Ответ: python

import random

x = [0]*50

for i in range(len(x)):

   x[i] = random.randint(-100, 100)

a = ''

for j in range(len(x)):

   if x[j] < 0:

       a = a + str(j) + ','

a = a[:-1]

f = open('test', 'w').write(a)