Why use "using namespace std;"

#1

what is the purpose of using “using namespace std;” just after the header file and otherwise as well?

#2

using: You are going to use it
namespace: To use what? A namespace
std: The std namespace (where features of the C++ Standard Library, such as string or vector, are declared).
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After you write this instruction, if the compiler sees string it will know that you may be referring to std::string, and if it sees vector, it will know that you may be referring to std::vector. (Provided that you have included in your compilation unit the header files where they are defined, of course.)
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If you don’t write it, when the compiler sees string or vector it will not know what you are refering to. You will need to explicitly tell it std::string or std::vector, and if you don’t, you will get a compile error