Infinite While loops in ruby - ruby

Question

I have the following program which should count the number of years it should take a population to grow to the desired size. Whenever I run this I get an infinite loop. Can someone help me identify my error?
def pop_growth(start, percent, desired)
year_count = 0
while start <= desired
year_count += 1
start = start + (start * (percent / 100))
end
return year_count
end

Answer 1

I'm sure that you are trying with Integers (instead floats), so you are losing precision try this
def pop_growth(start, percent, desired)
year_count = 0
while start <= desired
year_count += 1
start = start + (start * (percent.to_f / 100))
end
return year_count
end
and let me know if it works for you. if not can you send me your start, percent and desired values?

Answer 2

The proper answer is given by Horacio, let me rewrite this in idiomatic ruby:
def pop_growth start, percent, desired
(0..Float::INFINITY).inject(start) do |memo, years|
break years if memo > desired
memo *= (1.0 + percent / 100.0)
end
end
or, with infinite loop:
def pop_growth start, percent, desired
loop.each_with_object(years: 0, count: start) do |_, memo|
break memo[:years] if memo[:count] > desired
memo[:years] += 1
memo[:count] *= (1.0 + percent / 100.0)
end
end

Answer 3

Three ways.
#1 Solve equation
Solve desired = start * (1.0 + 0.01 * percent)**n for n:
def pop_growth(start, percent, desired)
Math.log(desired.to_f/start)/Math.log(1.0 + percent/100.0)
end
years = pop_growth(100, 10, 200)
#=> 7.272540897341713
years.ceil #=> 8 if desired.
#2 Compound until desire met
def pop_growth(start, percent, desired)
return 0 if start >= desired
alpha = 1.0 + 0.01 * percent
1.step.find { (start *= alpha) >= desired }
end
pop_growth 100, 10, 200
#=> 8
#3 Use recursion
def pop_growth(start, percent, desired, years=0)
return years if start >= desired
pop_growth(start*(1.0+0.01*percent), percent, desired, years+1)
end
pop_growth 100, 10, 200
#=> 8

Answer 4

Just add .to_f method to percent or divide by 100.0, which will convert the integer into float.
start + (start * (percent / 100))
When you are dividing, you need at least one float number in order to return the exact division answer, else Ruby will round it down to nearest whole number, which in this case percent / 100 will result in 0, assuming that the value in percent is less than 100. This will cause this statement start + (start * (percent / 100)) to become start = start + 0, which is why you are seeing the infinite loop.