Recursive function in bash

Question

I want to do a function that will return the factorial of a number in bash

Here\'s the current code that doesn\'t work, can anyone tell me what\'s wrong and how to c

Answer 1

#!/bin/bash

function factorial() 
{ 
   if (( $1 < 2 ))
   then
     echo 1
   else
     echo $(( $1 * $(factorial $(( $1 - 1 ))) ))
   fi
}

This will work better.

(It works up to 25, anyway, which should be enough to prove the point about recursion.)

For higher numbers, bc would be the tool to use, making the ninth line above:

echo "$1 * $(factorial $(( $1 - 1 )))" | bc

but you have to be a bit careful with bc --

$ factorial 260
38301958608361692351174979856044918752795567523090969601913008174806\
51475135399533485285838275429773913773383359294010103333339344249624\
06009974551133984962615380298039823284896547262282019684886083204957\
95233137023276627601257325925519566220247124751398891221069403193240\
41688318583612166708334763727216738353107304842707002261430265483385\
20637683911007815690066342722080690052836580858013635214371395680329\
58941156051513954932674117091883540235576934400000000000000000000000\
00000000000000000000000000000000000000000

was quite a strain on my poor system!

Answer 2

Another one implementation using echo instead of return

#!/bin/bash

factorial()
{
        if [ $1 -le 1 ]
        then
                echo 1
        else
                echo $[ $1 * `factorial $[$1-1]` ]
        fi
}
echo "factorial $1 = " `factorial $1`

Answer 3

echo-ing a result may be the only way to get a result for n > 5, but capturing the echo'ed result requires a subshell, which means the recursion will get expensive fast. A cheaper solution is to use a variable:

factorial() {
    local -i val=${val:-($1)}
    if (( $1 <= 1 )); then
        echo $val
        return
    fi
    (( val *= $1 - 1 ))
    factorial $(( $1 - 1 ))
}

If you want to be extra sure that val is unset when you start, use a wrapping function:

factorial() {
    local -i val=$1
    _fact() {
        if (( $1 <= 1 )); then
            echo $val
            return
        fi
        (( val *= $1 - 1 ))
        _fact $(( $1 - 1 ))
    }
    _fact $1
}

For comparison:

# My Method
$ time for i in {0..100}; do factorial $(( RANDOM % 21 )); done > /dev/null 

real    0m0.028s
user    0m0.026s
sys     0m0.001s

# A capturing-expression solution
$ time for i in {0..100}; do factorial $(( RANDOM % 21 )); done > /dev/null 

real    0m0.652s
user    0m0.221s
sys     0m0.400s

Answer 4

There are several syntax and a quite obvious logic one (return 0)

A working version is below:

#!/bin/bash

factorial()
{
    if (( $1 <= 1 )); then
        echo 1
    else
        last=$(factorial $(( $1 - 1 )))
        echo $(( $1 * last ))
    fi
}
factorial 5

You are missing:

1. return is bad (should use echo)

2. shbang line (is /bin/bash not /bash/bin)

3. Can't do arithmetic outside of (( )) or $(( )) (or let, but (( )) is preferred)

Answer 5

clear cat

fact()

{

        i=$1
        if [ $i -eq 0 -o $i -eq 1 ]
        then
                echo 1
        else
                f=`expr $i \- 1`
                f=$(fact $f)
                f=`expr $i \* $f`
                echo $f
        fi
}

read -p "Enter the number : " n

if [ $n -lt 0 ]

then

        echo "ERROR"

else

        echo "THE FACTORIAL OF $n : $(fact $n) "
fi

Answer 6

#-----------------factorial ------------------------
# using eval to avoid process creation
fac=25
factorial()
{
    if [[ $1 -le 1 ]]
    then
        eval $2=1
    else

        factorial $[$1-1] $2
        typeset f2
        eval f2=\$$2
        ((f2=f2*$1))
        eval $2=$f2

    fi
}
time factorial $fac res 
echo "factorial =$res"