PHP solution method of combination algorithm_PHP tutorial

Topic: Combination Algorithm: There is an array a with N elements. Now we are required to find the number of all combinations containing any element.
Answer: Let’s look at the rules first:
Assume this array is array(1,2,3,4,5) then M=5;
The possible combinations are:
1 number Number of combinations: 5
Number of combinations of 2 numbers: 4+3+2+1
Number of combinations of 3 numbers: 3+2+1
Number of combinations of 4 numbers: 2+1
The number of combinations of 5 numbers: 1
It looks familiar, it is a 9*9 multiplication table in reverse order. Except for the M combinations in the first row, other combinations are processed according to the multiplication table, and the two FOR statements are nested
Code:

Copy code The code is as follows:

$c = 5;
$a = $c;
for($i=1;$i<=$c;$i++){
for($k=$c-$i;$k>0;$k–){
$a +=$k;
}
}
echo $a;

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