# Introduction and a question about recursion

Patrick de Kruijf

Ranch Hand

Posts: 63

posted 6 years ago

Hello there,

This is my first post at this forum, always an exciting moment. As part of my study I need to learn (the basics i guess) Java, and thats fantastic. I have to work through Headfirst Java and a some other reader.

The first 4 chapters of Headfirst went ok, it was hard but finally I understood what they are trying to teach me. But now in chapter 5 thing are going the other way. Since my study is a homestudy (to busy to go to school, kids and stuff) It's hard to get in contact with my teachter, they only check my homework, wich I send them through mail, and that's about all the contact I have. So here I am posting my first question, and I'm sure it won't be the last.

In the reader I got from school, there is a class that shows me something about recursion. Here is the code:

I have some questions about the final recursive part between the final curly braces. I guess the first line declares a long primitive with the name factorial which is casted into an int called n.

The second line says: if n is equal to 0 return 1. Question: But where does that 1 returns to?

The third and fourth line are a mystery to me: Else return N x factorial (n-1); Question: How do I have to read that? How does this return work? Why is (n-1) typed after the long primitive factorial? What does it do?

Thanks you very very much for your help!

Patrick

This is my first post at this forum, always an exciting moment. As part of my study I need to learn (the basics i guess) Java, and thats fantastic. I have to work through Headfirst Java and a some other reader.

The first 4 chapters of Headfirst went ok, it was hard but finally I understood what they are trying to teach me. But now in chapter 5 thing are going the other way. Since my study is a homestudy (to busy to go to school, kids and stuff) It's hard to get in contact with my teachter, they only check my homework, wich I send them through mail, and that's about all the contact I have. So here I am posting my first question, and I'm sure it won't be the last.

In the reader I got from school, there is a class that shows me something about recursion. Here is the code:

I have some questions about the final recursive part between the final curly braces. I guess the first line declares a long primitive with the name factorial which is casted into an int called n.

The second line says: if n is equal to 0 return 1. Question: But where does that 1 returns to?

The third and fourth line are a mystery to me: Else return N x factorial (n-1); Question: How do I have to read that? How does this return work? Why is (n-1) typed after the long primitive factorial? What does it do?

Thanks you very very much for your help!

Patrick

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posted 6 years ago

If you are having trouble reading it in the program I would suggest doing a few of these by hand.

Try calculating:

7!

10!

and

15! by hand (! means factorial).

As to your other questions:

the first line

is a method declaration with consists of a return type (long in this case) a name (factorial in this case), and a list of parameters (values that are being passed into this function). int n is merely a placeholder for an integer value entering your function.

the second line:

returns the value of 1 to where the method was called. This is called a base case, it is what stops a recursive function from executing.

The third and fourth lines are more complex:

means that you return n(the value you passed to the function) * the result of the function with the value (n-1) passed in.

so if I called factorial(5): it would result in factorial(5) + factorial(4) + factorial(3) + factorial(2) + 1

Hope this helps,

Hunter

Try calculating:

7!

10!

and

15! by hand (! means factorial).

As to your other questions:

the first line

is a method declaration with consists of a return type (long in this case) a name (factorial in this case), and a list of parameters (values that are being passed into this function). int n is merely a placeholder for an integer value entering your function.

the second line:

returns the value of 1 to where the method was called. This is called a base case, it is what stops a recursive function from executing.

The third and fourth lines are more complex:

means that you return n(the value you passed to the function) * the result of the function with the value (n-1) passed in.

so if I called factorial(5): it would result in factorial(5) + factorial(4) + factorial(3) + factorial(2) + 1

Hope this helps,

Hunter

"If the facts don't fit the theory, get new facts" --Albert Einstein

Patrick de Kruijf

Ranch Hand

Posts: 63

posted 6 years ago

This means that when n = 1, your factorial function will return the literal value of 1. The value displayed on your screen will likely be a larger number, but if you look at all the recursive steps you get this for factorial(5):

5x4x3x2x1 = 120

Hunter

Patrick de Kruijf wrote:The myst is clearing up a bit, thanks for that.

You say returns the value of 1 to where the method was called. Does this mean that factorial = 1 ? or n = 1? Or both?

This means that when n = 1, your factorial function will return the literal value of 1. The value displayed on your screen will likely be a larger number, but if you look at all the recursive steps you get this for factorial(5):

5x4x3x2x1 = 120

Hunter

"If the facts don't fit the theory, get new facts" --Albert Einstein