Answer :
To assign the first element of a vector with the value 99 in C++, you can use the `at()` function provided by the vector class. Here's how you can do it:
1. You have a vector declaration `vector myVector(4);` which initializes a vector `myVector` with 4 integer elements. By default, these integers are initialized to 0 if no values are provided.
2. To set the first element of `myVector` to 99, you should use the `at()` function with an index of 0, because vector indices in C++ are zero-based.
3. The correct syntax for assigning a value to the first element using `at()` is:
```cpp
myVector.at(0) = 99;
```
This line of code changes the first element of `myVector` to 99. Let's review why the other options are incorrect:
- `myVector.at()` without specifying an index is incomplete and results in a syntax error because `at()` requires an index argument.
- `myVector.at(-1) = 99;` would raise an out-of-range exception because negative indices are not valid in standard C++ vectors.
- `myVector.at(1) = 99;` assigns the value 99 to the second element of the vector, not the first.
Therefore, `myVector.at(0) = 99;` is the correct option to assign the vector's first element with 99.
1. You have a vector declaration `vector myVector(4);` which initializes a vector `myVector` with 4 integer elements. By default, these integers are initialized to 0 if no values are provided.
2. To set the first element of `myVector` to 99, you should use the `at()` function with an index of 0, because vector indices in C++ are zero-based.
3. The correct syntax for assigning a value to the first element using `at()` is:
```cpp
myVector.at(0) = 99;
```
This line of code changes the first element of `myVector` to 99. Let's review why the other options are incorrect:
- `myVector.at()` without specifying an index is incomplete and results in a syntax error because `at()` requires an index argument.
- `myVector.at(-1) = 99;` would raise an out-of-range exception because negative indices are not valid in standard C++ vectors.
- `myVector.at(1) = 99;` assigns the value 99 to the second element of the vector, not the first.
Therefore, `myVector.at(0) = 99;` is the correct option to assign the vector's first element with 99.
