Bitwise operations in Python coming from C
I tried rewriting the small C program below in Python, but I am getting different outputs.
C version:
#include <stdio.h>
int main()
{
unsigned char data = 0x00;
unsigned char i;
unsigned char bit = 0x01;
unsigned char parity = 1;
unsigned char value = 0x1c;
for (i = 0; i < 8; i++)
{
data = data | bit;
bit = bit << 1;
parity = parity ^ (data & 0x01);
}
printf("data: %d bit: %d parity: %dn", data, bit, parity);
return 0;
}
Python version:
data = 0
bit = 1
parity = 1
value = int('1c', 16)
for i in range(8):
data = data | bit
bit = bit << 1
parity = parity ^ (data & 1)
print('data: {0} bit: {1} parity: {2}'.format(data, bit, parity))
And the outputs are:
C version
> $ ./test
data: 255 bit: 0 parity: 1
Python version
> $ python3 test.py
data: 255 bit: 256 parity: 1
What am I missing on Python bitwise operations?
python bitwise-operators
edited Nov 23 '18 at 3:52
Branson Camp
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
add a comment |
I tried rewriting the small C program below in Python, but I am getting different outputs.
C version:
#include <stdio.h>
int main()
{
unsigned char data = 0x00;
unsigned char i;
unsigned char bit = 0x01;
unsigned char parity = 1;
unsigned char value = 0x1c;
for (i = 0; i < 8; i++)
{
data = data | bit;
bit = bit << 1;
parity = parity ^ (data & 0x01);
}
printf("data: %d bit: %d parity: %dn", data, bit, parity);
return 0;
}
Python version:
data = 0
bit = 1
parity = 1
value = int('1c', 16)
for i in range(8):
data = data | bit
bit = bit << 1
parity = parity ^ (data & 1)
print('data: {0} bit: {1} parity: {2}'.format(data, bit, parity))
And the outputs are:
C version
> $ ./test
data: 255 bit: 0 parity: 1
Python version
> $ python3 test.py
data: 255 bit: 256 parity: 1
What am I missing on Python bitwise operations?
python bitwise-operators
edited Nov 23 '18 at 3:52
Branson Camp
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
2
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44
add a comment |
I tried rewriting the small C program below in Python, but I am getting different outputs.
C version:
#include <stdio.h>
int main()
{
unsigned char data = 0x00;
unsigned char i;
unsigned char bit = 0x01;
unsigned char parity = 1;
unsigned char value = 0x1c;
for (i = 0; i < 8; i++)
{
data = data | bit;
bit = bit << 1;
parity = parity ^ (data & 0x01);
}
printf("data: %d bit: %d parity: %dn", data, bit, parity);
return 0;
}
Python version:
data = 0
bit = 1
parity = 1
value = int('1c', 16)
for i in range(8):
data = data | bit
bit = bit << 1
parity = parity ^ (data & 1)
print('data: {0} bit: {1} parity: {2}'.format(data, bit, parity))
And the outputs are:
C version
> $ ./test
data: 255 bit: 0 parity: 1
Python version
> $ python3 test.py
data: 255 bit: 256 parity: 1
What am I missing on Python bitwise operations?
python bitwise-operators
edited Nov 23 '18 at 3:52
Branson Camp
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
I tried rewriting the small C program below in Python, but I am getting different outputs.
C version:
#include <stdio.h>
int main()
{
unsigned char data = 0x00;
unsigned char i;
unsigned char bit = 0x01;
unsigned char parity = 1;
unsigned char value = 0x1c;
for (i = 0; i < 8; i++)
{
data = data | bit;
bit = bit << 1;
parity = parity ^ (data & 0x01);
}
printf("data: %d bit: %d parity: %dn", data, bit, parity);
return 0;
}
Python version:
data = 0
bit = 1
parity = 1
value = int('1c', 16)
for i in range(8):
data = data | bit
bit = bit << 1
parity = parity ^ (data & 1)
print('data: {0} bit: {1} parity: {2}'.format(data, bit, parity))
And the outputs are:
C version
> $ ./test
data: 255 bit: 0 parity: 1
Python version
> $ python3 test.py
data: 255 bit: 256 parity: 1
What am I missing on Python bitwise operations?
python bitwise-operators
python bitwise-operators
edited Nov 23 '18 at 3:52
Branson Camp
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
edited Nov 23 '18 at 3:52
Branson Camp
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
edited Nov 23 '18 at 3:52
Branson Camp
79111
edited Nov 23 '18 at 3:52
Branson Camp
79111
edited Nov 23 '18 at 3:52
Branson Camp
79111
79111
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
asked Nov 23 '18 at 1:14
kolriekolrie
6,753105187
6,753105187
You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
2
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44
add a comment |
You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
2
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44
You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
2
2
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44
add a comment |
1 Answer
1
active
oldest
votes
As you can see, the only difference in the output is the value of the variable bit.
In your C program, the variable bit is declared as unsigned char. That means it takes on only the values 0 through 255. The last operation on bit in your code is
bit = bit << 1
Before the last time that line is executed, bit is 128. After that line, it "tries" to become 256 but that does not fit into an unsigned char. So overflow happens, not flagged by your program, and bit becomes 0.
