This was my answer to a question in which I was supposed to convert an iterative method to a recursive method. The teacher told me I cant use a-=1 as a parameter... So they gave me 0 points..
When I run this it works as it supposed to be..
Could someone tell me why its wrong?
public int do(int a){
if(a==0){
return 1 ;
}else{
return a * do(a-=1);
}
}
The problem with your code is that you're reading the value of a and reassigning it with a -= 1 in the same expression, but the order of these operations is not specified. The statement:
return a * do(a -= 1);
could be implemented as:
temp = a;
a -= 1;
return temp * do(a);
which will do what you were probably expecting, or:
a -= 1;
return a * do(a);
which will multiply by the decremented value of a rather than its original value.
The correct way to write your function is:
public int do(int a){
if(a==0){
return 1 ;
}else{
return a * do(a-1);
}
}
Just pass the result of the subtraction as an argument, don't reassign the variable at the same time.
-= operator in the example is pointless (hardly a reason to get 0 points), the statement that the order of evaluation is unspecified is false. The code in the question is actually correct according to the JLS. See stackoverflow.com/questions/6800590/…
I assume that this is in java?
I see two big problems with this snippet.
do is a reserved keyword. It may cause compilation errors, so you should name your method something else.-= in a parameter call seems quite ambiguous. Will the negation operator run before or after the multiplication by a? The more clear operator to use would be simply the - operator, and it would complete with the same result.That said, something like this might have been what the teacher was looking for:
public int calculateSomething(int a) {
if (a == 0) {
return 1;
} else {
return a * calculateSomething(a - 1);
}
}