A brief discussion on the fourth part of PHP---recursive functions_PHP tutorial

Many students feel dizzy when learning recursive functions and cannot figure out the principles and operating mechanisms of recursive functions. This article will explain the operating mechanisms and applications of recursive functions in detail.

So what is a recursive function?

A recursive function is a self-calling function. It calls itself directly or indirectly in the function body, but the conditions for self-calling need to be set. If the conditions are met, the function itself will be called. If not, the self-calling of this function will be terminated, and then Returning the control of the current process to the upper-level function for execution may still be difficult to understand if I explain it to you this way.

Okay, let’s explain it to you with an example from Teacher Gao Luofeng’s "Explaining PHP in Details".

function test($n){
echo $n." ";
if($n>0){
test($n-1);
}else{
echo "<-->";
}
echo $n." ";
}
test(10);
?>

Let’s first think about it, what is the final output of this example?

Okay, let’s take a look at the output of this function:

10 9 8 7 6 5 4 3 2 1 0 <--> 0 1 2 3 4 5 6 7 8 9 10

How about it? I wonder if the result is what everyone expected?

Okay, let me explain to you step by step...

The first step is to execute test(10), echo 10, and then because 10>0, execute test(9), and there is still echo 10 that has not been executed in time

The second step is to execute test(9), echo 9, and then because 9>0, execute test(8), and there is also echo 9 that has not been executed in time

The third step is to execute test(8), echo 8, and then because 8>0, execute test(7). There is also echo 8 that has not been executed in time

The fourth step is to execute test (7), echo 7, and then because 7>0, execute test (6). There is also echo 7 that has not been executed in time

The fifth step is to execute test (6), echo 6, and then because 6>0, execute test (5). There is also echo 6 that has not been executed in time

.............

The tenth step is to execute test(0) and echo 0. At this time, the condition of 0>0 is not met. The test() function is no longer executed, but echo "<-->" is executed, and the subsequent echo is executed. 0

10 9 8 7 6 5 4 3 2 1 0 <--> 0 1 2 3 4 5 6 7 8 9 10

At this time, the output content is as shown in the red part above. At this time, the function no longer calls itself and begins to return the control of the process to the upper layer function for execution

That is, the last echo that just started to be executed before all the test() functions had time to output

The process is like this:

In the first to tenth steps of function execution, the function outputs the green part. If the red part has not yet been output, it should call itself to perform the operation, and so on, until the process is executed to the point where the conditions for calling itself are no longer met. , output "<-->". At this time, the process should execute the code output by "not in time" previously.

Just like the games we usually play, if you kill a monster, a treasure will drop out, but there are other monsters waiting for you to destroy. You have to destroy all the monsters before you can come back and pick up the treasures one by one.

How about explaining it to everyone like this, do you understand?
Some students will ask again. After I executed all the test functions, I finally output 0

That is, output it here,

10 9 8 7 6 5 4 3 2 1 0 <--> 0

Then why the next output is 1 instead of 10,

Regarding this issue, in order to help everyone understand, let me give you another example:

Look at the following code:

function one($num){
echo $num;
two($num-1);
echo $num;
}
function two($num){
echo $num;
three($num-1);
echo $num;
}
function three($num){
echo $num;
}
one(3);
?>

The above code decomposes the test() function. Let’s think about:

When executing the one(3) function, like the test() function, first output 3, and then call the two(2) function,

Note that the following 3 has not been output at this time,

Next, execute the two(2) function, output 2, and call the three(1) function. Similarly, there is no time to output the following 2,

Execute three(1) and output 1 directly without calling other functions,

At this time, we wonder if the two() function just has not been executed yet. OK, then execute the unfinished part of the two() function. After the two() function is executed, it will output the following 2, and then start executing one () The part of the function that has not been executed is the output of 3 below. At this time, all functions have been executed.

Then, the output result is:

3 2 1 2 3

How about it, would it be easier for everyone to understand it this way?

If you still don’t understand, please leave a question below and give your valuable suggestions. I will make corresponding improvements and explanations.