title: Average annual change?
body: <p>I am working on a program that will pull the midyear population for the United States during the years 1950 through 1990. The first line pulls for 50, the next line has the population for 51, and so on. The program pulls the information, then displays the year with the greatest increase in population, the smallest increase in population, and the average annual change in population during the range of dates.</p>
  
  <p>I can do the first two, but the average annual change is not displaying correctly. I was given a hint, and apparently I am missing a line at 30. The average annual change just keeps displaying as zero? :(</p>
  
  <pre><code>def main():
      #setup variables
      yearly_change = []
      change=0.0
      total_change=0
      average_change=0
      greatest_increase=0
      smallest_increase=0
      greatest_year=0
      smallest_year=0
      BASE_YEAR=1950
      try:
          #open the file for reading
          input_file = open("USPopulation.txt", "r")
          #read all the lines in the in file into a list
          yearly_population= input_file.readlines()
          #turn all read lines into a number
          for i in range(len(yearly_population)):
              yearly_population[i] = float(yearly_population[i])
          #calculate the change in population size for each two years
          for i in range(1,len(yearly_population)):
              change = yearly_population[i] - yearly_population[i-1]
  
              #MISSING SINGLE LINE HERE? 
  
              #if this is the first year, set trackers to its value
              if i==1:
                  greatest_increase = change
                  smallest_increase = change
                  greatest_year = 1
                  smallest_year = 1
              #this is not the first change in population size
              #update the trackers if relevent
              else:
                  if change&gt;greatest_increase:
                      greatest_increase = change
                      greatest_year = i
                  elif change&lt;smallest_increase:
                      smallest_increase = change
                      smallest_year = i
              total_change = float(sum(yearly_change))
              average_change = total_change/40
              print("The average annual change in population during the time period is",\
                    format(average_change, '.2f'))
              print("The year with the greatest increase in population was",
  BASE_YEAR+greatest_year)
              print("The year with the smallest increase in population was",
  BASE_YEAR+smallest_year)
              input_file.close()
      except IOError:
          print("The file could not be found")
      except IndexError:
          print("There was an indexing error")
      except:
          print("An error occurred")
  
  main()
  </code></pre>
  
  <p>As requested, here are a few lines of the input file: </p>
  
  <pre><code>151868
  153982
  156393
  158956
  161884
  165069
  168088
  171187
  174149
  177135
  179979
  </code></pre>
  
  <p>This is just a basic .txt file, with the first line being the population of the States in 1950, the second being the population in 1951, and so on. </p>