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Why does Python start at index -1 when indexing a list from the end?
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list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"
print(list[-4]) # Number -4 is "a"
python list
New contributor
add a comment |
list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"
print(list[-4]) # Number -4 is "a"
python list
New contributor
27
Don't uselist
as a variable name, it's the name of a standard class.
– Barmar
10 hours ago
8
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
1
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago
add a comment |
list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"
print(list[-4]) # Number -4 is "a"
python list
New contributor
list = ["a", "b", "c", "d"]
print(list[3]) # Number 3 is "d"
print(list[-4]) # Number -4 is "a"
python list
python list
New contributor
New contributor
edited 16 mins ago
Aaron Hall♦
185k53309264
185k53309264
New contributor
asked 18 hours ago
abrahamabraham
21626
21626
New contributor
New contributor
27
Don't uselist
as a variable name, it's the name of a standard class.
– Barmar
10 hours ago
8
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
1
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago
add a comment |
27
Don't uselist
as a variable name, it's the name of a standard class.
– Barmar
10 hours ago
8
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
1
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago
27
27
Don't use
list
as a variable name, it's the name of a standard class.– Barmar
10 hours ago
Don't use
list
as a variable name, it's the name of a standard class.– Barmar
10 hours ago
8
8
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
1
1
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago
add a comment |
7 Answers
7
active
oldest
votes
To explain it in another way, because -0
is equal to 0
, if backward starts from 0
, it is ambiguous to the interpreter.
If you are confused about -
, and looking for another way to index backwards more understandably, you can try ~
, it is a mirror of forward:
arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c
The typical usages for ~
are like "swap mirror node" or "find median in a sort list":
def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]
def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2
~
actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like ~
:
In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.
For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.
def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
add a comment |
list[-1]
Is short hand for:
list[len(list)-1]
The len(list)
part is implicit. That's why the -1
is the last element. That goes for any negative index - the subtraction from len(list)
is always implicit
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
add a comment |
Because -0
in Python is 0
.
With 0
you get first element of list and
with -1
you get the last element of the list.
list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"
I think-0
is0
pretty much everywhere.
– Koray Tugay
10 hours ago
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not0
anymore. @Barmar
– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
add a comment |
This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
"Think about a[2] and a[-2]" isa
the list, or an element of the list?
– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
add a comment |
Another explanation:
Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.
Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.
New contributor
add a comment |
This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i
elements. -1
referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.
For any list xs
and index i
positive or negative, the expression
xs[i]
will either have the same value as the expression below or produce an IndexError
:
xs[i % len(xs)]
The index of the last element is -1 + len(xs)
which is congruent to -1
mod len(xs)
. For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.
In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs)
or smaller than -1-len(xs)
are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.
add a comment |
In a comment, abraham asked
I wish python has another mechanism to get list backward.
For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:
lst = ["a", "b", "c", "d"]
print(lst[::-1][0]) # "d"
print(lst[::-1][1]) # "c"
New contributor
2
True, but not really answer the question "why".
– Valentino
15 hours ago
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
add a comment |
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7 Answers
7
active
oldest
votes
7 Answers
7
active
oldest
votes
active
oldest
votes
active
oldest
votes
To explain it in another way, because -0
is equal to 0
, if backward starts from 0
, it is ambiguous to the interpreter.
If you are confused about -
, and looking for another way to index backwards more understandably, you can try ~
, it is a mirror of forward:
arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c
The typical usages for ~
are like "swap mirror node" or "find median in a sort list":
def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]
def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2
~
actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like ~
:
In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.
For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.
def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
add a comment |
To explain it in another way, because -0
is equal to 0
, if backward starts from 0
, it is ambiguous to the interpreter.
If you are confused about -
, and looking for another way to index backwards more understandably, you can try ~
, it is a mirror of forward:
arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c
The typical usages for ~
are like "swap mirror node" or "find median in a sort list":
def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]
def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2
~
actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like ~
:
In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.
For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.
def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
add a comment |
To explain it in another way, because -0
is equal to 0
, if backward starts from 0
, it is ambiguous to the interpreter.
If you are confused about -
, and looking for another way to index backwards more understandably, you can try ~
, it is a mirror of forward:
arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c
The typical usages for ~
are like "swap mirror node" or "find median in a sort list":
def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]
def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2
~
actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like ~
:
In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.
For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.
def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums
To explain it in another way, because -0
is equal to 0
, if backward starts from 0
, it is ambiguous to the interpreter.
If you are confused about -
, and looking for another way to index backwards more understandably, you can try ~
, it is a mirror of forward:
arr = ["a", "b", "c", "d"]
print(arr[~0]) # d
print(arr[~1]) # c
The typical usages for ~
are like "swap mirror node" or "find median in a sort list":
def reverse(arr):
for i in range(len(arr)//2):
arr[i], arr[~i] = arr[~i], arr[i]
def median(arr):
mid = len(arr) // 2
return (arr[mid] + arr[~mid]) / 2
~
actually is a math trick of inverse code and complement code, and it is more easy to understand in some situations.
