fatal error: iostream.h no such file or directory [duplicate]

That header doesn't exist in standard C++. It was part of some pre-1990s compilers, but it is certainly not part of C++.

Use #include <iostream> instead. And all the library classes are in the std:: namespace, for example std::cout.

Also, throw away any book or notes that mention the thing you said.

Using standard C++ calling (note that you should use namespace std for cout or add using namespace std;)

#include <iostream>

int main()
{
    std::cout<<"Hello World!\n";
    return 0;
}

You should be using iostream without the .h.

Early implementations used the .h variants but the standard mandates the more modern style.

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