Is this program having any sequence point issues?

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自闭症患者 2021-01-07 14:21
#include
 int main()
 {  
       int i=7,j;
       j=(i++,++i,j*i); 
       return 0;
}

j=(i++,++i,j*i);Is this well defined ? Let m

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  • 2021-01-07 14:48

    In your code " ," will be work as sequence point.

    so in this

    j=(i++,++i,j*i);
    

    expression would be work from left to right.
    so at first i++ then ++i and then j*i

    at the last j*i would be stored in j;

    but lastly your result would be elegant because " j " have no predefined data
    so undefined value would be stored in j.

    if you don't use " () "

    your code would be work as single statement such as

    j=i++;
    ++i;
    j*i;
    
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  • 2021-01-07 14:54

    Yes, It's well defined. sequence point

    comma operator in C

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  • 2021-01-07 15:07

    This expression is OK because the comma operator is a sequence point:

    (i++, ++i, j*i)
    

    However do not confuse it with the following where the comma is not acting as a sequence point:

    somefunction(i++, ++i, j*i)
    

    What about j = i++ * ++i

    The multiplication operator is not a sequence point.


    (Excuse me hijacking your answer)

    From §3.4 of ISO 9899:1999 (C Standard):

    3.4 behavior

    external appearance or action

    3.4.1 implementation-defined behavior

    unspecified behavior where each implementation documents how the choice is made

    EXAMPLE An example of implementation-defined behavior is the propagation of the high-order bit when a signed integer is shifted right.

    3.4.2 locale-specific behavior

    behavior that depends on local conventions of nationality, culture, and language that each implementation documents

    EXAMPLE An example of locale-specific behavior is whether the islower function returns true for characters other than the 26 lowercase Latin letters.

    3.4.3 undefined behavior

    behavior, upon use of a nonportable or erroneous program construct or of erroneous data, for which this International Standard imposes no requirements

    NOTE Possible undefined behavior ranges from ignoring the situation completely with unpredictable results, to behaving during translation or program execution in a documented manner characteristic of the environment (with or without the issuance of a diagnostic message), to terminating a translation or execution (with the issuance of a diagnostic message).

    EXAMPLE An example of undefined behavior is the behavior on integer overflow.

    3.4.4 unspecified behavior

    behavior where this International Standard provides two or more possibilities and imposes no further requirements on which is chosen in any instance

    EXAMPLE An example of unspecified behavior is the order in which the arguments to a function are evaluated.

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