# Find an equation that uses all 9 digits (1-9)

I used Python to write the following program:

"""
A desired equation includes a 4-digit integer multiplied with a single-digit integer
to give another 4-digit integer and all 9 digits(1-9) are used in the equation.
"""

import random

found=False

full_range=range(1,10)

four_digit_num=""
multiple=""
for i in range(4):
four_digit_num+=str(full_range.pop(random.randint(1,len(full_range)-1)))

single_digit_num=full_range.pop(random.randint(1,len(full_range)-1))

for i in range(4):
multiple+=str(full_range.pop(random.randint(1,len(full_range)-1)))

if int(four_digit_num)*single_digit_num==int(multiple):
found=True

#output the equation
print "The desired equation is:"
print four_digit_num+"*"+str(single_digit_num)+"="+multiple


Is there any better way to perform this task?

• Without getting into the math (because I don't do math). four_digit_num="" multiple="" refactors to four_digit_num=multiple="" Jun 27 '14 at 2:35

It is extremely inefficient to do this randomly; you could end up checking the same numbers again and again. For a more structured approach, you could use itertools.permutations:

from itertools import permutations

for nums in permutations("123456789"):
a = int("".join(nums[:4]))
b = int(nums[4])
c = int("".join(nums[5:]))
if a * b == c:
print "{0} * {1} == {2}".format(a, b, c)
break # optional - there's more than one answer!


Also, note that:

found=False
...
if int(four_digit_num)*single_digit_num==int(multiple):
found=True


would be neater as:

while True:
...
if int(four_digit_num) * single_digit_num == int(multiple):
break


def check(i, j, c):
istr = list(str(i))
cstr = list(str(c))
if "0" in istr: istr.remove("0")
if "0" in cstr: cstr.remove("0")
Jval = ["%d"%j]
Set_String = set(istr + cstr + Jval)
return len(Set_String) == 9

for i in range(1000, 10000):
for j in range(2, 10):
c = i * j
if c < 10000 and check(i, j, c):
print "The desired equation is: %d * %d = %d" %(min(i,c), j , max(i,c))i/j, max(i,j))

• Can't you just make i go in [1000, 10000[ and j go in [2, 10[ and then consider i, j, i*j ? You should shave off useless checks by considering when the product i*j becomes bigger than 9999. Jun 24 '14 at 7:09