"""File:     histo.py
   Purpose:  Construct a histogram of a user-input collection of values

   Run:      python histo.py

   Input:    n:     number of user-input values
             vals:  the values from which a histogram will be constructed
                    (floats)
             b:     number of bars in the user-input collection
             min_v: the smallest possible value considered in the
                    histogram.  Note that it's possible that 
                    min_v > min(vals).
             max_v: the largest possible value considered in the
                    histogram.  Note that it's possible that
                    max_v < max(vals).
   Output:   A histogram of X's with b bars covering the values in
             vals that are >= min_v and < max_v
"""

verbose = True

#----------------------------------------------------------------------
def get_vals(vals, n):
   """Read the n user-input values into the vals list"""

   print "Enter the values, one per line"
   for i in range(n):
      x = float(raw_input(""))
      vals.append(x)
   if verbose:
      print "vals =", vals

#----------------------------------------------------------------------
def bar_bounds(bounds, b, min_v, max_v):
   """Find the upper bound for each bar"""
   bar_width = (max_v - min_v)/b
   for i in range(1, b+1):
      bounds.append(min_v + i*bar_width)
   if verbose:
      print "bounds =", bounds


#----------------------------------------------------------------------
def find_subint(x, min_v, bounds):
   """Determine which bin the value x belongs to"""
   if x < min_v or x >= bounds[len(bounds)-1]:
      return None
   for i in range(len(bounds)):
      if x < bounds[i]:
         return i 
   return None # Never executed

#----------------------------------------------------------------------
def bar_heights(vals, min_v, bounds, bar_counts):
   """Determine the number of values in each subinterval"""
   for x in vals:
      subint = find_subint(x, min_v, bounds) 
      if subint != None:
         bar_counts[subint] = bar_counts[subint] + 1
   if verbose:
      print "bar_counts =", bar_counts

#----------------------------------------------------------------------
def find_int_len(x):
   """Determine the number of characters to the left of the decimal
      point in the float x"""
   count = 0
   y = abs(int(x))
   while y > 0:
      y = y/10
      count = count+1
   if x < 0:
      count = count+1
   return count


#----------------------------------------------------------------------
def create_format(min_val, max_val):
   """Create a format string that can be used to print the upper
      and lower bounds of the bars"""
   len_min = find_int_len(min_val)
   len_max = find_int_len(max_val)
   if len_min > len_max:
      field_width = len_min+2  # For decimal and one place to right of decimal
   else:
      field_width = len_max+2  # For decimal and one place to right of decimal
   format_s = '%' + str(field_width) + '.1f'
   return format_s


#----------------------------------------------------------------------
def print_histo(min_v, bounds, bar_counts):
   """Print each subinterval and a bar indicating the number of
      values that fall in the subinterval"""

   s = create_format(min_v, bounds[len(bounds)-1])

   for i in range(len(bounds)):
      upper_bound = bounds[i]
      if i == 0:
         lower_bound = min_v
      else:
         lower_bound = bounds[i-1]
      bar = bar_counts[i]*'X'
      print s % lower_bound, "--", s % upper_bound, bar

#----------------------------------------------------------------------
# Main program 

# Get data
n = int(raw_input("How many values will you input?\n   "))
vals = []
get_vals(vals, n)

# Get histogram information
b = int(raw_input("How many bars should there be in the histogram?\n   "))
min_v = float(raw_input("What's the smallest value in the histogram?\n   "))
max_v = float(raw_input("What's the largest value in the histogram?\n   "))
bounds = []
bar_bounds(bounds, b, min_v, max_v)
bar_counts = [0]*b

# Count number of values in each subinterval
bar_heights(vals, min_v, bounds, bar_counts)

print_histo(min_v, bounds, bar_counts)