Arithmetic promotion: floating-point to integer?

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Bob Li 2011-10-13

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Hi,

[quote "MATLAB - Programming Fundamentals R2011b.pdf"] Arithmetic operations that involve both integers and floating-point always result in an integer data type. MATLAB rounds the result, when necessary, according to the default rounding algorithm. The example below yields an exact answer of 1426.75 which MATLAB then rounds to the next highest integer:

int16(325) * 4.39
ans =
1427

[/quote]

In C/C++, the arithmetic promotion rule is to upper ↑: char to int, int unsigned int, int to float, float to double, etc.

Why Matlab designs to be to lower (int)?

Bob

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Walter Roberson 2011-10-13

The Algol Coercing patents were still in effect at the time MATLAB was designed?

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采纳的回答

Mike Hosea 2011-10-13

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I don't know. Since I work in code generation, these sorts of behaviors are a regular nuisance to me, but maybe I can imagine why. In MATLAB, most users do almost everything with doubles. When programming in MATLAB language, the average user doesn't even think about numeric types. I wasn't around then, but I think integers were originally added to the language primarily because it was burdensome to represent things like images as large arrays of doubles. Having added integers to the language to save on storage, you wouldn't want something simple like multiplying a large int8 array by 2 (that's double precision 2.0 in MATLAB) to generate a large double precision array out of a large integer array. I guess they preferred the current behavior to requiring, in that case, multiplication by int8(2). This rationale is, of course, less compelling when we're talking about scalars, but having inconsistent rules for scalars would be madness. -- Mike

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Mike Hosea 2011-10-15

Sounds like Jan's answer helped you, but let me just say that MATLAB language is implemented in software with "just-in-time" compilation in some contexts. It is written to use the hardware intelligently, but integer arithmetic and promotion does what the MATLAB software says it does, not what some other language says it ought to do. It is like doing floating point in software. You can make it do whatever you want, e.g. use BCD instead of binary IEEE. The data itself is stored in the usual number of bits, but the operations that work on it are more complex. Obviously if you're going to eschew the behaviors that are supported most directly by the hardware, there is likely to be a run-time cost for defining your own behavior and not "going with the flow." C'est la vie. In code generation we have options to turn off integer saturation, so if you turn it off (thereby warranting that your algorithm does not rely on it), then we can probably generate the kind of C code that you wanted. This would be more direct if MATLAB would add C-integer types that are defined to wrap and promote like C integers. Alas, they do not exist, but we can dream and cajole.

Bob Li 2011-10-17

Walter,

Thanks for answer. "My questions" page only shows something like:

2answers

0votes

No rich formatting for posts?

Asked 4 days ago by Bob Li

Accepted Answer from Walter Roberson

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But I cannot find any information about which one has last been updated?

Walter Roberson 2011-10-17

Your My Questions page should (for you) currently list 7 questions. They are ordered from most recently updated (by anyone, including via comments) to least recently updated. What you list above is just the information for one of the questions: there should be 6 above that one.

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