C/C++ Reversing Vector in C++: Which Loop is Correct?

[ineedhelpnow]

which loop is correct for reversing a vector? please help.
A.
for (i = 0 ; i < NUM_ELEMENTS ; ++i ) {
tmpValue = revVctr .at (i ) ;
revVctr .at (i ) = revVctr .at (NUM_ELEMENTS - 1 - i ) ;
revVctr .at (NUM_ELEMENTS - 1 - i ) = tmpValue ;
}

B.

for (i = 0 ; i < (NUM_ELEMENTS / 2 ) ; ++i ) {
tmpValue = revVctr .at (i ) ;
revVctr .at (i ) = revVctr .at (NUM_ELEMENTS - 1 - i ) ;
revVctr .at (NUM_ELEMENTS - 1 - i ) = tmpValue ;
}

C.
for (i = 0; i < NUM_ELEMENTS; ++i) {
revVctr.at(i) = revVctr.at(NUM_ELEMENTS - i);
}

D.
for (i = 0 ; i < NUM_ELEMENTS ; ++i ) {
revVctr .at (i ) = revVctr .at (NUM_ELEMENTS - 1 - i ) ;
}

 

[Evgeny.Makarov]

An ideal way to solve this would be to take a sample vector with, say, 5 different elements and trace all four code snippets on this vector. Tracing a code on paper is an important skill to master. If you are short on time or have questions how to do this, please let us know. I just want to point out that in fragment C, when [m]i == 0[/m], we have [m]NUM_ELEMENTS - i == NUM_ELEMENTS[/m], so [m]revVctr.at(NUM_ELEMENTS - i)[/m] will produce the array-out-of-bounds error, if I understand correctly.

 

[ineedhelpnow]

the answer was B. (which i guessed correctly) otherwise i believe it would reverse twice.

 

[Evgeny.Makarov]

You are right. Code A reverses the vector twice, code C produces an error and code D overwrites the first half by the reversed second half but does not change the second half.