List comprehension iterating over two lists is not working as expected [duplicate]

This question already has an answer here:

Nested list comprehension with two lists

5 answers

I want to iterate over two lists. The first list contains some browser user-agents and the second list contains versions of those browsers. I want to filter out only those user-agents whose version is greater than 60.

Here is how my list comprehension looks:

[link for ver in version for link in useragents if ver > 60]

The problem with this list is that it prints same user-agent multiple times. I wrote the following using the zip function, which works fine:

for link, ver in zip(useragents, version):

    if ver > 60:

        # append to list

        print(link)

Why is my list comprehension returning unexpected results?

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

marked as duplicate by coldspeed python
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Jan 29 at 18:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

This question already has an answer here:

Nested list comprehension with two lists

5 answers

Here is how my list comprehension looks:

[link for ver in version for link in useragents if ver > 60]

The problem with this list is that it prints same user-agent multiple times. I wrote the following using the zip function, which works fine:

for link, ver in zip(useragents, version):

    if ver > 60:

        # append to list

        print(link)

Why is my list comprehension returning unexpected results?

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

marked as duplicate by coldspeed python
Users with the python badge can single-handedly close python questions as duplicates and reopen them as needed.

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Jan 29 at 18:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

This question already has an answer here:

Nested list comprehension with two lists

5 answers

Here is how my list comprehension looks:

[link for ver in version for link in useragents if ver > 60]

The problem with this list is that it prints same user-agent multiple times. I wrote the following using the zip function, which works fine:

for link, ver in zip(useragents, version):

    if ver > 60:

        # append to list

        print(link)

Why is my list comprehension returning unexpected results?

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

This question already has an answer here:

Nested list comprehension with two lists

5 answers

Here is how my list comprehension looks:

[link for ver in version for link in useragents if ver > 60]

The problem with this list is that it prints same user-agent multiple times. I wrote the following using the zip function, which works fine:

for link, ver in zip(useragents, version):

    if ver > 60:

        # append to list

        print(link)

Why is my list comprehension returning unexpected results?

This question already has an answer here:

Nested list comprehension with two lists

5 answers

python list-comprehension

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

edited Jan 29 at 14:35

gary

3,85822548

edited Jan 29 at 14:35

gary

3,85822548

edited Jan 29 at 14:35

gary

3,85822548

asked Jan 29 at 12:29

Viktor

352215

asked Jan 29 at 12:29

Viktor

352215

asked Jan 29 at 12:29

Viktor

352215

marked as duplicate by coldspeed python
Users with the python badge can single-handedly close python questions as duplicates and reopen them as needed.

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Jan 29 at 18:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

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Jan 29 at 18:24

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

add a comment |

5 Answers
5

active

oldest

votes

Your first list comprehension is equivalent to:

res = 

for ver in version:

    for link in useragents:

        if ver > 60:

            res.append(link)

Notice you have nested loop with time complexity O(n²), i.e. you are iterating over every combination of version and useragents. That's not what you want, assuming your version and useragents lists are aligned.

The equivalent of your for loop is the following list comprehension:

res = [link for link, ver in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:34

jpp

102k2165116

add a comment |

[link for (link, ver) in zip(useragents, version) if ver > 60]

You still have to zip the two lists together.

answered Jan 29 at 12:33

shuebner

815

add a comment |

This

[link for ver in version for link in useragents if ver > 60]

is not the same as zip. It's not iterating through the two sequences in parallel. It's iterating through all combinations of those two sequences.

It is as if you wrote:

for ver in version:

    for link in useragents:

        if ver > 60:

            # append(link)

So if both sequences had length 5, there would be 25 combinations (some of which are filtered out by the condition ver > 60).

When you want to go through sequences in parallel, zip is the way to do it, even in a comprehension.

[link for (link, ver) in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:36

khelwood

32.1k74465

add a comment |

Alternatively you can use the function compress() in combination with map(), where you check some condition:

from itertools import compress



filter_ = map(lambda x: x > 60, version)

list(compress(useragents, filter_))

Example:

s = 'ABCDEFG'

nums = range(len(s))



filter_ = map(lambda x: x > 3, nums)

print(list(compress(s, filter_)))

# ['E', 'F', 'G']

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

add a comment |

Can't be sure on what's happening, without your data, but in general, "double" list comprehension is not the same as zip, but rather a double loop, i.e.

