Perl - grep in foreach does not work with $_ but was with variable
I want to delete some defined elements from a array. I already have a solution with grep and one element: @big = grep { ! /3/ } @big. In case of several elements I want to put them in an array and using foreach. @big is the array from which I want delete elements from @del:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach $i (@del) {@big = grep { ! /$i/ } @big; print "@bign"}"
Here is the output:
1 3 4 5
1 3 5
This works fine for me. If I want to use default variable $_ it does not work:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach (@del) {@big = grep { ! /$_/ } @big; print "@bign"}"
This gives no output. Any idea what happens?
arrays perl foreach grep
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
add a comment |
I want to delete some defined elements from a array. I already have a solution with grep and one element: @big = grep { ! /3/ } @big. In case of several elements I want to put them in an array and using foreach. @big is the array from which I want delete elements from @del:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach $i (@del) {@big = grep { ! /$i/ } @big; print "@bign"}"
Here is the output:
1 3 4 5
1 3 5
This works fine for me. If I want to use default variable $_ it does not work:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach (@del) {@big = grep { ! /$_/ } @big; print "@bign"}"
This gives no output. Any idea what happens?
arrays perl foreach grep
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
2
foreachandgrepboth clobber$_. And if you re-assign@bigon every iteration, your code probably isn't doing what you want anyway!
– Matt Jacob
Nov 22 '18 at 15:45
1
@MattJacob why not? it deletes one element at every iteration over@del(or more, if more elements are matching). it might not be the fastest solution, but it's correct
– tinita
Nov 22 '18 at 17:04
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05
add a comment |
I want to delete some defined elements from a array. I already have a solution with grep and one element: @big = grep { ! /3/ } @big. In case of several elements I want to put them in an array and using foreach. @big is the array from which I want delete elements from @del:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach $i (@del) {@big = grep { ! /$i/ } @big; print "@bign"}"
Here is the output:
1 3 4 5
1 3 5
This works fine for me. If I want to use default variable $_ it does not work:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach (@del) {@big = grep { ! /$_/ } @big; print "@bign"}"
This gives no output. Any idea what happens?
arrays perl foreach grep
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
I want to delete some defined elements from a array. I already have a solution with grep and one element: @big = grep { ! /3/ } @big. In case of several elements I want to put them in an array and using foreach. @big is the array from which I want delete elements from @del:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach $i (@del) {@big = grep { ! /$i/ } @big; print "@bign"}"
Here is the output:
1 3 4 5
1 3 5
This works fine for me. If I want to use default variable $_ it does not work:
perl -e "@big = (1,2,3,4,5); @del = (2,4);
foreach (@del) {@big = grep { ! /$_/ } @big; print "@bign"}"
This gives no output. Any idea what happens?
arrays perl foreach grep
arrays perl foreach grep
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
asked Nov 22 '18 at 15:41
giordanogiordano
1,03821634
1,03821634
2
foreachandgrepboth clobber$_. And if you re-assign@bigon every iteration, your code probably isn't doing what you want anyway!
– Matt Jacob
Nov 22 '18 at 15:45
1
@MattJacob why not? it deletes one element at every iteration over@del(or more, if more elements are matching). it might not be the fastest solution, but it's correct
– tinita
Nov 22 '18 at 17:04
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05
add a comment |
2
foreachandgrepboth clobber$_. And if you re-assign@bigon every iteration, your code probably isn't doing what you want anyway!
– Matt Jacob
Nov 22 '18 at 15:45
1
@MattJacob why not? it deletes one element at every iteration over@del(or more, if more elements are matching). it might not be the fastest solution, but it's correct
– tinita
Nov 22 '18 at 17:04
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05
2
2
foreach and grep both clobber $_. And if you re-assign @big on every iteration, your code probably isn't doing what you want anyway!– Matt Jacob
Nov 22 '18 at 15:45
foreach and grep both clobber $_. And if you re-assign @big on every iteration, your code probably isn't doing what you want anyway!– Matt Jacob
Nov 22 '18 at 15:45
1
1
@MattJacob why not? it deletes one element at every iteration over
@del (or more, if more elements are matching). it might not be the fastest solution, but it's correct– tinita
Nov 22 '18 at 17:04
@MattJacob why not? it deletes one element at every iteration over
@del (or more, if more elements are matching). it might not be the fastest solution, but it's correct– tinita
Nov 22 '18 at 17:04
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05
add a comment |
1 Answer
1
active
oldest
votes
As @MattJacob points out, there are two conflicting uses for $_:
foreach (@del) {...}
This implicitly uses $_ as the loop variable. From the foreach docs:
The foreach keyword is actually a synonym for the for keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Your grep command also uses $_, always, as part of its processing. So if you had a value in $_ as part of your foreach, and it was replaced in the grep ...?
From the grep docs:
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element)
Please do check the grep docs, since there is an explicit warning about modifying a list that you are iterating over. If you are trying to iteratively shrink the list, you might consider some alternative ways to process it. (For example, could you build a single pattern for grep that would combine all the values in your @del list, and just process the @big list one time? This might be quicker, too.)
