javascript factorial with recursion
Question
I am trying this simple code to calculate factorial of 5. But I am getting "undefined" as the result. I am aware of other methods but what is wrong with this?
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title> Learning </title>
<head>
<body>
<h2> Welcome<h2>
<p id="demo"></p>
<script>
var fact=5;
function calfact(num)
{
if(num!=1)
{
fact=fact*(num-1);
num=num-1;
calfact(num);
}
else
{
return fact;
}
}
document.getElementById("demo").innerHTML=calfact(5);
</script>
</body>
</html>
2 answers
solution1
1 2017-04-15 13:47:29
var fact=5;
function calfact(num){
if(num!=1){
fact=fact*(num-1);
num=num-1;
return calfact(num);//the missing thing
}else{
return fact;//why fact? i think it should be 1
}
}
By the way, your approach is maybe working, but really bad style.May do this:
function calfact(num){
if(num!=1){
return calfact(num-1)*num;
}else{
return 1;
}
}
Or short:
calfact=num=>num==1?1:calfact(num-1)*num;
solution2
0 ACCPTED 2017-04-15 13:44:32
If you want a result from a recursive function, all code paths through the function must return something. Your code isn't returning anything in the num!=1 case. It should be returning the result of calling itself, eg (see the *** line):
var fact=5;
function calfact(num)
{
if(num!=1)
{
fact=fact*(num-1);
num=num-1;
return calfact(num); // ***
}
else
{
return fact;
}
}
Your function is using a global variable, which isn't a great idea as it means the funtion isn't self-contained; and isn't a true factorial function, because you're effectively using two inputs ( fact — the global and num , the argument)
If you want a true factorial, you don't need a global variable, just work from the argument itself:
function factorial(num) { if (num < 0) { throw new Error("num must not be negative"); } if (num <= 1) { // Both 1! and 0! are defined as 1 return 1; } return num * factorial(num - 1); } console.log(factorial(5)); // 120
Or of course, more compactly:
function factorial(num) {
if (num < 0) {
throw new Error("num must not be negative");
}
return num <= 1 ? 1 : num * factorial(num - 1);
}
(More about 0!: https://en.wikipedia.org/wiki/Factorial )
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