**Steps**
1. Square the numbers that are greater than zero.
2. Multiply by 3 every third number.
3. Multiply by -1 every fifth number.
4. Return the sum of the sequence.

**Example**   
`{ -2, -1, 0, 1, 2 }` returns `-6`
```
1. { -2, -1, 0, 1 * 1, 2 * 2 }
2. { -2, -1, 0 * 3, 1, 4 }
3. { -2, -1, 0, 1, -1 * 4 }
4. -6
```

P.S.: The sequence consists only of integers. And try not to use "for", "while" or "loop" statements.

Difficulty: introductory

Starter code:

def calc(a):
	

Solutions:
def calc(a):
    return sum( x**(1 + (x>=0)) * (1 + 2*(not i%3)) * (-1)**(not i%5) for i,x in enumerate(a,1))

def calc(a):
    for i, n in enumerate(a):
        if a[i] > 0: a[i] *= a[i]
        if i % 3 == 2: a[i] = a[i] * 3
        if i % 5 == 4: a[i] = a[i] * -1
    return sum(a)

def calc(a):
    print(a)
    
    for i in range(len(a)):
        if a[i] > 0:
            a[i] *= a[i]
            
    for i in range(2,len(a),3):
        a[i] *= 3 
        
    for i in range(4,len(a),5):
        a[i] *= -1
        
    return sum(a)

def calc(a):
    sum = 0
    for i in range(len(a)):
        value = a[i]
        if value > 0: value *= value
        if (i+1) % 3 == 0: value *= 3
        if (i+1) % 5 == 0: value = -value
        sum += value
    return sum

def mod(e):
    i, n = e
    return (n**2 if n > 0 else n) * (3 if i % 3 == 0 else 1) * (-1 if i % 5 == 0 else 1)

def calc(lst):
    return sum(map(mod, enumerate(lst, 1)))
    
#   without loop, as asked   
##############################


# there with a comprehension (so a for):
#def calc(lst):
#    return sum((n**2 if n > 0 else n) * (3 if i % 3 == 0 else 1) * (-1 if i % 5 == 0 else 1) for i, n in enumerate(lst, 1))


def calc(a):
    return sum([i * (i if i > 0 else 1) * (3 if k%3 == 2 else 1) * (-1 if k%5 == 4 else 1) for k, i in enumerate(a)])

fifth  = lambda i, x: x if i%5 else -x
third  = lambda i, x: x if i%3 else 3*x
square = lambda x   : x if x<0 else x*x

def calc(a):
    return sum(fifth(i, third(i, square(x))) for i,x in enumerate(a, 1))

from itertools import cycle
from operator import mul

muls=(1,1,3,1,-1,3,1,1,3,-1,1,3,1,1,-3)

def calc(a):
    return sum(map(mul,map(lambda n:n*n if n>0 else n,a),cycle(muls)))

from itertools import cycle

def calc(lst):
    return sum((a ** 2 if a > 0 else a) * b * c
        for a, b, c in zip(lst, cycle((1, 1, 3)), cycle((1, 1, 1, 1, -1))))

def calc(a):
    return sum(i**(1,2)[i>0]*(1,3)[idx%3==2]*(1,-1)[idx%5==4] for idx,i in enumerate(a))