Python: Sorting array returns unintuitive order

Question

I have the two inputs and my output

Sorted_Array = ['Historic Rate', 'Overnight', '1M', '3M', '6M', '1Y', '2Y', '3Y', '4Y', '5Y', '6Y', '7Y', '8Y', '9Y', '10Y', '12Y', '15Y']

Input = ['6M', '2Y', '7Y', '1Y']

Output = ['7Y', '1Y', '6M', '2Y']

the output is unintuitive, it should be

Actual_Output = ['6M','1Y','2Y','7Y']

the code I use is:

Ouput = [x for _, x in sorted(zip(Sorted_Array,Input), key=lambda pair: pair[0])]
print(Output)

Can anyone see where this has gone wrong?

Why would you expect that output? I think your code is working properly.... — toti08
– toti08, Commented Oct 2, 2018 at 10:02
you are sorting your sorted list, not using it to sort the the second list — Frayal
– Frayal, Commented Oct 2, 2018 at 10:04
Have you looked at what zip(Sorted_Array,Input) looks like ? The output is what one would expect. I don't know what you're trying to do but obviously you're doing it wrong ;-) — bruno desthuilliers
– bruno desthuilliers, Commented Oct 2, 2018 at 10:15

ikkuh · Accepted Answer · 2018-10-02 22:32:00Z

2

You are currently pairing the items of Input with the 4 first items of Sorted_Array:

>>> temp = list(zip(Sorted_Array, Input)))
[('Historic Rate', '6M'), ('Overnight', '2Y'), ('1M', '7Y'), ('3M', '1Y')]

Then you sort with a lambda function based on the first items of these tuples. Which is in the following order:

>>> sorted(temp, key=lambda pair: pair[0])
[('1M', '7Y'), ('3M', '1Y'), ('Historic Rate', '6M'), ('Overnight', '2Y')]

It should now be clear how you end up with the result you have. To sort based on the order of the elements in Sorted_Array can the index function:

>>> sorted(Input, key=lambda item: Sorted_Array.index(item))
['6M', '1Y', '2Y', '7Y']

Note: the index function raises an exception if the item is not present in the list.

edited Oct 2, 2018 at 22:32

answered Oct 2, 2018 at 10:19

ikkuh

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2 Comments

deadvoid Over a year ago

you mean raises a ValueError if there is no such item present in the list.

78282219 Over a year ago

Thanks, I didn't realise I was doing this

deadvoid · Accepted Answer · 2018-10-02 10:13:52Z

0

sorted(zip(Sorted_Array,Input), key=lambda pair: pair[0]) sort the result based on the key, the first member of the pair, from which you extracted the second member

>>>> Output = [x for x in sorted(zip(Sorted_Array,Input), key=lambda pair: pair[0])]
>>>> print(Output)
[('1M', '7Y'), ('3M', '1Y'), ('Historic Rate', '6M'), ('Overnight', '2Y')]
>>>> Output = [x for _, x in sorted(zip(Sorted_Array,Input), key=lambda pair: pair[0])]
>>>> print(Output)
['7Y', '1Y', '6M', '2Y']

you need to change the key to pair[1]

>>>> Output = [x for _, x in sorted(zip(Sorted_Array,Input), key=lambda pair: pair[1])]
>>>> print(Output)
['1Y', '2Y', '6M', '7Y']

answered Oct 2, 2018 at 10:13

deadvoid

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2 Comments

toti08 Over a year ago

That's not the expected output, the OP says it should be ['6M','1Y','2Y','7Y'].

deadvoid Over a year ago

well, i'd say the OP expected it to be intuitive, which I'd argue sorted by the actual first char 1, 2, 6 7 is more intuitive than using the 2nd char M, Y, Y, Y. But my actual intention is to pointed out the usage of key=lamda pair: pair[0] there (in line with his question: Can anyone see where this has gone wrong?).

Frayal · Accepted Answer · 2018-10-02 11:59:32Z

0

in python 2.x:

def dist(x,y):
     return sorted_array.index(x) - sorted_array.index(y)

output = sorted(input, cmp=dist)

In python 3.x:

def func(input):
    output = input
    for i in range(len(output)):
         try:
            output[i] = sorted_array.index(output[i])
         except Exception as e:
            output[i] = len(sorted_array)+1
    return output

output = [ x for x,y in sorted((input,func(input),key = lambda k: k[1])]

this should work

edited Oct 2, 2018 at 11:59

answered Oct 2, 2018 at 10:08

Frayal

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3 Comments

Frayal Over a year ago

the sorted-array containing the order you are following to sort

toti08 Over a year ago

You mean what the OP called Sorted_Array?

Frayal Over a year ago

yes it is the Sorted_array of OP. i ll change it for better understanding

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