[a for b in bs for a in as]

is equivalent to

for b in bs:

    for a in as:

        lst.append(a)

answered Jan 29 at 12:35

Slam

5,53712435

add a comment |

5 Answers
5

active

oldest

votes

5 Answers
5

active

oldest

votes

Your first list comprehension is equivalent to:

res = 

for ver in version:

    for link in useragents:

        if ver > 60:

            res.append(link)

The equivalent of your for loop is the following list comprehension:

res = [link for link, ver in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:34

jpp

102k2165116

add a comment |

Your first list comprehension is equivalent to:

res = 

for ver in version:

    for link in useragents:

        if ver > 60:

            res.append(link)

The equivalent of your for loop is the following list comprehension:

res = [link for link, ver in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:34

jpp

102k2165116

add a comment |

Your first list comprehension is equivalent to:

res = 

for ver in version:

    for link in useragents:

        if ver > 60:

            res.append(link)

The equivalent of your for loop is the following list comprehension:

res = [link for link, ver in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:34

jpp

102k2165116

Your first list comprehension is equivalent to:

res = 

for ver in version:

    for link in useragents:

        if ver > 60:

            res.append(link)

The equivalent of your for loop is the following list comprehension:

res = [link for link, ver in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:34

jpp

102k2165116

answered Jan 29 at 12:34

jpp

102k2165116

answered Jan 29 at 12:34

jpp

102k2165116

answered Jan 29 at 12:34

jpp

102k2165116

add a comment |

[link for (link, ver) in zip(useragents, version) if ver > 60]

You still have to zip the two lists together.

answered Jan 29 at 12:33

shuebner

815

add a comment |

[link for (link, ver) in zip(useragents, version) if ver > 60]

You still have to zip the two lists together.

answered Jan 29 at 12:33

shuebner

815

add a comment |

[link for (link, ver) in zip(useragents, version) if ver > 60]

You still have to zip the two lists together.

answered Jan 29 at 12:33

shuebner

815

[link for (link, ver) in zip(useragents, version) if ver > 60]

You still have to zip the two lists together.

answered Jan 29 at 12:33

shuebner

815

answered Jan 29 at 12:33

shuebner

815

answered Jan 29 at 12:33

shuebner

815

answered Jan 29 at 12:33

shuebner

815

add a comment |

This

[link for ver in version for link in useragents if ver > 60]

is not the same as zip. It's not iterating through the two sequences in parallel. It's iterating through all combinations of those two sequences.

It is as if you wrote:

for ver in version:

    for link in useragents:

        if ver > 60:

            # append(link)

So if both sequences had length 5, there would be 25 combinations (some of which are filtered out by the condition ver > 60).

When you want to go through sequences in parallel, zip is the way to do it, even in a comprehension.

[link for (link, ver) in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:36

khelwood

32.1k74465

add a comment |

This

[link for ver in version for link in useragents if ver > 60]

is not the same as zip. It's not iterating through the two sequences in parallel. It's iterating through all combinations of those two sequences.

It is as if you wrote:

for ver in version:

    for link in useragents:

        if ver > 60:

            # append(link)

So if both sequences had length 5, there would be 25 combinations (some of which are filtered out by the condition ver > 60).

When you want to go through sequences in parallel, zip is the way to do it, even in a comprehension.

[link for (link, ver) in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:36

khelwood

32.1k74465

add a comment |

This

[link for ver in version for link in useragents if ver > 60]

is not the same as zip. It's not iterating through the two sequences in parallel. It's iterating through all combinations of those two sequences.

It is as if you wrote:

for ver in version:

    for link in useragents:

        if ver > 60:

            # append(link)

So if both sequences had length 5, there would be 25 combinations (some of which are filtered out by the condition ver > 60).

When you want to go through sequences in parallel, zip is the way to do it, even in a comprehension.

