How does list comparison work?

Question

I want to compare two lists. For example:

a = [8,9,9,11] 

b = [8,7,20,10]

if a >= b :

   print "true"

Why does this print "true"? I want to compare the values vertically like this:

8 >= 8 is true

9 >= 7 is true

9 >= 20 is false but the program return true

11 >= 10 is true

Can you clarify what output you're looking for. Do you just want a result of `False`, or are you looking for a sequence, e.g., `(True, True, False, True)`? — Mark Dickinson, Nov 29 '17 at 08:26
By default, tuples (or lists) are compared similar to how strings are compared: the comparison checks corresponding items until it finds a pair that aren't equal (or one collection runs out of items), and then the comparison stops. — PM 2Ring, Nov 29 '17 at 08:31
@Ev.Kounis because first time two list element are not equal is `9` and `7` so it returns True and ignores other elements — ᴀʀᴍᴀɴ, Nov 29 '17 at 08:35

score 1 · Answer 1 · answered Nov 29 '17 at 08:31

1

You can use list comprehension and all function as follows:

code:

a = 8,9,9,11 
b = 8,7,20,10
print all([(a > b) for a, b in zip(a,b)])

output:

False

answered Nov 29 '17 at 08:31

Mahesh Karia

2

You should pass a generator expression to `all` or `any`, not a list comprehension. Both of those functions short-circuit, so they stop processing as soon as the result is clear. If you feed them a list comp the whole list has to be constructed before they start work, but if you feed them a gen exp then only as many items as are needed to determine the result will be generated. – PM 2Ring Nov 29 '17 at 08:35

ᴀʀᴍᴀɴ · Accepted Answer · 2017-11-29T08:43:14.270

0

You can use a list comprehension for comparing two lists element wise then use all function to check all of comparison are True:

a = [8,9,9,11]
b = [8,7,20,10]
all(a[i]>=b[i] for i in range(len(a))) # False

edited Nov 29 '17 at 08:43

answered Nov 29 '17 at 08:32

ᴀʀᴍᴀɴ

1

Please see [my comment to Mahesh Karia](https://stackoverflow.com/questions/47548113/how-to-compare-8-9-9-11-vs-8-7-20-10#comment82053005_47548232) – PM 2Ring Nov 29 '17 at 08:36
1

Also, the OP (probably) doesn't want to compare every pair of elements, so that double for loop is incorrect. – PM 2Ring Nov 29 '17 at 08:38
@PM2Ring , Yeah I see your point , but since can not use `break` in list comprehension , how can I stop that when first `False` occurs (with list comprehension of course)? – ᴀʀᴍᴀɴ Nov 29 '17 at 08:40
1

You use a generator expression: `all(a[i]>=b[i] for i in range(len(a)))`. Even better, use direct iteration instead of indices: `all(a >= b for a, b in zip(a, b))`. And that will safely handle tuples that aren't the same length. – PM 2Ring Nov 29 '17 at 08:42

2 Answers2