times. The array_slice function works, but it doesn't give random values. What is the correct (most efficient) function for this task.
The code right now is
$seedkeys = (...array.....);
for ($i=0; $i<1000; $i++) {
$random = array_rand($seedkeys);
$randseed[$i] = $seedkeys[$random];
}//for close
TIA
Well, there are a few alternatives. I'm not sure which is the fastest since you're dealing with a sizable array, but you may want to try them out:
You can use shuffle, which will randomize the entire array. This will likely have the best performance since you're consuming a significant portion of the array (10%).
shuffle($seedkeys);
$result = array_slice($seedkeys, 0, 1000);
You could use array_rand (as you already said) in the manor that Tom Haigh specifies. This will require copying the keys, so if you're dealing with a significant portion of the source array, this may not be the fastest. (Note the use of array_flip, it's needed to allow the usage of array_intersect_key:
$keys = array_flip(array_rand($seedkeys, 1000));
$result = array_intersect_key($seedkeys, $keys);
If memory is tight, the best solution (besides the MySQL one) would be a loop since it doesn't require arrays to be copied at all. Note that this will be slower, but if the array contains a lot of information, it may offset the slowness by being more memory efficient (since it only ever copies exactly what it returns)...
$result = array();
for ($i = 0; $i < 1000; $i++) {
$result[] = $seedkeys[array_rand($seedkeys)];
}
You could do it in MySQL (assuming that the data for the array starts from MySQL). Be aware this is simple, but not that efficient (See Jan Kneschke's post)...
SELECT * FROM `foo` ORDER BY RAND() LIMIT 1000;
You could use array_rand() to get multiple items ?
$random_keys = array_rand($seedkeys, 1000);
shuffle($random_keys);
This will give you an array of random keys, so to get an array of values you need to do something like this:
$result = array();
foreach ($random_keys as $rand_key) {
$result[] = $seedkeys[$rand_key];
}
You could instead use array_intersect_key():
$result = array_intersect_key($seedkeys, array_flip($random_keys));