what is the order of execution, associativity in c?

Question

Why am I getting different output for variables c and d? how do I explain e? can any one give a clue?

 #include <stdio.h>
 #include <string.h> 

 main()
 {
    int i=10,d=10,e=10;
    float c;
    c=(float)++i + ++i;
    d= ++d  + ++d;
    e= ++e  + ++e + ++e;
    printf("d=%d\n c=%f\n e=%d ",d,c,e);       
 }

This is the output.

d=24
c=23.000000
e=37

Answer 1

why am I getting different output for variable c and d ?

Because of the Undefined Behavior of the program. Statement

c=(float)++i + ++i;  

is trying to modify i twice between two sequence point. Between two sequence point modification can be done only once to a variable.

C-FAQ: 3.8:

Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be accessed only to determine the value to be stored.

C11: 5.1.2.3 Program execution:

The presence of a sequence point between the evaluation of expressions A and B implies that every value computation and side effect associated with A is sequenced before every value computation and side effect associated with B.

Same for the statements

d= ++d  + ++d;            //Modifying d three times
e= ++e  + ++e + ++e;      //Modifying e four times

---

Side Note:

C11: Annexe C: Sequence points:

The following are the sequence points described in 5.1.2.3:
— Between the evaluations of the function designator and actual arguments in a function call and the actual call. (6.5.2.2).
— Between the evaluations of the first and second operands of the following operators: logical AND && (6.5.13); logical OR || (6.5.14); comma , (6.5.17).
— Between the evaluations of the first operand of the conditional ?: operator and whichever of the second and third operands is evaluated (6.5.15).
— The end of a full declarator: declarators (6.7.6);
— Between the evaluation of a full expression and the next full expression to be evaluated. The following are full expressions: an initializer that is not part of a compound literal (6.7.9); the expression in an expression statement (6.8.3); the controlling expression of a selection statement (if or switch) (6.8.4); the controlling expression of a while or do statement (6.8.5); each of the (optional) expressions of a for statement (6.8.5.3); the (optional) expression in a return statement (6.8.6.4).
— Immediately before a library function returns (7.1.4).
— After the actions associated with each formatted input/output function conversion specifier (7.21.6, 7.29.2).
— Immediately before and immediately after each call to a comparison function, and also between any call to a comparison function and any movement of the objects passed as arguments to that call (7.22.5).
§