does matlab have a problem with modular integer arithmetic?

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alexander sharp
alexander sharp 2017-3-24
评论： Walter Roberson 2017-3-24
>> x = -78907292 * 1941317253; >> y = 2^32 +1
y =
4.29496729700000e+009
>> x
x =
-153.184087347109e+015
>> mod(x,y)
ans =
1.51932828600000e+009
correct answer:
= 1519328274 (python) and others...

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回答（3 个）

Walter Roberson
Walter Roberson 2017-3-24
Try
X = int64(-78907292) * int64(1941317253)
Remember that the default data type is double not one of the integer data classes.
Roger Stafford
Roger Stafford 2017-3-24
编辑：Roger Stafford 2017-3-24
As has so often been pointed out in this forum, matlab’s “double” in everyone’s computers possesses a significand (mantissa) consisting of 53 binary digits. Consequently it is incapable of representing the above product -78907292*1941317253 exactly. For that reason the errors it must necessarily make will certainly be manifest using the mod function as given here. Have a heart! Or better still use the symbolic forms of numbers for such calculations.
alexander sharp
alexander sharp 2017-3-24
an integer is an integer is an integer - by any other name. it is wrong
  1 个评论
Walter Roberson
Walter Roberson 2017-3-24
MATLAB frequently allows people to use abbreviated forms. In MATLAB your line
x = -78907292 * 1941317253;
is considered to be an abbreviated form of
x = times(-78907292.0, 1941317253.0);
An integer might, as you say, be an integer, but you did not enter any integers.

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