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
Thanks! If I had a look at the foreach or grep docs I would have avoid to use$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of@del? For exampe,$del = '2|4'and then:@big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.
– giordano
Nov 23 '18 at 10:21
add a comment |
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As @MattJacob points out, there are two conflicting uses for $_:
foreach (@del) {...}
This implicitly uses $_ as the loop variable. From the foreach docs:
The foreach keyword is actually a synonym for the for keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Your grep command also uses $_, always, as part of its processing. So if you had a value in $_ as part of your foreach, and it was replaced in the grep ...?
From the grep docs:
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element)
Please do check the grep docs, since there is an explicit warning about modifying a list that you are iterating over. If you are trying to iteratively shrink the list, you might consider some alternative ways to process it. (For example, could you build a single pattern for grep that would combine all the values in your @del list, and just process the @big list one time? This might be quicker, too.)
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
Thanks! If I had a look at the foreach or grep docs I would have avoid to use$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of@del? For exampe,$del = '2|4'and then:@big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.
– giordano
Nov 23 '18 at 10:21
add a comment |
As @MattJacob points out, there are two conflicting uses for $_:
foreach (@del) {...}
This implicitly uses $_ as the loop variable. From the foreach docs:
The foreach keyword is actually a synonym for the for keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Your grep command also uses $_, always, as part of its processing. So if you had a value in $_ as part of your foreach, and it was replaced in the grep ...?
From the grep docs:
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element)
Please do check the grep docs, since there is an explicit warning about modifying a list that you are iterating over. If you are trying to iteratively shrink the list, you might consider some alternative ways to process it. (For example, could you build a single pattern for grep that would combine all the values in your @del list, and just process the @big list one time? This might be quicker, too.)
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
Thanks! If I had a look at the foreach or grep docs I would have avoid to use$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of@del? For exampe,$del = '2|4'and then:@big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.
– giordano
Nov 23 '18 at 10:21
add a comment |
As @MattJacob points out, there are two conflicting uses for $_:
foreach (@del) {...}
This implicitly uses $_ as the loop variable. From the foreach docs:
The foreach keyword is actually a synonym for the for keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Your grep command also uses $_, always, as part of its processing. So if you had a value in $_ as part of your foreach, and it was replaced in the grep ...?
From the grep docs:
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element)
Please do check the grep docs, since there is an explicit warning about modifying a list that you are iterating over. If you are trying to iteratively shrink the list, you might consider some alternative ways to process it. (For example, could you build a single pattern for grep that would combine all the values in your @del list, and just process the @big list one time? This might be quicker, too.)
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
As @MattJacob points out, there are two conflicting uses for $_:
foreach (@del) {...}
This implicitly uses $_ as the loop variable. From the foreach docs:
The foreach keyword is actually a synonym for the for keyword, so you can use either. If VAR is omitted, $_ is set to each value.
Your grep command also uses $_, always, as part of its processing. So if you had a value in $_ as part of your foreach, and it was replaced in the grep ...?
From the grep docs:
Evaluates the BLOCK or EXPR for each element of LIST (locally setting $_ to each element)
Please do check the grep docs, since there is an explicit warning about modifying a list that you are iterating over. If you are trying to iteratively shrink the list, you might consider some alternative ways to process it. (For example, could you build a single pattern for grep that would combine all the values in your @del list, and just process the @big list one time? This might be quicker, too.)
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
answered Nov 22 '18 at 15:57
aghastaghast
11.2k11035
11.2k11035
Thanks! If I had a look at the foreach or grep docs I would have avoid to use$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of@del? For exampe,$del = '2|4'and then:@big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.
– giordano
Nov 23 '18 at 10:21
add a comment |
Thanks! If I had a look at the foreach or grep docs I would have avoid to use$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of@del? For exampe,$del = '2|4'and then:@big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.
– giordano
Nov 23 '18 at 10:21
Thanks! If I had a look at the foreach or grep docs I would have avoid to use
$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of @del? For exampe, $del = '2|4' and then: @big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.– giordano
Nov 23 '18 at 10:21
Thanks! If I had a look at the foreach or grep docs I would have avoid to use
$_. The advantage to put the question on SO is that you suggest to do this task in another way. What do you mean with build a single pattern for? Is that a regular expression which match all elements of @del? For exampe, $del = '2|4' and then: @big = grep { ! /$del/ } @big;. This is a good idea and so simple. It is embarassing not to get it myself. Interesting would be how to cope if putting in a pattern is not possible.– giordano
Nov 23 '18 at 10:21
add a comment |
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2
foreachandgrepboth clobber$_. And if you re-assign@bigon every iteration, your code probably isn't doing what you want anyway!– Matt Jacob
Nov 22 '18 at 15:45
1
@MattJacob why not? it deletes one element at every iteration over
@del(or more, if more elements are matching). it might not be the fastest solution, but it's correct– tinita
Nov 22 '18 at 17:04
@tinita Oops, you're right. That's what I get for browsing SO and trying to cook for Thanksgiving at the same time.
– Matt Jacob
Nov 22 '18 at 18:05