Problem with factorial recursive function

Question

Stu145 2015-1-28

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回答： Abdimalik 2022-12-14

Whenever I run the code for a matrix of n values, like n=1:10, only the last 2 factorials are displayed while the rest are 0's. Would anyone be able to tell me where I have gone wrong?

Code:

 function x = fact(n)
    if n<=1
       x = 1;    
    else
       x = n .* fact(n-1) ;
    end
 end

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Answer 1

Titus Edelhofer 2015-1-28

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Hi Arun,

your code does not work for arrays because of the "if n<=1". You would need to do something like

idx = (n>1);
x(~idx) = 1;
x(idx) = n(idx).* ...

Titus

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John D'Errico 2015-1-28

For how large of a value are you trying to compute the factorial? Be careful, as this could be important, since that too would blow up the recursion limits.

Regardless, as you have written it, the code will still run forever. What happens when all values of n are <= 1?

The function as you wrote it will still call fact, but with an empty array for n. so you will have fact calling fact, each time with an empty argument n.

I would add a test to stop that from happening.

Shubham Tyagi 2016-8-15

I'm compiling this code with a 2*4 matrix but (dimensions doesn't match in val(i)=r(i).*fact(r(i)-1);) type error is coming.

function val=fact(r)

i=(r>1); val(~i)=1; if r==1 return end val(i)=r(i).*fact(r(i)-1); end

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Answer 2

Image Analyst 2015-1-31

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Not sure what you did, but it worked fine for me. I put all this (both functions) into test.m

function test
clc;
for k = 1 : 10
  kFactorial = fact(k);
  fprintf('%d! = %d\n', k, kFactorial);
end
function x = fact(n)
if n<=1
  x = 1;
else
  x = n .* fact(n-1) ;
end

In the command window you'll see:

1! = 1
2! = 2
3! = 6
4! = 24
5! = 120
6! = 720
7! = 5040
8! = 40320
9! = 362880
10! = 3628800

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Image Analyst 2016-8-15

I don't know what matrix you're talking about. Please give the code you are going to run to call your function, and show what output you'd expect to see.

If you want fact n to print out an array from the first integer to the last integer, the fact() function will need to be modified since now it's just expecting a single number, not a vector or matrix.

Torsten 2016-8-25

Reshape everything to vectors and reshape back to matrices, if necessary.

Best wishes

Torsten.

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Answer 3

Titus Edelhofer 2016-8-25

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Hi,

two minor changes are needed:

function x = fact(n)
x = ones(size(n));
idx = (n>1);
if any(idx)
  x(idx) = n(idx).*fact(n(idx)-1);
end

First, you need to initialize x as an array of the same size as n (because for fact(n-1) otherwise the array could be smaller), giving the error you observed.

Second, protect the recursive call to fact(n-1) by doing this only if there is anything to do...

Titus

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Titus Edelhofer 2016-8-26

Calling for a matrix works fine, btw.:

fact([2 3 4; 5 6 7])
ans =
         2           6          24
       120         720        5040

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Answer 4

Anil Sarode 2017-12-26

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编辑：Walter Roberson 2017-12-26

sorry i copied your code :

this is the one way to solve your problem :

function file: from where you make function call each time using for loop :

n=1:10;
y=[];
for i=1:length(n)
 y(i)=fact(i) ;  
 end

function: which do desire task

   function [x] = fact(n)
    if n<=1
       x = 1;    
    else
       x = n .* fact(n-1) ;
    end
 end

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Answer 5

Abdimalik 2022-12-14

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You don't need to read input or print anything. Your task is to complete the function factorial() which takes an integer N as input parameters and returns an integer, the factorial of N.