I was coding PHP when suddenly Im being confused with variable scopes.
If I do the loop like this ...
function foo()
{
$ctr = 0;
for ($i = 0 ; $i > 8 ; $i++)
{
$ctr = $ctr + 1;
}
return $ctr;
}
$ctr returns a 0.
But if I do the loop like this ...
function foo()
{
$collections = //This has 7 counts;
$ctr = 0;
foreach ($collections as $collection)
{
$ctr = $ctr + 1;
}
return $ctr;
}
CTR IS RETURNING 7!
So what's is the problem in the first loop?
The for loop you are trying to do seems to be a bit wrong.
for ($i = 0 ; $i > 8 ; $i++)
^^^^^^
Means, Set $i to 0. For as long as $i is bigger than 8, do this loop then increment.
Since $i is set to 0, the condition is never met, so the loop is never executed.
Change $i > 8 to $i < 8.
Your loop condition:
for ($i = 0 ; $i > 8 ; $i++)
^^^^^^
since the loop starts at 0, you have 0 > 8, which is false, and your loop terminates immediately. Remember, loops terminate when that 2nd argument becomes false. it has to be TRUE for the loop body to execute.
The problem in the first loop might be hard to spot, I must say.
It's about the $i > 8, your code doesn't even enter the loop. Invert the operator, ($i = 0 ; $i < 8 ; $i++)
This worked for me.
Related
what I am trying to achieve here is to be able to loop from 0 to 100 (100, 98, 96, 94 ...) but has to stop at 0. What is doing right now is it passes 0 and -2 -4 which crashes the server. What am I doing wrong?
for ($i = 100; $i <= 100; $i--){
echo $i--;
echo "<br>";
}
Maybe a little explanation would be useful.
The middle part of the for loop $i <= 100 is what makes it infinite. That expression is checked before each iteration of the loop, and the loop will continue as long as that expression evaluates to true.
Since you set $ito 100 in the first section of the loop, and you're doing nothing except making it smaller, it will always be <= 100, forever.
The loop will work fine just the way you have it written if you change the continuation condition.
for ($i = 100; $i >= 0; $i--){
echo $i--;
echo "<br>";
}
That way it will continue until $i is reduced to less than zero, then $i >= 0 will be false, and the loop will end.
The third argument in for loop is what will be executed at the end of the loop. So:
for ($i = 100; $i >= 0; $i -= 2){
echo "$i<br>";
}
Will do the trick
As you can read here https://secure.php.net/manual/en/control-structures.for.php
At the end of each iteration, expr3 is evaluated (executed).
Alternatively:
<?php
foreach(range(100, 0, -2) as $n) {
echo $n;
}
for ($i = 100; $i >= 0; $i-=1){
echo $i--;
echo "<br>";
}
I figured it out somehow. Has been studying JavaScript for a year; Loop is still confusing to me
Recently, my exams got over. My last exam was based on PHP. I got the following question for my exam:
"Convert the following script using for loop without affecting the output:-"
<?php
//Convert into for loop
$x = 0;
$count = 10;
do
{
echo ($count. "<BR>");
$count = $count - 2;
$x = $x + $count;
}
while($count < 1)
echo ($x);
?>
Please help me as my computer sir is out of station and I am really puzzled by it.
Well, If I understand well, You have to use "for" loop, instead of "do...while", but the printed text must not change.
Try:
$count = 10;
$x = 0;
$firstRun = true;
for(; $count > 1 || $firstRun;) {
$firstRun = false;
echo ($count . "<BR>");
$count -= 2;
$x = $x + $count;
}
echo ($x);
By the way loop is unnecessary, because $count will be greater than 1 after the first loop, so the while will get false.
EDIT
$firstRun to avoid infiniteLoop
$count in loop
EDIT
Fixed code for new requirement
Removed unnecessary code
Hmmm I don't know if your teacher wanted to own you... but the do{} will execute only once since $count is never < 1.
