# -1^(even) = +1
# -1^(odd)  = -1
print("how to alternate positive / negative numbers:")
sign = 1
number = 5
for x in range(1, 8):
    sign = (-1)*sign
    value = number * sign
    print(value)

print("one way to work with odd numbers:")
for x in range(1, 10, 2):
    print(x)

print("another way to work with odd numbers:")
startOdd = 1
for x in range(5): # number of next odd numbers to find
    print(startOdd)
    startOdd = startOdd + 2

print("one way to work with even numbers:")
for x in range(2, 11, 2):
    print(x)

print("another way to work with even numbers:")
startEven = 2
for x in range(5): # number of next even numbers to find
    print(startEven)
    startEven = startEven + 2

print("multi-tasking:")
evenNumber = 0
oddNumber = 1
for x in range(0, 10):
    evenNumber = evenNumber + 2
    #evenNumber += 2 # (an equivalent way to express above)
    
    oddNumber = oddNumber + 2
    #oddNumber += 2  # (an equivalent way to express above)
    
    print("x:", x, "evenNumber:", evenNumber, "oddNumber:", oddNumber)