Discussion about whether should use python tricks like ~
:
In my opinion, if it is a code maintained by yourself, you can use any trick to avoid potential bug or achieve goal easier, because of maybe a high readability and usability. But in team work, avoid using 'too clever' code, may bring troubles to your co-workers.
For example, here is one concise code from Stefan Pochmann to solve this problem. I learned a lot from his code. But some are just for fun, too hackish to use.
def findStrobogrammatic(self, n):
nums = n % 2 * list('018') or ['']
while n > 1:
n -= 2
# n < 2 is so genius here
nums = [a + num + b for a, b in '00 11 88 69 96'.split()[n < 2:] for num in nums]
return nums
edited 9 hours ago
answered 18 hours ago
recnacrecnac
1,9222524
1,9222524
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
add a comment |
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
Comments are not for extended discussion; this conversation has been moved to chat.
– Samuel Liew♦
2 hours ago
add a comment |
list[-1]
Is short hand for:
list[len(list)-1]
The len(list)
part is implicit. That's why the -1
is the last element. That goes for any negative index - the subtraction from len(list)
is always implicit
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
add a comment |
list[-1]
Is short hand for:
list[len(list)-1]
The len(list)
part is implicit. That's why the -1
is the last element. That goes for any negative index - the subtraction from len(list)
is always implicit
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
add a comment |
list[-1]
Is short hand for:
list[len(list)-1]
The len(list)
part is implicit. That's why the -1
is the last element. That goes for any negative index - the subtraction from len(list)
is always implicit
list[-1]
Is short hand for:
list[len(list)-1]
The len(list)
part is implicit. That's why the -1
is the last element. That goes for any negative index - the subtraction from len(list)
is always implicit
answered 18 hours ago
DroidX86DroidX86
2,261919
2,261919
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
add a comment |
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
5
5
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
In my opinion this answer is better than the accepted one.
– NicolasB
14 hours ago
3
3
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
Be aware that list[-n] and list[len(list)-n] are only equivilent for values of n between 1 and len(list). This becomes especially important when slicing rather than indexing.
– plugwash
9 hours ago
add a comment |
Because -0
in Python is 0
.
With 0
you get first element of list and
with -1
you get the last element of the list.
list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"
I think-0
is0
pretty much everywhere.
– Koray Tugay
10 hours ago
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not0
anymore. @Barmar
– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
add a comment |
Because -0
in Python is 0
.
With 0
you get first element of list and
with -1
you get the last element of the list.
list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"
I think-0
is0
pretty much everywhere.
– Koray Tugay
10 hours ago
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not0
anymore. @Barmar
– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
add a comment |
Because -0
in Python is 0
.
With 0
you get first element of list and
with -1
you get the last element of the list.
list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"
Because -0
in Python is 0
.
With 0
you get first element of list and
with -1
you get the last element of the list.
list = ["a", "b", "c", "d"]
print(list[0]) # "a"
print(list[-1]) # "d"
edited 18 hours ago
answered 18 hours ago
AshishAshish
1,457630
1,457630
I think-0
is0
pretty much everywhere.
– Koray Tugay
10 hours ago
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not0
anymore. @Barmar
– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
add a comment |
I think-0
is0
pretty much everywhere.
– Koray Tugay
10 hours ago
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not0
anymore. @Barmar
– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
I think
-0
is 0
pretty much everywhere.– Koray Tugay
10 hours ago
I think
-0
is 0
pretty much everywhere.– Koray Tugay
10 hours ago
2
2
@KorayTugay Except in floating point.
– Barmar
10 hours ago
@KorayTugay Except in floating point.
– Barmar
10 hours ago
I guess that is not
0
anymore. @Barmar– Koray Tugay
10 hours ago
I guess that is not
0
anymore. @Barmar– Koray Tugay
10 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
@KorayTugay It is. The binary representation is even all 0s. Floating point also just has another 0, where the sign bit is 1.
– curiousdannii
2 hours ago
add a comment |
This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
"Think about a[2] and a[-2]" isa
the list, or an element of the list?
– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
add a comment |
This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
"Think about a[2] and a[-2]" isa
the list, or an element of the list?
– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
add a comment |
This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
This is the mnemonic method I use. It is just an approach of what is happening, but it works.
Don't think of those as indexes. Think of them as offsets on a circular list.
Let's use the list x = [a,b,c,d,e,f,g,h] as an example. Think about x[2] and x[-2]:
You start at offset zero. If you move two steps forward, you're going from a to b (0 to 1), and them from b to c (1 to 2).
If you move two steps backward, you're going from a to h (0 to -1), and then from h to g (-1 to -2)
edited 10 mins ago
answered 8 hours ago
T. SarT. Sar
181110
181110
"Think about a[2] and a[-2]" isa
the list, or an element of the list?
– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
add a comment |
"Think about a[2] and a[-2]" isa
the list, or an element of the list?
– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
"Think about a[2] and a[-2]" is
a
the list, or an element of the list?– detly
3 hours ago
"Think about a[2] and a[-2]" is
a
the list, or an element of the list?– detly
3 hours ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
"a" is an hypothetical list which contains the values a-h! I'll clarify!