The output of your teacher's code is:
10
8
I presume there was a mistake in the code and the while would be while($count > 1) which would make more sense (since it's weird to ask for a loop to output only 10 8) and would result in this output:
10
8
6
4
2
20
Then a good for() loop would have been :
$x = 0;
$count = 10;
for($i = $count; $i > 1; $i -= 2)
{
$count -= 2;
echo $i . "<br>";
$x += $count;
}
echo $x;
Which will output the same values.
If you can, ask your teacher for this, and comment the answer ^^ ahahah
I am taking a demo test at codility.com.
I tried the following PHP test code:
function solution($A) {
$min = 0;
$size = count($A)-1;
for($i=0;$i<5;$i++){
if($i=0)
$min=$A[0];
}
return $min;
}
The script takes around 3.03s to execute where they have set the maximum execution time to 2.00s.
And if i comment the FOR LOOP it works properly.
Any idea ?
You are overwritting your $i variable here:
if($i=0)
it should be
if($i==0)
because you have wirte if($i=0) it is assignment operator not comparision operator. make it correct if($i==0)
function solution($A) {
$min = 0;
$size = count($A)-1;
for($i=0;$i<5;$i++){
if($i==0)
$min=$A[0];
}
return $min;
}
Your for loop looks like this:
for($i = 0; $i < 5; $i++)
This means: initialize $i to 0, and until $i is 5 or more, execute the loop and increment $i.
But, you wrote this:
if($i = 0)
To compare $i and 0, you should've used ==, not =. This sets $i to 0. The if is not executed, as 0 is equal to false. Then $i is incremented to 1. 1 is less than 5, so the loop is executed forever.
Use if($i == 0) to fix it.
There is a mistake that everyone has pointed out which is if($i=0) should be if($i==0).
But I have one concern. Why there is a for loop when it is simply returning $min which is $A[0] ?
So I have a query, and though my query can possibly return 20 results, I only want it to show the first 9 results. There is a dumb reason I'm not just limiting the query results to 9, for this purpose I need to know how to stop the while function if $i reaches 9.
Code is
$i = 0;
while($array = mysql_fetch_array($queryresults) && $i <= '9')
{
echo $array['id'];
$i++;
}
How do I get it to stop putting out more echoes after 9th result? Thank you!
Putting the limiting condition first will also make sure you only fetch 9 rows, not and 10 and one discard because && is called lazy (if the first part is false, it wont even look at the next part).
$i = 0;
while($i < 9 && $array = mysql_fetch_array($queryresults)){
echo $array['id'];
$i += 1;
}
or if you want to be fancy:
$i = 9;
while($i-- > 0 && $array = mysql_fetch_array($queryresults)){
echo $array['id'];
}
But I find this error prone ..
$i = 0;
while($array = mysql_fetch_array($queryresults) && $i++ < 9)
echo $array['id'];
}
Notes:
No need to put 9 in quotes
You can increment-after-compare by doing $i++ in the while loop condition (although you don't have to, you could also put it after echo $array['id'] in the while loop body).
Careful! You want < 9 and not <= 9 since starting at zero, or you'll get 10 iterations of the loop.
Cheers
Your $i++ needs to be inside the while loop
You should also have $i <= 9 instead of $i <= '9' but it should still work as is
'9' needs to be 9
$i = 0;
while($array = mysql_fetch_array($queryresults) && $i < 9){
echo $array['id'];
$i += 1;
}
How are they different? Here's what I'm thinking, but I'm not sure....
If you use pre-incrementation, for example in a for loop with ++j, then you are basically saying: "Make a copy of the value of j for use in the loop, then increment j, then go through the statements in the loop with the copy of j." If you are using post-incrementation in the same loop j++, then you are basically saying: "Make a copy of the value of j for use in the loop, then go through the statements in the loop with the copy of j, then increment j."
The reason I'm unsure is because I've created a for loop that multiplies the value of j by 10 and then outputs the result for j=1 through j=12, using both post- and pre-incrementation. The human readable output is exactly the same with post- and pre-incrementation. I'm thinking, 'How are the outputs exactly the same if there isn't some kind of copy operation involved?'
