Check the io directory on the page https://bitbucket.org/dclong/cpp-learn/src for some illustrative examples for the following discussions.

1. You can format output of numbers using functions std::setpresicion and std::setw in the header.

2. When you read data from a file into an array or write data from an array to a file, you can just use the address of the array. However, if a vector (e.g., vector x) is used instead of an array, you have to use the address of the underlying data in the vector x which is &x[0]. Also, when you read/write data into/from a vector, you have use the size of the vector x multiplied by the memory used by each element to calculate the total number of bytes, i.e.

x.size() * sizeof(x[0]);

or if x is a vector of double values you can use

x.size() * sizeof(double);

You cannot use sizeof(x), because it is the size of the vector itself (without counting the underlying array) and is always the same (12 bytes).