I have the PHP code below:
<?php
$length = $_GET["length"];
$maxValue = $_GET["maxValue"];
$distribution = array();
for($j = 0; $j < $maxValue; $j++) {
$distribution[j] = 5;
}
$x = 0;
$x++;
for($j = 0; $j < $maxValue; $j++) {
echo $distribution[j] , " ";
}
echo $x;
?>
$x starts as 0 and is incremented by 1. However, just below $x is incremented, I am also incrementing the first element of the "distribution" array - $distribution[0]. And it's not working. It worked fine when I was initializing the elements (set them to 5).
Any ideas on why it might now be working? I am probably referencing the array element wrong. But this seems inconsistent.
When you say $distribution[j] -> php doesn't understand the j as a variable - but rather as an undefined constant
It looks like you are trying to say $distribution[$j] - which is partially -why your increments aren't working - -
The other reason would be that you aren't ever calling $distribution[$j]++ --- so there is no incrementation happening...
Related
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 want to increment the int on the end of my string which together makes up the complete value.
$btnid = 'btnid1';
for($i = 1; $i < $countP; $i++) {
$btnid = 'btnid' . ++;
}
I tried different types of concatenation but I can't seem to get it to work if I just set it to 1 it works but I need the string there too.
Just append $i to the string btnid in each loop iteration.
$string = 'btnid';
for($i = 1; $i < $countP; $i++) {
$btnid = $string . $i;
}
As a side note (and seriously, DO NOT actually do this) so long as you don't go past btnid9, you can just increment the string.
$btnid = 'btnid1';
for($i = 1; $i < 9; $i++) {
$btnid++;
}
echo $btnid; // btnid9
If you go over, things get a bit weird:
$btnid++;
echo $btnid; // btnie0
Manual page: http://php.net/manual/en/language.operators.increment.php
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
hello i'm beginner for programming I've got a homework. googled it but couldnt find anything...
i need to get the total value of numbers from 1 to 10. this need to be done in loop. but couldn't figure which loop should i use. if you can also give me an example code thats would be great.
This is a homework question, I'm not sure why people are just giving you an answer to copy-paste.
Achieving the sum of numbers 1..10 is pretty simple. You will need to initialise an empty int var before your loop, and for each iteration from 0 up to and including 10 you will add your int var to the current iteration.
For example:
sum = 0;
for num in range 1 to 10:
sum = sum + num;
<?php
$start = 0; // set the variable that will hold our total
for($i=1;$i<11;$i++){ // set a loop, read here: http://php.net/manual/en/control-structures.for.php for more info
$start += $i; // add $i to our start value
}
echo $start; // display our final value
I would use a for loop.
$total = 0;
for($i = 1; $i <= 10; $i++){
$total += $i;
}
Using the for loop:
<?php
$sum = 0;
for($i = 1; $i <= 10; $i++){
$sum += $i;
}
Using the foreach loop:
<?php
$sum = 0;
foreach(range(1,10) as $num){
$sum += $num;
}
echo $sum; // prints 55
And disregarding your assignment, here is an easier way:
echo array_sum(range(1,10));
What's the difference between ++$i and $i++ in PHP?
++$i is pre-increment whilst $i++ post-increment.
pre-increment: increment variable i first and then de-reference.
post-increment: de-reference and then increment i
"Take advantage of the fact that PHP
allows you to post-increment ($i++)
and pre-increment (++$i). The meaning
is the same as long as you are not
writing anything like $j = $i++,
however pre-incrementing is almost 10%
faster, which means that you should
switch from post- to pre-incrementing
when you have the opportunity,
especially in tight loops and
especially if you're pedantic about
micro-optimisations!"
- TuxRadar
For further clarification, post-incrementation in PHP has been documented as storing a temporary variable which attributes to this 10% overhead vs. pre-incrementation.
++$i increments $i, but evaluates to the value of $i+1
$i++ increments $i, but evaluates to the old value of $i.
Here's an example:
$i = 10;
$a = $i++;
// Now $a is 10, and $i is 11
$i = 10;
$a = ++$i;
// Now $a is 11, and $i is 11
There is sometimes a slight preformance cost for using $i++. See, when you do something like
$a = $i++;
You're really doing this:
$temporary_variable = $i;
$i=$i+1;
$a=$temporary_variable;
++$i is pre-incrementation
$i is incremented
the new value is returned
$i++ is post-incrementation
the value of $i copied to an internal temporary variable
$i is incremented
the internal copy of the old value of $i is returned
++$i //first increment $i then run line
$i++ //first run line then increment $i
this example elplains simply
<?php
$x = 10;
echo $x++. ' '.$x; // the result is 10 and 11
echo '<br>';
$y = 10;
echo ++$y. ' ' .$y; // the result is 11 and 11
// so the $x++ is not showing +1 at first but the next time
// and the ++y is showing +1 first time but not increasing next
in this case there is no difference:
for($i = 0;$i<3;++$i)var_dump $i;
/*
int(0)
int(1)
int(2)
*/
for($i = 0;$i<3;$i++)var_dump $i;
/*
int(0)
int(1)
int(2)
*/
but:
for($i = 0;$i<3; $j = ++$i )var_dump($j);
/*
NULL
int(1)
int(2)
*/
for($i = 0;$i<3; $j = $i++ )var_dump($j);
/*
NULL
int(0)
int(1)
*/
Difference is: ++$i will increment $i variable and return updated value, while $i++ will return original value, so increment it.