So, I'm guessing the difference between pre- and post-incrementation truly becomes important, in php, when I use references (which act as pointers in php) rather than names for return values? This would be because copies of references aren't made, so pre-incrementation would be: "Increment j, then go through the statements in the loop with the changed value of j, then increment j again...," whereas post-incremetation would look like: "Use the value of j for the statements in the loop, then change the value of j, then go through the loop with the new value of j..."
Pre- or post-incrementing do not magically delay things until later. It's simply inline shorthand.
// pre-increment
$var = 5;
print(++$var); // increments first, then passes value (now 6) to print()
// post-increment
$var = 5;
print($var++); // passes value (still 5) to print(), then increments
Now let's look at a loop.
for ($i = 0; $i < 9; $i++) {
print($i);
}
The last part of the loop declaration (the $i++) is simply the statement to execute after each time through the loop. It "passes" the value to nowhere, then increments it. $i isn't used anywhere at that time. Later when the next statement is executed (print($i);), the value of $i has already increased.
// add 1, then do nothing with $i
for ($i = 0; $i < 9; ++$i) {}
// do nothing with $i, then add 1
for ($i = 0; $i < 9; $i++) {}
Whichever way you do it, $i will be the same within the loop.
If it helps, you can think of them as small routines that kind of do this:
// ++$i
{
$i = $i + 1;
return $i;
}
// $i++
{
return $i;
$i = $i + 1;
}
As I reread your question, I think the confusion is more with how the loop works than how increment operators work. Keeping in mind that the increment is a straightforward, all-at-once operation, here's how third expression in the loop works.
// here's a basic loop
for ($i = 0; $i < 9; $i++) {
// do loop stuff
print($i);
}
// this is exactly what happens
for ($i = 0; $i < 9; ) {
// do loop stuff
print($i);
$i++;
}
Just because that last line can be put in the loop declaration doesn't give it any special powers. There are no references or anything used behind the scenes. The same $i variable is seen both inside and outside the loop. Every statement inside or outside the loop directly looks up the value of $i when necessary. That's it. No funny business.
When doing $x++, you are post-incrementing... This means that the incrementation will only occur after the statement has been evaluated.
So, given the following code:
$x = 10; $y = 0; $z = 5;
$y = $z * $x++;
PHP does this:
$x = 10; $y = 0; $z = 5;
$y = $z * $x++;
// Ignore Post-Increment, Evalutate
$y = $z * $x;
$y = 5 * 10;
// Now Increment x - POST-INCREMENT
$x = $x + 1;
$x = 10 + 1;
$x = 11;
// Continue evaluating statement
$y = 5 * 10;
$y = 50;
When doing ++$x, you are pre-incrementing... This means that the incrementation will occur before the statement is evaluated:
$x = 10; $y = 0; $z = 5;
$y = $z * ++$x;
// Do Pre-Increment
$x = $x + 1;
$x = 10 + 1;
$x = 11;
// Evaluate
$y = $z * $x;
$y = 5 * 11;
$y = 55;
In the case of a for loop in PHP, PHP evaluates a for loop as follows:
for($i = 0; $i < 30; $i++) {
doSomething();
}
// Is evaluated EXACTLY as such by PHP
$i = 0;
while($i < 30) {
doSomething();
$i++;
}
The first expression ($i = 0) is evaluated (executed) once unconditionally at the beginning of the loop.
In the beginning of each iteration, $i < 30 is evaluated. If it evaluates to TRUE, the loop continues and the nested statement(s) are executed. If it evaluates to FALSE, the execution of the loop ends.
At the end of each iteration, $i++ is evaluated (executed) as an independent expression.
Therefore, post-incrementing or pre-incrementing a variable as the third expression in the loop doesn't have an effect on the behavior of it. In this simple case, both expressions will behave exactly the same.
However, in a complex loop such as the following:
for($i = $j = 0; $i < 30; $i += ++$j) {
$j = getResult($j);
}
Post-incrementing or pre-incrementing $j directly affects the value of $i according to the examples above. In this case, you need to choose exactly what you want to do.
$i = 0;
echo $i++;
echo $i;
$j=0;
echo ++$j;
echo $j;
Pre increment display incremented value. But Post increment display value then increment. About code will output 01 and 11