Python: bool(-1) is returning true. Why does this happen?
I decided to cast -1 into a bool to check what the outcome was.
Contrary to my Expectation that bool(-1) would return FALSE it returns TRUE.
Why is this the case?
python boolean
edited Nov 14 '18 at 9:18
D.Mendes
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
add a comment |
I decided to cast -1 into a bool to check what the outcome was.
Contrary to my Expectation that bool(-1) would return FALSE it returns TRUE.
Why is this the case?
python boolean
edited Nov 14 '18 at 9:18
D.Mendes
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
1
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31
add a comment |
I decided to cast -1 into a bool to check what the outcome was.
Contrary to my Expectation that bool(-1) would return FALSE it returns TRUE.
Why is this the case?
python boolean
edited Nov 14 '18 at 9:18
D.Mendes
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
I decided to cast -1 into a bool to check what the outcome was.
Contrary to my Expectation that bool(-1) would return FALSE it returns TRUE.
Why is this the case?
python boolean
python boolean
edited Nov 14 '18 at 9:18
D.Mendes
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
edited Nov 14 '18 at 9:18
D.Mendes
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
edited Nov 14 '18 at 9:18
D.Mendes
11810
edited Nov 14 '18 at 9:18
D.Mendes
11810
edited Nov 14 '18 at 9:18
D.Mendes
11810
11810
asked Nov 14 '18 at 8:21
VenkatVenkat
342
asked Nov 14 '18 at 8:21
VenkatVenkat
342
asked Nov 14 '18 at 8:21
VenkatVenkat
342
342
1
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31
add a comment |
1
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31
1
1
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31
add a comment |
2 Answers
2
active
oldest
votes
Based on Python 3 documentation, most of the built-in objects considered false are:
- Constants defined to be false: None and False.
- Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
- Empty sequences and collections: '', (), , {}, set(), range(0)
Notice that it only specifies zero to be false, instead of any value that is equal to or lower than 0. This is also common in various other programming languages.
edited Nov 14 '18 at 9:24
jpp
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
1
...and to make the answer complete, I'd also mention thatbool()uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values
– NPE
Nov 14 '18 at 9:22
add a comment |
TL;DR
Yes, this is expected. Most programming languages only consider 0 to be false.
Elaborated
This makes sense when you consider how -1 is represented in the processor. Typically you convert a positive number to it's negative counterpart by using 2's complement. 2's complent represents a negative binary number by inverting all the bits and adding 1. Say we have a 4-bit architecture, the decimal 1 is represented as 0001. To get its negative representation, invert all the bits and add 1, thus 0001 becomes 1110 + 1 = 1111.
When we want to know if a value is false, we can or all its inputs, if the result is 0, it's false, otherwise it's true, and thus it makes perfect sense that -1 == true.
The reson that we use 2's complement, as appose to 1's complement (where we just invert all bits without adding 1) is exacly because we do not want to be able to represent both 0 and -0.
answered Nov 14 '18 at 8:34
CortexCortex
7010
add a comment |
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2 Answers
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oldest
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2 Answers
2
active
oldest
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active
oldest
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active
oldest
votes
Based on Python 3 documentation, most of the built-in objects considered false are:
- Constants defined to be false: None and False.
- Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
- Empty sequences and collections: '', (), , {}, set(), range(0)
Notice that it only specifies zero to be false, instead of any value that is equal to or lower than 0. This is also common in various other programming languages.
edited Nov 14 '18 at 9:24
jpp
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
1
...and to make the answer complete, I'd also mention thatbool()uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values
– NPE
Nov 14 '18 at 9:22
add a comment |
Based on Python 3 documentation, most of the built-in objects considered false are:
- Constants defined to be false: None and False.
- Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
- Empty sequences and collections: '', (), , {}, set(), range(0)
Notice that it only specifies zero to be false, instead of any value that is equal to or lower than 0. This is also common in various other programming languages.
edited Nov 14 '18 at 9:24
jpp
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
1
...and to make the answer complete, I'd also mention thatbool()uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values
– NPE
Nov 14 '18 at 9:22
add a comment |
Based on Python 3 documentation, most of the built-in objects considered false are:
- Constants defined to be false: None and False.
- Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
- Empty sequences and collections: '', (), , {}, set(), range(0)
Notice that it only specifies zero to be false, instead of any value that is equal to or lower than 0. This is also common in various other programming languages.
edited Nov 14 '18 at 9:24
jpp
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
Based on Python 3 documentation, most of the built-in objects considered false are:
- Constants defined to be false: None and False.
