由于列表索引超出范围而出现错误

Question

I have the following program to calculate the percentage of CO2 every year. I get an error, "list index out of range" , once the entire program runs on the following line:

m= co2level[i+1][1] 

Code:

# Program to calculate percentage of CO2 every year     

co2level = [(2001,320.93),(2003,322.16),(2004,328.07),
             (2006,323.91),(2008,341.47),(2009,348.22)]
i = 0

while i!=len(co2level):

   m= co2level[i+1][1] # I am getting error here as list index out of range
   n= co2level[i][1]
   percentage=((m-n)/n)*100
   print " Change in percentage of CO2 in year %r is"%co2level[i][0],percentage

   i+=1

Answer 1

You could avoid the bounds error like this:

co2level = [(2001,320.93),(2003,322.16),(2004,328.07),
             (2006,323.91),(2008,341.47),(2009,348.22)]
i = 1

while i != len(co2level):
   old = co2level[i-1][1]
   act = co2level[i][1]

   percentage=((act-old)/old)*100
   print (" Change in percentage of CO2 in year %r is " %co2level[i][0],percentage)

   i+=1

You start with the 1st year you can get a "delta" for and inspect the one before this. You compare act to old so you are not going over the bound.

Answer 2

Python is 0 indexed; that means for a list with n elements, the first element of a list is at index 0 and the last element is at index n - 1

Consider the following snippent from your code:

while i!=len(co2level):
   m= co2level[i+1][1] 

The line m = co2level[i+1][1] implies that you start iterating from the element at index 1 (2003,322.16) and tries to get the item at index 6 at the end which causes the error. Morover you have an error in how you assigned m and n . To correct this you can do:

i = 0
while i!=len(co2level):
   m= co2level[i][0] # I am getting error here as list index out of range
   n= co2level[i][1]
   percentage=((m-n)/n)*100
   print" Change in percentage of CO2 in year %r is"%co2level[i][0],percentage
   i+=1

The more pythonic way (using for loops) will be

for i in co2level:
    m = i[0]
    n = i[1]
    percentage=((m-n)/n)*100
    print" Change in percentage of CO2 in year %r is"%co2level[i][0],percentage

better still:

for i in co2level:
    m, n = i
   percentage=((m-n)/n)*100
   print" Change in percentage of CO2 in year %r is"%co2level[i][0],percentage