CODEWITHSUNDEEP
phpfor-loop
Darius
Darius

Reputation: 612

Double equals in for loop?

Why doesn't the double equals work in a PHP for loop? The following works:

$cnt = 5;
for ($z = 0; $z <= $cnt; $z++) {
    echo $z."!<br />";
}

But why doesn't this work?

$cnt = 5;
for ($z = 0; $z == $cnt; $z++) {
    echo $z."!<br />";
}

Upvotes: 1

Views: 4035

Answers (5)

Ted Hopp
Ted Hopp

Reputation: 234795

The loop executes only if the condition evaluates to true. At the first iteration, $z == $cnt is false, so the loop never executes.

A common loop strategy is to use a sentinel value:

$cnt = 5;
$stop = $cnt + 1;
for ($z = 0; $z != $stop; $z++) {
    . . .
}

Note that the comparison is negative (!= or !==). Using a sentinel is usually unnecessary for numerical loop variables (I wouldn't recommend it for your posted code), but is useful for other situations (e.g., when $stop represents null, an illegal value, etc.). It's particularly helpful when the loop variable changes in a pattern that is not easy to characterize succinctly.

Upvotes: 3

Barmar
Barmar

Reputation: 780724

The syntax of for is:

for (<init>; <condition>; <increment>)

The loop tests <condition> before each iteration. If it's true, it executes that iteration; if it's false, the loop terminates.

In your case, since $z == $cnt is false before the first iteration, the loop terminates immediately.

To do what you want, invert the test. You also need to bump the end value up by one, since the original version used <=, not <. Note that in both cases, the loop executes $cnt+1 times, because you start from 0.

for ($z = 0; $z != $cnt+1; $z++)

Upvotes: 1

abden003
abden003

Reputation: 1335

A for loop works by looping until a condition is made false. In the first example the loop will execute until z = 5 after that z will not longer be less than or equal to cnt (z starts at 0 and increments through each loop). In the second example you have $z == $cnt as your condition, and since z = 0 and cnt = 5 the loop will stop automatically because the condition is made false. You can use not equals instead like following:

$cnt = 6;
for ($z = 0; $z != $cnt; $z++) {
    echo $z."!<br />";
}

Upvotes: 1

ibreznik
ibreznik

Reputation: 169

Let us look at this from the computer's perspective:

If I am a computer this is what you told me to do:

  • set $cnt = 5;
  • set $z = 0;
  • check if $z equals $cnt
  • if so do whatever is in the loop, then increment $z

Trouble is, 5 does not equal 0 and never will, so the loop will simply be skipped. If you had $cnt = $z+1 inside the loop this would be an infinite loop.

So, you see, == works just fine, it simply doesn't do what you think it should do.

Hope this helps!

Upvotes: 1

Tim
Tim

Reputation: 8606

because on the first iteration $z != $cnt, so the loop stops immediately.

Upvotes: 1

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