add without carry

Hugh

I would like to perform an addition without carrying of two integers...
I've got no idea how to do this in python, although I've been using it
for web/cgi/db work for a few years now.

Any help would be great.

Hugh

Sep 15 '06 #1

Subscribe Reply

5178

Marc 'BlackJack' Rintsch

In <11************ **********@i42g 2000cwa.googleg roups.com>, Hugh wrote:

I would like to perform an addition without carrying of two integers...
I've got no idea how to do this in python, although I've been using it
for web/cgi/db work for a few years now.

Your description is a bit vague. What is `without carrying`? Do you want
to limit the result to a given bit length with a "wrap around" behavior if
the number gets too large to fit into those bits? Then this should do the
trick:

a = (b + c) & (2**bits - 1)

Ciao,
Marc 'BlackJack' Rintsch

Sep 15 '06 #2

John Machin

Hugh wrote:

I would like to perform an addition without carrying of two integers...
I've got no idea how to do this in python, although I've been using it
for web/cgi/db work for a few years now.

In multiword addition in assembly language, one uses a normal ADD
instruction on the lowest-order pair of words, and an ADC (or ADDC or
whatever) i.e add *with* carry which adds in the carry bit from the
previous operation for each subsequent pair of words. IOW, addition
*without* carrying is normal, and I don't recall hearing the expression
before ......

Do you possibly mean: add two 32-bit integers such the result fits in
32 bits i.e. discard the generated carry bit, keep the low-order
32-bits?

If so, that's simply (a + b) & 0xFFFFFFFF (in Python as well as a few
other languages).

If you meant a different number of bits, well the answer for 8 bits
would be (a + b) & 0xFF.

Otherwise, you need to provide some examples of two integers and what
"addition without carrying" produces.

HTH,
John

Sep 15 '06 #3

Hugh

Sorry, here's an example...

5+7=12

added without carrying, 5+7=2

i.e the result is always less than 10

I've been thinking some more about this and my brain is starting to
work something out... I just wondered if there was a function in python
math to do this automatically.. .

Hugh

Sep 15 '06 #4

MonkeeSage

Hugh wrote:

Sorry, here's an example...

5+7=12

added without carrying, 5+7=2

i.e the result is always less than 10

def add(a, b, c=10):
an = a + b
if an >= c:
an -= c
return an

add(5, 7) # = 2

?

Regards,
Jordan

Sep 15 '06 #5

Bryan Olson

Hugh wrote:

Sorry, here's an example...

5+7=12

added without carrying, 5+7=2

i.e the result is always less than 10

Are you looking for bitwise exclusive or? In Python it's
the '^' operator. For example:

print 5 ^ 7
--
--Bryan

Sep 15 '06 #6

Hugh

Thankyou everyone this gives me something to work with.

Hugh

Sep 15 '06 #7

Peter Otten

Hugh wrote:

Sorry, here's an example...

5+7=12

added without carrying, 5+7=2

i.e the result is always less than 10

I've been thinking some more about this and my brain is starting to
work something out... I just wondered if there was a function in python
math to do this automatically.. .

>>(5 + 7) % 10

2

In this context '%' is called 'modulo operator'. What John and Marc
suggested is basically the same operation for the special case of binary
numbers, i. e.

a % b == a & (b-1)

if a >= 0 and b == 2**N.

Peter

Sep 15 '06 #8

Hugh

Peter,

That was what I was thinking along the lines of, It's been two years
since I finished my CS degree and working in mechanical engineering
means I've nearly forgotten it all! :(

Thanks, I'll write a function in my app to handle this...

Hugh

>(5 + 7) % 10

2

In this context '%' is called 'modulo operator'. What John and Marc
suggested is basically the same operation for the special case of binary
numbers, i. e.

a % b == a & (b-1)

if a >= 0 and b == 2**N.

Peter

Sep 15 '06 #9

Bruno Desthuilliers

Bryan Olson wrote:

Hugh wrote:
>Sorry, here's an example...

5+7=12

added without carrying, 5+7=2

i.e the result is always less than 10

Are you looking for bitwise exclusive or? In Python it's
the '^' operator. For example:

print 5 ^ 7

>>10 ^ 21

31

Not really "less than 10"...

--
bruno desthuilliers
python -c "print '@'.join(['.'.join([w[::-1] for w in p.split('.')]) for
p in 'o****@xiludom. gro'.split('@')])"

Sep 15 '06 #10

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