How to separate a Python list into two lists, according to some aspect of the elements

Question

I have a list like this:

[[8, \"Plot\", \"Sunday\"], [1, \"unPlot\", \"Monday\"], [12, \"Plot\", \"Monday\"], [10, \"Plot\", \"Tuesday\"], [4, \"unPlot\", \"         


        

Answer 1

I have a helper function for the general case of partitioning a list in two:

def partition(iterable, condition):
        def partition_element(partitions, element):
            (partitions[0] if condition(element) else partitions[1]).append(element)
            return partitions
        return reduce(partition_element, iterable, ([], []))

For example:

>>> partition([1, 2, 3, 4], lambda d: d % 2 == 0)
([2, 4], [1, 3])

Or in your case:

>>> partition(your_list, lambda i: i[1] == "Plot")

Answer 2

Try:

yourList=[[8, "Plot", "Sunday"], [1, "unPlot", "Monday"], [12, "Plot", "Monday"], [10, "Plot", "Tuesday"], [4, "unPlot", "Tuesday"], [14, "Plot", "Wednesday"], [6, "unPlot", "Wednesday"], [1, "unPlot", "Thursday"], [19, "Plot", "Thursday"], [28, "Plot", "Friday"], [10, "unPlot", "Friday"], [3, "unPlot", "Saturday"]]
plotList=[]
unPlotList=[]

for i in yourList:
    if "Plot" in i:
        plotList.append(i)
    else:
        unPlotList.append(i)

or shorter with comprehension:

plotList = [i for i in yourList if "Plot" in i]
unPlotList = [i for i in yourList if "unPlot" in i]

Answer 3

You can also do it with the filter command:

list1 = filter(lambda x: x[1] == "Plot", list)
list2 = filter(lambda x: x[1] == "unPlot", list)

Answer 4

You could use list comprehensions, e.g.

# old_list elements should be tuples if they're fixed-size, BTW
list1 = [(X, Y, Z) for X, Y, Z in old_list if Y == 'Plot']
list2 = [(X, Y, Z) for X, Y, Z in old_list if Y == 'unPlot']

If you want to traverse the input list only once, then maybe:

def split_list(old_list):
    list1 = []
    list2 = []
    for X, Y, Z in old_list:
        if Y == 'Plot':
            list1.append((X, Y, Z))
        else:
            list2.append((X, Y, Z))
    return list1, list2

Answer 5

data = [[8, "Plot", "Sunday"], [1, "unPlot", "Monday"], [12, "Plot", "Monday"], [10, "Plot", "Tuesday"], [4, "unPlot", "Tuesday"], [14, "Plot", "Wednesday"], [6, "unPlot", "Wednesday"], [1, "unPlot", "Thursday"], [19, "Plot", "Thursday"], [28, "Plot", "Friday"], [10, "unPlot", "Friday"], [3, "unPlot", "Saturday"]]

res = {'Plot':[],'unPlot':[]}
for i in data: res[i[1]].append(i)

This way you iterate the list once

Answer 6

Use list comprehension:

l = [[8, "Plot", "Sunday"], [1, "unPlot", "Monday"], [12, "Plot", "Monday"], [10, "Plot", "Tuesday"], [4, "unPlot", "Tuesday"], [14, "Plot", "Wednesday"], [6, "unPlot", "Wednesday"], [1, "unPlot", "Thursday"], [19, "Plot", "Thursday"], [28, "Plot", "Friday"], [10, "unPlot", "Friday"], [3, "unPlot", "Saturday"]]

list1 = [x for x in l if x[1] == "Plot"]

list2 = [x for x in l if x[1] == "unPlot"]