How to add integers without correction?

Question

Dejia Kong 2021-9-14

1
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1452834-how-to-add-integers-without-correction

评论： Jan 2021-9-23

when using integer class such as int8 in matlab, and some calculation result is bigger than the upper range (for int8 , the upper range is 127), matlab would automatically set the value to the maximum.

Help Center - Largest and Smallest Values for Integer Classes

eg.

a=int8(126);
b=int8(2);
c=a+b; % matlab would gives c=127, since 128 exceed the upper range of int8

but in some cases, i want to get a negetive result, like many programming languages do

i want c to be -128 because 01111110+00000010=10000000 and it's -128 for int8

this could be done by a "bitadd" like

function int1 = bitadd(int1,int2)
while (int2 ~=0)
    carry=bitand(int1,int2);
    int1=bitxor(int1,int2);
    int2=bitshift(carry,1);
end
end

however this is very inefficient, since a while loop in the function

Setting the result to maximum should be additional correction, so is there a faster method to do bitwise add without such correction?

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Jan 2021-9-14

1
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1452834-how-to-add-integers-without-correction#answer_787219

在 MATLAB Online 中打开

a = int8(126);
b = int8(2);
tic
for k = 1:1e4
    c = bitadd1(a, b);
end
toc
Elapsed time is 0.141335 seconds.
tic
for k = 1:1e4
    c = bitadd2(a, b);
end
toc
Elapsed time is 0.068172 seconds.
function int1 = bitadd1(int1,int2)
while (int2 ~=0)
    carry = bitand(int1,int2);
    int1  = bitxor(int1,int2);
    int2  = bitshift(carry,1);
end
end
function c = bitadd2(a, b)
m = typecast(int16(a) + int16(b), 'int8');
c = m(1);
end

3 个评论
显示 1更早的评论隐藏 1更早的评论

Dejia Kong 2021-9-15

在 MATLAB Online 中打开

thanks for your bitadd2 and introducing typecast to me

in fact i want some code as fast as

tic;
for i=1:1e4
    c=a+b;
end
toc;

since i think setting the result to maximum is additional correction and would take extra time.

But now i think when i have such performance requirement, it's better to use c++/fortran instead of matlab.

Jan 2021-9-23

It will be much faster to use int16 in general and crop the higher bits during the addition. In my tests this is 10 times faster, but I did not get the correct results when the overflow appears.

请先登录，再进行评论。