# How can I compare two lists in python and return matches

## How can I compare two lists in python and return matches

Not the most efficient one, but by far the most obvious way to do it is:

```
>>> a = [1, 2, 3, 4, 5]
>>> b = [9, 8, 7, 6, 5]
>>> set(a) & set(b)
{5}
```

if order is significant you can do it with list comprehensions like this:

```
>>> [i for i, j in zip(a, b) if i == j]
[5]
```

(only works for equal-sized lists, which order-significance implies).

Use set.intersection(), its fast and readable.

```
>>> set(a).intersection(b)
set([5])
```

#### How can I compare two lists in python and return matches

A quick performance test showing Lutzs solution is the best:

```
import time
def speed_test(func):
def wrapper(*args, **kwargs):
t1 = time.time()
for x in xrange(5000):
results = func(*args, **kwargs)
t2 = time.time()
print %s took %0.3f ms % (func.func_name, (t2-t1)*1000.0)
return results
return wrapper
@speed_test
def compare_bitwise(x, y):
set_x = frozenset(x)
set_y = frozenset(y)
return set_x & set_y
@speed_test
def compare_listcomp(x, y):
return [i for i, j in zip(x, y) if i == j]
@speed_test
def compare_intersect(x, y):
return frozenset(x).intersection(y)
# Comparing short lists
a = [1, 2, 3, 4, 5]
b = [9, 8, 7, 6, 5]
compare_bitwise(a, b)
compare_listcomp(a, b)
compare_intersect(a, b)
# Comparing longer lists
import random
a = random.sample(xrange(100000), 10000)
b = random.sample(xrange(100000), 10000)
compare_bitwise(a, b)
compare_listcomp(a, b)
compare_intersect(a, b)
```

These are the results on my machine:

```
# Short list:
compare_bitwise took 10.145 ms
compare_listcomp took 11.157 ms
compare_intersect took 7.461 ms
# Long list:
compare_bitwise took 11203.709 ms
compare_listcomp took 17361.736 ms
compare_intersect took 6833.768 ms
```

Obviously, any artificial performance test should be taken with a grain of salt, but since the `set().intersection()`

answer is *at least as fast* as the other solutions, and also the most readable, it should be the standard solution for this common problem.