- Zero of any numeric type: 0, 0.0, 0j, Decimal(0), Fraction(0, 1)
- Empty sequences and collections: '', (), , {}, set(), range(0)
Notice that it only specifies zero to be false, instead of any value that is equal to or lower than 0. This is also common in various other programming languages.
edited Nov 14 '18 at 9:24
jpp
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
edited Nov 14 '18 at 9:24
jpp
100k2161111
edited Nov 14 '18 at 9:24
jpp
100k2161111
edited Nov 14 '18 at 9:24
jpp
100k2161111
100k2161111
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
answered Nov 14 '18 at 8:36
AndreasAndreas
1,90731018
1,90731018
1
...and to make the answer complete, I'd also mention thatbool()uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values
– NPE
Nov 14 '18 at 9:22
add a comment |
1
...and to make the answer complete, I'd also mention thatbool()uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values
– NPE
Nov 14 '18 at 9:22
1
1
...and to make the answer complete, I'd also mention that
bool() uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values– NPE
Nov 14 '18 at 9:22
...and to make the answer complete, I'd also mention that
bool() uses the standard truth testing protocol that you quote from. docs.python.org/3/library/stdtypes.html#boolean-values– NPE
Nov 14 '18 at 9:22
add a comment |
TL;DR
Yes, this is expected. Most programming languages only consider 0 to be false.
Elaborated
This makes sense when you consider how -1 is represented in the processor. Typically you convert a positive number to it's negative counterpart by using 2's complement. 2's complent represents a negative binary number by inverting all the bits and adding 1. Say we have a 4-bit architecture, the decimal 1 is represented as 0001. To get its negative representation, invert all the bits and add 1, thus 0001 becomes 1110 + 1 = 1111.
When we want to know if a value is false, we can or all its inputs, if the result is 0, it's false, otherwise it's true, and thus it makes perfect sense that -1 == true.
The reson that we use 2's complement, as appose to 1's complement (where we just invert all bits without adding 1) is exacly because we do not want to be able to represent both 0 and -0.
answered Nov 14 '18 at 8:34
CortexCortex
7010
add a comment |
TL;DR
Yes, this is expected. Most programming languages only consider 0 to be false.
Elaborated
This makes sense when you consider how -1 is represented in the processor. Typically you convert a positive number to it's negative counterpart by using 2's complement. 2's complent represents a negative binary number by inverting all the bits and adding 1. Say we have a 4-bit architecture, the decimal 1 is represented as 0001. To get its negative representation, invert all the bits and add 1, thus 0001 becomes 1110 + 1 = 1111.
When we want to know if a value is false, we can or all its inputs, if the result is 0, it's false, otherwise it's true, and thus it makes perfect sense that -1 == true.
The reson that we use 2's complement, as appose to 1's complement (where we just invert all bits without adding 1) is exacly because we do not want to be able to represent both 0 and -0.
answered Nov 14 '18 at 8:34
CortexCortex
7010
add a comment |
TL;DR
Yes, this is expected. Most programming languages only consider 0 to be false.
Elaborated
This makes sense when you consider how -1 is represented in the processor. Typically you convert a positive number to it's negative counterpart by using 2's complement. 2's complent represents a negative binary number by inverting all the bits and adding 1. Say we have a 4-bit architecture, the decimal 1 is represented as 0001. To get its negative representation, invert all the bits and add 1, thus 0001 becomes 1110 + 1 = 1111.
When we want to know if a value is false, we can or all its inputs, if the result is 0, it's false, otherwise it's true, and thus it makes perfect sense that -1 == true.
The reson that we use 2's complement, as appose to 1's complement (where we just invert all bits without adding 1) is exacly because we do not want to be able to represent both 0 and -0.
answered Nov 14 '18 at 8:34
CortexCortex
7010
TL;DR
Yes, this is expected. Most programming languages only consider 0 to be false.
Elaborated
This makes sense when you consider how -1 is represented in the processor. Typically you convert a positive number to it's negative counterpart by using 2's complement. 2's complent represents a negative binary number by inverting all the bits and adding 1. Say we have a 4-bit architecture, the decimal 1 is represented as 0001. To get its negative representation, invert all the bits and add 1, thus 0001 becomes 1110 + 1 = 1111.
When we want to know if a value is false, we can or all its inputs, if the result is 0, it's false, otherwise it's true, and thus it makes perfect sense that -1 == true.
The reson that we use 2's complement, as appose to 1's complement (where we just invert all bits without adding 1) is exacly because we do not want to be able to represent both 0 and -0.
answered Nov 14 '18 at 8:34
CortexCortex
7010
answered Nov 14 '18 at 8:34
CortexCortex
7010
answered Nov 14 '18 at 8:34
CortexCortex
7010
answered Nov 14 '18 at 8:34
CortexCortex
7010
7010
add a comment |
add a comment |
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1
Generally (for many programming languages) only zero is considered false. Everything non-zero is true.
– Some programmer dude
Nov 14 '18 at 8:22
Generally (for many programming languages) there is detailed information to be found online, so you don't have to check and see what happens. See docs.python.org/3.7/library/functions.html#bool of the official documentation.
– usr2564301
Nov 14 '18 at 9:31