Question

I am new to python and I have a list of years and values for each year. What I want to do is check if the year already exists in a dictionary and if it does, append the value to that list of values for the specific key.

So for instance, I have a list of years and have one value for each year:

2010  
2  
2009  
4  
1989  
8  
2009  
7  

What I want to do is populate a dictionary with the years as keys and those single digit numbers as values. However, if I have 2009 listed twice, I want to append that second value to my list of values in that dictionary, so I want:

2010: 2  
2009: 4, 7  
1989: 8  

Right now I have the following:

d = dict()  
years = []  

(get 2 column list of years and values)

for line in list:    
    year = line[0]   
    value = line[1]  

for line in list:  
    if year in d.keys():  
        d[value].append(value)  
    else:  
        d[value] = value  
        d[year] = year  

Answer 1

If I can rephrase your question, what you want is a dictionary with the years as keys and an array for each year containing a list of values associated with that year, right? Here’s how I’d do it:

years_dict = dict()

for line in list:
    if line[0] in years_dict:
        # append the new number to the existing array at this slot
        years_dict[line[0]].append(line[1])
    else:
        # create a new array in this slot
        years_dict[line[0]] = [line[1]]

What you should end up with in years_dict is a dictionary that looks like the following:

{
    "2010": [2],
    "2009": [4,7],
    "1989": [8]
}

In general, it’s poor programming practice to create “parallel arrays”, where items are implicitly associated with each other by having the same index rather than being proper children of a container that encompasses them both.