Can someone explain to me this works?

Question

I have a user defined function that returns the factorial of the number entered. Here is the function:

int factorial_calc(int m1)
{
if(m1==1)
{
 return 1;
}else 
return m1*factorial_calc(m1-1);
}

Now the program works and there seems to be nothing wrong with the program.

What I am asking for here is can someone explain how the code calculates the factorial and gives us the output?

Thanks for all your help! -Sayan. EDIT: Sorry in my hurry I didn't check what I had typed. I corrected it now. Both are factorial_calc now.

Double EDIT: What I really want to know is not about recursion. I know that when the code works (example: let's say m1=5) then it takes 5 and stores it in the variable present in main(). And then does it over and over again till it reaches 0. What I want to know is how does it know that each of the values are going to be multiplied? We only store values in the variables, not math operators. So, what am I missing??

Read https://en.wikipedia.org/wiki/Recursion_(computer_science) — s7amuser, Jun 23 '16 at 15:50
Recursivity. It continues calculating the factorial until it is 1. Also, what is `m`? `fact_calc` and `factorial_calc` are different things — Andrew Li, Jun 23 '16 at 15:51
Also, this code doesn't work. The line `return m1*fact_calc(m1-1)` should read `return m1*factorial_calc(m1-1)`. Please only post working code. — Scott Mermelstein, Jun 23 '16 at 15:51
Do need to learn how to use the debugger and then you can step through it and see for yourself. — AnthonyLambert, Jun 23 '16 at 15:53
return m1*fact_calc(m1-1); will replace m1 by value and call back function fact_calc with starting value minus 1 at the end it do for example 5*(fact_calc(4)= 4*(fact_calc(3)= 3*(fact_calc(2)=1*1))) so at the end 5*4*3*2*1 — Destrif, Jun 23 '16 at 15:54
Possible duplicate of [Understanding recursion](http://stackoverflow.com/questions/717725/understanding-recursion) — Scott Mermelstein, Jun 23 '16 at 15:54
@AnthonyLambert It's not that easy to understand from the debugger what recursion is... — Eugene Sh., Jun 23 '16 at 15:54
@Eugene the debugger is your friend..... I would of thought it was very quickly self evident. — AnthonyLambert, Jun 23 '16 at 16:07
@AnthonyLambert Debugger doesn't tell me how the code calculates the factorial and gives me the value. But thank you for your feedback nonetheless! :) — returnNULL, Jun 23 '16 at 16:14

score 1 · Accepted Answer · answered Jun 23 '16 at 15:56

Recursion: simplify the problem, solve the simpler one

Example: the multiplication of all the numbers from N to 1
1 * 2 * 3 * 4 * 5 * 6 * ... * N

Simplification: the multiplication of all the numbers from N to 1 is N multiplied by the multiplication of all the numbers from N - 1 to 1 which is simpler than the example
N * 1 * 2 * 3 * 4 * 5 * 6 * ... * (N-1)

in pseudo-code

factorial(N) = N * factorial(N - 1)

but with the simplest case in code

if (N == 1) return 1;

Put these statements in a function and you have the solution.

Can someone explain to me this works?

1 Answers1