– T. Sar
10 mins ago
add a comment |
Another explanation:
Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.
Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.
New contributor
add a comment |
Another explanation:
Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.
Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.
New contributor
add a comment |
Another explanation:
Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.
Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.
New contributor
Another explanation:
Your finger points to the first element. The index decides how many places you shift your finger to the right. If the number is negative, you shift your finger to the left.
Of course, you cant step to the left from the first element, so the first step to the left wraps around to the last element.
New contributor
New contributor
answered 12 hours ago
OscarOscar
213
213
New contributor
New contributor
add a comment |
add a comment |
This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i
elements. -1
referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.
For any list xs
and index i
positive or negative, the expression
xs[i]
will either have the same value as the expression below or produce an IndexError
:
xs[i % len(xs)]
The index of the last element is -1 + len(xs)
which is congruent to -1
mod len(xs)
. For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.
In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs)
or smaller than -1-len(xs)
are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.
add a comment |
This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i
elements. -1
referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.
For any list xs
and index i
positive or negative, the expression
xs[i]
will either have the same value as the expression below or produce an IndexError
:
xs[i % len(xs)]
The index of the last element is -1 + len(xs)
which is congruent to -1
mod len(xs)
. For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.
In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs)
or smaller than -1-len(xs)
are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.
add a comment |
This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i
elements. -1
referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.
For any list xs
and index i
positive or negative, the expression
xs[i]
will either have the same value as the expression below or produce an IndexError
:
xs[i % len(xs)]
The index of the last element is -1 + len(xs)
which is congruent to -1
mod len(xs)
. For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.
In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs)
or smaller than -1-len(xs)
are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.
This idiom can be justified using modular arithmetic. We can think of indices as referring to a cell in a list obtained by walking forward i
elements. -1
referring to the last element of the list is a natural generalization of this, since we arrive at the last element in the list if we walk backwards one step from the start of the list.
For any list xs
and index i
positive or negative, the expression
xs[i]
will either have the same value as the expression below or produce an IndexError
:
xs[i % len(xs)]
The index of the last element is -1 + len(xs)
which is congruent to -1
mod len(xs)
. For example, in an array of length 12, the canonical index of the last element is 11. 11 is congruent to -1 mod 12.
In Python, though, arrays are more often used as linear data structures than circular ones, so indexes larger than len(xs)
or smaller than -1-len(xs)
are out of bounds since there's seldom a need for them and the effects would be really counterintuitive if the size of the array ever changed.
answered 2 hours ago
Gregory NisbetGregory Nisbet
3,23321334
3,23321334
add a comment |
add a comment |
In a comment, abraham asked
I wish python has another mechanism to get list backward.
For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:
lst = ["a", "b", "c", "d"]
print(lst[::-1][0]) # "d"
print(lst[::-1][1]) # "c"
New contributor
2
True, but not really answer the question "why".
– Valentino
15 hours ago
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
add a comment |
In a comment, abraham asked
I wish python has another mechanism to get list backward.
For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:
lst = ["a", "b", "c", "d"]
print(lst[::-1][0]) # "d"
print(lst[::-1][1]) # "c"
New contributor
2
True, but not really answer the question "why".
– Valentino
15 hours ago
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
add a comment |
In a comment, abraham asked
I wish python has another mechanism to get list backward.
For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:
lst = ["a", "b", "c", "d"]
print(lst[::-1][0]) # "d"
print(lst[::-1][1]) # "c"
New contributor
In a comment, abraham asked
I wish python has another mechanism to get list backward.
For another way to access lists backwards, one could simply first reverse the list and then access the elements as usual:
lst = ["a", "b", "c", "d"]
print(lst[::-1][0]) # "d"
print(lst[::-1][1]) # "c"
New contributor
edited 13 hours ago
New contributor
answered 15 hours ago
Michael EngenMichael Engen
12
12
New contributor
New contributor
2
True, but not really answer the question "why".
– Valentino
15 hours ago
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
add a comment |
2
True, but not really answer the question "why".
– Valentino
15 hours ago
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
2
2
True, but not really answer the question "why".
– Valentino
15 hours ago
True, but not really answer the question "why".
– Valentino
15 hours ago
4
4
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
It's a waste of ressources to create a whole new list in order to retrieve a single element.
– Eric Duminil
14 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
What is the use of showing another way to solve a task when it is all of, syntaxly, logically and computationally more complex than the standard method?
– Sam
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
@Sam Why use C when we got Assembly?
– mid
10 hours ago
add a comment |
abraham is a new contributor. Be nice, and check out our Code of Conduct.
abraham is a new contributor. Be nice, and check out our Code of Conduct.
abraham is a new contributor. Be nice, and check out our Code of Conduct.
abraham is a new contributor. Be nice, and check out our Code of Conduct.
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27
Don't use
list
as a variable name, it's the name of a standard class.– Barmar
10 hours ago
8
It isn't starting at 1, it's starting at -1. ?!?
– Thomas Weller
6 hours ago
1
mod arithmetic should really be mentioned on this page somewhere...
– Nacht
2 hours ago