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

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  • Thread starter ineedhelpnow
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    C++ Vector
In summary, code B is the correct loop for reversing a vector as it iterates only through the first half of the vector and swaps elements with their respective mirrored positions in the second half. This results in the entire vector being reversed.
  • #1
ineedhelpnow
651
0
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 ) ;
}
 
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  • #2
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.
 
  • #3
the answer was B. (which i guessed correctly) otherwise i believe it would reverse twice.
 
  • #4
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.
 
  • #5


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.
 

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

What is the purpose of reversing a vector in C++?

Reversing a vector in C++ allows for the elements in the vector to be rearranged in the opposite order. This can be useful for various purposes such as displaying data in a different format or implementing algorithms that require the vector to be in reverse order.

What are the different ways to reverse a vector in C++?

There are multiple ways to reverse a vector in C++. One way is to use the built-in reverse function from the <algorithm> library. Another way is to use a for loop and swap the first and last elements until the middle is reached. A third way is to use the reverse iterators provided by the vector container.

Which loop is the correct way to reverse a vector in C++?

The best loop to use for reversing a vector in C++ depends on the specific situation and personal preference. However, the most commonly used and efficient loop is the for loop that iterates from both ends of the vector and swaps the elements until the middle is reached.

Can you provide an example of reversing a vector in C++?

Yes, here is an example using the for loop method:

vector<int> numVec = {1, 2, 3, 4, 5};
for(int i = 0; i < numVec.size()/2; i++){
int temp = numVec[i];
numVec[i] = numVec[numVec.size()-1-i];
numVec[numVec.size()-1-i] = temp;
}

After this code is executed, the vector will be reversed and the elements will be in the following order: {5, 4, 3, 2, 1}.

Are there any potential pitfalls to be aware of when reversing a vector in C++?

Yes, when reversing a vector in C++, it is important to consider the type of elements in the vector. If the vector contains objects rather than primitive data types, the objects may need to have a defined copy constructor and assignment operator to ensure proper copying and swapping during the reversal process. Additionally, when using the reverse iterators method, the vector must be non-const and have a bidirectional iterator.

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