Unexpected output of PHP recursive function: Why does a simple addition function print '85' instead of '8'?

Detailed explanation of PHP recursive function: analyzing the unexpected output of the addition function

This article analyzes an example of a PHP recursive function to explain why its output results do not match expectations. The code is as follows:

 <?php function sd($a=3,$b=2){
   $c = $a $b;
   if($c < 6){
        sd($a,$c); 
   }
   echo $c;
}
sd();
?>

The function sd() accepts two parameters a and b , and the default values ​​are 3 and 2 respectively. The function calculates the sum of a and b and assigns the value to c . If c is less than 6, then recursively call itself and pass a and the new c values ​​as parameters. Finally, the function prints the value of c .

After executing sd() , the output result is "85", rather than the expected "8". This is because there is a bias in understanding the order of recursive calls and echo statement execution.

The program execution process is as follows:

1. The first call to sd() is called, a=3 , b=2 , c = 3 2 = 5 . Since c , the function calls itself recursively and the parameters become <code>sd(3, 5) .
2. In recursive calls, a=3 , b=5 , c = 3 5 = 8 . At this time c >= 6 , the recursion ends. echo $c; print 8.
3. The program returns to the location where the first call sd() is called. echo $c; prints the c value at the first call, which is 5.

Therefore, the final output is "85". It is not that the variable c is not overwritten, but that the echo statement is at the end of the function and is executed every time the recursive ends, resulting in two prints.

To get the result "8", you need to modify the function logic, such as placing the echo statement in if conditional statement, or printing the final result only at the end of recursion. The modified code can be as follows:

 <?php function sd($a=3,$b=2){
   $c = $a $b;
   if($c < 6){
        return sd($a,$c); 
   }
   return $c;
}
echo sd();
?>

This modified version uses the return statement to return the value of c , ensuring that the final result is printed only after the recursion is over.

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