$prefix = 1;
$postfix = 1;
echo ++$prefix; // 2
echo $postfix++; // 1
To explain jldupont's point:
$i = 1;
$x = $i++;
echo $x; // prints 1
$x = ++$i;
echo $x; // prints 3
Another way of looking at pre and post incrementing is that it's shorthand for combining 2 statements.
Pre-incrementing
// long form
$y = $y + 1;
$x = $y; // any statement using $y
// shorthand
$x = ++$y; // the same statement using $y
Post-incrementing
// long form
$x = $y; // any statement using $y
$y = $y + 1;
// shorthand
$x = $y++; // the same statement using $y
$i++ is known as post-increment. It increments the value of $i only after assigning the original value of $i to $j first.
++$i is known as pre-increment. It increments the value of $i before assigning the value to $j, so the updated value of $i will be assigned to $j.
Hence,
$i = 4;
$j = $i++;
// Now, $i = 5 and $j = 4
$i = 4;
$j = ++$i;
// Now, $i = 5 and $j = 5
These theories apply in a similar manner for decrementing as well.
Hope this helps!
It's probably best-illustrated by an example...
Post-increment:
$zero = 0;
$n = $zero++; //$n is zero
Pre-increment:
$zero = 0;
$n = ++$zero; //$n is one
Short answer:
Prefix increases the value and returns the value increased
Postfix increases the value and returns the value before it was increased
Prefix is faster
Long answer: If you think a little about it, how you would implement those yourself, you will probably realize why prefix is faster. Truth to be told, postfix is actually (often) implemented using prefix:
const T T::operator ++ (int) // postfix
{
T orig(*this);
++(*this); // call prefix operator
return (orig);
}
Avoid postfix unless you have a specific reason not to. The difference in speed can be quite a lot for complex datatypes.
I actually looked this up a few days ago. Heres my source.
The main purpose of the post-fix increment operator is usage like this:
while(*condition*)
$array[$i++] = $something;
This is a very elegant way, how to get around some array iterations.
Breakdown:
Variable $something will be assigned to the array element indexed with $i
Variable $i will be incremented
Iteration is at the end, condition will be checked
In all other cases, you should use the prefix operator. It makes the code much more clear (You can be sure, that you already work with the incremented value of particular variable).
I ran the following code to test if ++$i is 10% faster than $i++. I admit, the code does not have a stable outcome but even then I should at least have seen some numbers near the 10%. The highest I got was 4-4.5% approximately.
<?php
$randomFloat = rand(0, 10) / 10;
$before1 = microtime(true);
for($i=0; $i <1000000; ++$i){
$rand = (rand(0, 10) / 10) * (rand(0, 10) / 10);
}
$after1 = microtime(true);
echo 'it took '.($after1-$before1) . ' seconds fot ++$i<br />';
$before2 = microtime(true);
for($i=0; $i <1000000; $i++){
$rand = (rand(0, 10) / 10) * (rand(0, 10) / 10);
}
$after2 = microtime(true);
echo 'it took '.($after2-$before2) . ' seconds fot $i++<br /><br />';
echo '++$i is '.((($after1-$before1)*100)/($after2-$before2)-100).'% faster than $i++';
Both operators still do what their syntax implies: to increment. Regardless of prefix or postfix, the variable is sure to be incremented by 1. The difference between the two lies in their return values.
1. The prefix increment returns the value of a variable after it has been incremented.
2. On the other hand, the more commonly used postfix increment returns the value of a variable before it has been incremented.
// Prefix increment
let prefix = 1;
console.log(++prefix); // 2
console.log(prefix); // 2
// Postfix increment
let postfix = 1;
console.log(postfix++); // 1
console.log(postfix); // 2
To remember this rule, I think about the syntax of the two. When one types in the prefix increment, one says ++x. The position of the ++ is important here. Saying ++x means to increment (++) first then return the value of x, thus we have ++x. The postfix increment works conversely. Saying x++ means to return the value of x first then increment (++) it after, thus x++.