Need help with if statement and error messages

[nm1206]

Hello everyone, was wondering if you could help me figure out how to do the if statement to show these if the user inputs certain things.

  #include <iostream>
    #include <string>
    #include <istream>
    using namespace std;


    int main()
    {
	int unitNumber;
	string unit;

	// request user to input a value to convert with unit name
	
	{
		cout << "Value to convert = "; //getting input
		cin >> unitNumber >> unit;
		if (!(cin >> unitNumber))
		{
		cout << "Error: You did not enter a recognizable number.\n";

		}
		else if (cin && unitNumber < 0);
		{
	       cout << "Error: The input value must be a non-negative value.\n";
		}
		if (cin >> unit = "")
		{
			cout << "Error: No unit type provided";
		}
	}

[coder777]

		if (!(cin >> unitNumber))
		{
		cout << "Error: You did not enter a recognizable number.\n";

		}
		else if (cin && unitNumber < 0) // Important: no ; at this point!
		{
	       cout << "Error: The input value must be a non-negative value.\n";
		}
		cin >> unit;
		if (unit.empty())
		{
			cout << "Error: No unit type provided";
		}

Note that
= is always assignment (even within an if clause)
== is comparison.