[link for (link, ver) in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:36

khelwood

32.1k74465

This

[link for ver in version for link in useragents if ver > 60]

is not the same as zip. It's not iterating through the two sequences in parallel. It's iterating through all combinations of those two sequences.

It is as if you wrote:

for ver in version:

    for link in useragents:

        if ver > 60:

            # append(link)

So if both sequences had length 5, there would be 25 combinations (some of which are filtered out by the condition ver > 60).

When you want to go through sequences in parallel, zip is the way to do it, even in a comprehension.

[link for (link, ver) in zip(useragents, version) if ver > 60]

answered Jan 29 at 12:36

khelwood

32.1k74465

answered Jan 29 at 12:36

khelwood

32.1k74465

answered Jan 29 at 12:36

khelwood

32.1k74465

answered Jan 29 at 12:36

khelwood

32.1k74465

add a comment |

Alternatively you can use the function compress() in combination with map(), where you check some condition:

from itertools import compress



filter_ = map(lambda x: x > 60, version)

list(compress(useragents, filter_))

Example:

s = 'ABCDEFG'

nums = range(len(s))



filter_ = map(lambda x: x > 3, nums)

print(list(compress(s, filter_)))

# ['E', 'F', 'G']

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

add a comment |

Alternatively you can use the function compress() in combination with map(), where you check some condition:

from itertools import compress



filter_ = map(lambda x: x > 60, version)

list(compress(useragents, filter_))

Example:

s = 'ABCDEFG'

nums = range(len(s))



filter_ = map(lambda x: x > 3, nums)

print(list(compress(s, filter_)))

# ['E', 'F', 'G']

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

add a comment |

Alternatively you can use the function compress() in combination with map(), where you check some condition:

from itertools import compress



filter_ = map(lambda x: x > 60, version)

list(compress(useragents, filter_))

Example:

s = 'ABCDEFG'

nums = range(len(s))



filter_ = map(lambda x: x > 3, nums)

print(list(compress(s, filter_)))

# ['E', 'F', 'G']

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

Alternatively you can use the function compress() in combination with map(), where you check some condition:

from itertools import compress



filter_ = map(lambda x: x > 60, version)

list(compress(useragents, filter_))

Example:

s = 'ABCDEFG'

nums = range(len(s))



filter_ = map(lambda x: x > 3, nums)

print(list(compress(s, filter_)))

# ['E', 'F', 'G']

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

edited Jan 29 at 21:10

answered Jan 29 at 12:41

Mykola Zotko

1,865518

answered Jan 29 at 12:41

Mykola Zotko

1,865518

answered Jan 29 at 12:41

Mykola Zotko

1,865518

add a comment |

Can't be sure on what's happening, without your data, but in general, "double" list comprehension is not the same as zip, but rather a double loop, i.e.

[a for b in bs for a in as]

is equivalent to

for b in bs:

    for a in as:

        lst.append(a)

answered Jan 29 at 12:35

Slam

5,53712435

add a comment |

Can't be sure on what's happening, without your data, but in general, "double" list comprehension is not the same as zip, but rather a double loop, i.e.

[a for b in bs for a in as]

is equivalent to

for b in bs:

    for a in as:

        lst.append(a)

answered Jan 29 at 12:35

Slam

5,53712435

add a comment |

Can't be sure on what's happening, without your data, but in general, "double" list comprehension is not the same as zip, but rather a double loop, i.e.

[a for b in bs for a in as]

is equivalent to

for b in bs:

    for a in as:

        lst.append(a)

answered Jan 29 at 12:35

Slam

5,53712435

Can't be sure on what's happening, without your data, but in general, "double" list comprehension is not the same as zip, but rather a double loop, i.e.

[a for b in bs for a in as]

is equivalent to

for b in bs:

    for a in as:

        lst.append(a)

answered Jan 29 at 12:35

Slam

5,53712435

answered Jan 29 at 12:35

Slam

5,53712435

answered Jan 29 at 12:35

Slam

5,53712435

answered Jan 29 at 12:35

Slam

5,53712435

add a comment |

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