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.

 

[LudusRex]

B. is the correct loop for reversing a vector. It uses a temporary variable to store the value of each element as it is swapped with its corresponding element on the opposite end of the vector. This loop iterates only through half of the vector, as the elements are swapped in pairs. Option A. and D. do not correctly reverse the vector, as they do not swap the elements in the correct order. Option C. is also incorrect, as it does not use a temporary variable to store the values and will result in incorrect values being assigned to the vector.