In the Python program, the variable bit is simply an integer, int, which has no maximum size. So there is no overflow, and bit does become 256.
There are several ways to overcome that difference in Python. You could force bit to stay in the desired range by instead using
bit = (bit << 1) % 256
or perhaps
bit = (bit << 1) & 255
You could instead make bit to be the equivalent of an unsigned char variable. As a comment says, you could use the ctypes module, I believe. You could also use numpy (I'm more familiar with that).
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
|
show 1 more comment
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1 Answer
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1 Answer
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votes
As you can see, the only difference in the output is the value of the variable bit.
In your C program, the variable bit is declared as unsigned char. That means it takes on only the values 0 through 255. The last operation on bit in your code is
bit = bit << 1
Before the last time that line is executed, bit is 128. After that line, it "tries" to become 256 but that does not fit into an unsigned char. So overflow happens, not flagged by your program, and bit becomes 0.
In the Python program, the variable bit is simply an integer, int, which has no maximum size. So there is no overflow, and bit does become 256.
There are several ways to overcome that difference in Python. You could force bit to stay in the desired range by instead using
bit = (bit << 1) % 256
or perhaps
bit = (bit << 1) & 255
You could instead make bit to be the equivalent of an unsigned char variable. As a comment says, you could use the ctypes module, I believe. You could also use numpy (I'm more familiar with that).
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
|
show 1 more comment
As you can see, the only difference in the output is the value of the variable bit.
In your C program, the variable bit is declared as unsigned char. That means it takes on only the values 0 through 255. The last operation on bit in your code is
bit = bit << 1
Before the last time that line is executed, bit is 128. After that line, it "tries" to become 256 but that does not fit into an unsigned char. So overflow happens, not flagged by your program, and bit becomes 0.
In the Python program, the variable bit is simply an integer, int, which has no maximum size. So there is no overflow, and bit does become 256.
There are several ways to overcome that difference in Python. You could force bit to stay in the desired range by instead using
bit = (bit << 1) % 256
or perhaps
bit = (bit << 1) & 255
You could instead make bit to be the equivalent of an unsigned char variable. As a comment says, you could use the ctypes module, I believe. You could also use numpy (I'm more familiar with that).
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
|
show 1 more comment
As you can see, the only difference in the output is the value of the variable bit.
In your C program, the variable bit is declared as unsigned char. That means it takes on only the values 0 through 255. The last operation on bit in your code is
bit = bit << 1
Before the last time that line is executed, bit is 128. After that line, it "tries" to become 256 but that does not fit into an unsigned char. So overflow happens, not flagged by your program, and bit becomes 0.
In the Python program, the variable bit is simply an integer, int, which has no maximum size. So there is no overflow, and bit does become 256.
There are several ways to overcome that difference in Python. You could force bit to stay in the desired range by instead using
bit = (bit << 1) % 256
or perhaps
bit = (bit << 1) & 255
You could instead make bit to be the equivalent of an unsigned char variable. As a comment says, you could use the ctypes module, I believe. You could also use numpy (I'm more familiar with that).
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
As you can see, the only difference in the output is the value of the variable bit.
In your C program, the variable bit is declared as unsigned char. That means it takes on only the values 0 through 255. The last operation on bit in your code is
bit = bit << 1
Before the last time that line is executed, bit is 128. After that line, it "tries" to become 256 but that does not fit into an unsigned char. So overflow happens, not flagged by your program, and bit becomes 0.
In the Python program, the variable bit is simply an integer, int, which has no maximum size. So there is no overflow, and bit does become 256.
There are several ways to overcome that difference in Python. You could force bit to stay in the desired range by instead using
bit = (bit << 1) % 256
or perhaps
bit = (bit << 1) & 255
You could instead make bit to be the equivalent of an unsigned char variable. As a comment says, you could use the ctypes module, I believe. You could also use numpy (I'm more familiar with that).
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
edited Nov 23 '18 at 11:47
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
answered Nov 23 '18 at 1:26
Rory DaultonRory Daulton
14k41933
14k41933
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
|
show 1 more comment
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
1
1
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
damn beat me to it (just barely). have an upvote.
– CoffeeTableEspresso
Nov 23 '18 at 1:27
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
Exactly, Python is not aware about "small" integer types…
– agg3l
Nov 23 '18 at 1:43
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@RoryDaulton Maybe you should rephrase it, considering google will match this some day: "In the Python program, bit is just an integer."
– agg3l
Nov 23 '18 at 1:47
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@agg3l: I rephrased it. Is that what you meant?
– Rory Daulton
Nov 23 '18 at 1:48
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
@RoryDaulton Confusing naming of variables in Python code sample (assuming byte-aware integer types, which are not really there) by kolrie has already and may lead to more confusion later. As long as Python is not low-level language by design. That was my idea
– agg3l
Nov 23 '18 at 1:55
|
show 1 more comment
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You're missing 'ctypes'
– agg3l
Nov 23 '18 at 1:17
2
You are missing that your type is larger than a byte. As such: 128 << 1 = 256 and does not overflow to 0
– planetmaker
Nov 23 '18 at 1:25
@planetmaker, yep; ctypes module is somewhat awkward but consistent way to deal with it. Although not "pythonic" at all )
– agg3l
Nov 23 '18 at 1:44