How do I add integers from a list of lists if the string in the list of lists is the same?-CodePudding

I want to be able to add certain numbers in lists in a list together depending on if the first item in the lists within the list match. For example:

myList = [['Mar', 5], ['Feb', 29], ['Mar', 7], ['Jan', 20], ['Feb', 3], ['Feb', 4], ['Mar', 12], ['Jan', 2], ['Jan', 16]]

If the the first item in each list ('Mar', 'Feb', or 'Jan') match, I want to add the numbers of that list together and ultimately get a list of lists that looks like this:

newList = [['Mar', 24], ['Feb', 36], ['Jan', 38]]

However, I'm fairly new to programming and I'm not sure how to approach this. Any advice on how to achieve this would be greatly appreciated.

CodePudding user response：

If you have the list as an given input you can add up the values using a dictionary. If you can choose the data structure for the input yourself, consider using a dictionary straight away.

myList = [['Mar', 5], ['Feb', 29], ['Mar', 7], ['Jan', 20], ['Feb', 3], ['Feb', 4], ['Mar', 12], ['Jan', 2], ['Jan', 16]]

months = dict()
for [month, day] in myList:
    if month in months:
        months[month]  = day
    else:
        months[month] = day

# output dictionary (you may wanna use this for further computation)
print(months)

# if you need it sorted (and as a list)
sorted_month = sorted(months.items(), key=lambda item: item[1])
print(sorted_month)

Expected output:

{'Mar': 24, 'Feb': 36, 'Jan': 38}
[('Mar', 24), ('Feb', 36), ('Jan', 38)]

CodePudding user response：

it looks like you are using python, but i will answer in pseudocode anyway. In the future read stackoverflow help section to learn how to ask a good question, and tag appropriately

when coding, we are breaking down problems.

myList = [['Mar', 5], ['Feb', 29], ['Mar', 7], ['Jan', 20], ['Feb', 3], ['Feb', 4], ['Mar', 12], ['Jan', 2], ['Jan', 16]]

if you gave me the task physically, i might create a little tally, of each month and their current running total. (this is what @Mushroomator has provided code for)

another option is to search for the months im looking for and go through the list, finding a total for that specific month as i do. e.g. find jan total by going to first entry, discarding if not jan, adding to a total if jan, and then moving onto the next month

i actually have many more, think how would you break this down?

to code the second one:

monthsICareAbout = [‘jan’,’etc?’]
newList=[]
for month in monthsICareAbout
     running_total=0 (because its fresh per month)
     for data in myList
          if data[0] == month
              running_total  = data[1] (adding the number part)
      newList.add([month, running_total])

CodePudding user response：

This should work:

myList = [['Mar', 5], ['Feb', 29], ['Mar', 7], ['Jan', 20], ['Feb', 3], ['Feb', 4], ['Mar', 12], ['Jan', 2], ['Jan', 16]]
totals = {}
for i in myList:
    totals.setdefault(i[0], 0)
    totals[i[0]]  = i[1]
newList = [[*i] for i in totals.items()]
print(newList)

Output:

[['Mar', 24], ['Feb', 36], ['Jan', 38]]