A mess. Literally.

[BlueBomberXTJ]

After a while of messing around while trying to get the smallest to largest double to show it finally worked. There is one bug though. The first value always says 0 is the smallest number. Other than that it works like a charm.
The true problem is trying to do this with units of measurement. I just
can't find a way to do this... This is my horrible attempt of trying to piece
it together and show the number a long side it's unit of measurement.
Would anyone be so kind as to show me this in the simplest way possible?
I'm doing this from a book so I have to learn how to do it the hard way before
I move on to my lesson on vectors etc. So if, while, for, else, etc will
be a big help instead of all the other things I don't even understand yet.

  #include <iostream>
#include <string>
using namespace std;
int main ()
{
	
	double entered = 0;
	double previous = 0;
	double largest = 0;
	double smallest = 0;
	double base = 0;
	char unit = '?';
	char previous_unit = '?';
	char smallest_unit = '?';
	char largest_unit = '?';
	char m;
	char cm;
	char in;
	char ft;
	
	double s_unit_ft_to_ft = smallest * 1;
	double s_unit_in_to_in = smallest * 1;
	double s_unit_m_to_m = smallest * 1;
	double s_unit_cm_to_cm = smallest * 1;
	double s_unit_m_to_cm = smallest * 100;
	double s_unit_in_to_cm = smallest * 2.54;
	double s_unit_ft_to_in = smallest * 12;
	double s_unit_ft_to_cm = smallest * 30.48;
	double s_unit_ft_to_m = smallest * 0.3048;
	double s_unit_m_to_in = smallest * 0.0254;
	
	double l_unit_ft_to_ft = largest * 1;
	double l_unit_in_to_in = largest * 1;
	double l_unit_m_to_m = largest * 1;
	double l_unit_cm_to_cm = largest * 1;
	double l_unit_m_to_cm = largest * 100;
	double l_unit_in_to_cm = largest * 2.54;
	double l_unit_ft_to_in = largest * 12;
	double l_unit_ft_to_cm = largest * 30.48;
	double l_unit_ft_to_m = largest * 0.3048;
	double l_unit_m_to_in = largest * 0.0254;
	
	cout << "Input a number followed by a unit. I shall show the largest and smallest out of all." << endl;
	cout << "--------------------------------------Unit Key--------------------------------------" << endl;
	cout << "Meter = m";
	cout << "\n";
	cout << "Centimeter = cm";
	cout << "\n";
	cout << "Inch = in";
	cout << "\n";
	cout << "Feet = ft";
	cout << "\n";
	
	cin >> base;
	cout << "Input unit for this number" << endl;
	cin >> unit;
	previous = base;
	previous_unit = unit;
	
	while (cin >> entered) {
		if (entered < base, unit == 'm'){
			base = entered;
			smallest = entered;
			smallest_unit = unit;
			cout << "Smallest so far is " << smallest << smallest_unit << endl;
			cout << "Largest so far is " << largest << largest_unit << endl;
			}
		if (entered > previous){
			largest = entered;
			previous = entered;
			largest_unit = unit;
			cout << "Smallest so far is " << smallest << smallest_unit << endl;
			cout << "Largest so far is " << largest << largest_unit << endl;
			}	
	}

}	

[Superdude]

Not quite sure what you are trying to do here. Are you trying to output the biggest number of those entered? Or relative to the unit of measure?

Expound, please.

[BlueBomberXTJ]

Largest and smallest number with units of measurement.

[xismn]

Like superdude, I don't know what you're trying to do. If you give us more info or maybe post the example from the book, we'd be able to help more.

Some things I see from looking at your code:

unit, m, cm, in, ft, previous_unit, smallest_unit and largest_unit are of type char, which means they can only hold one character.

Lines 21-41 are not equations as you might think, they are assignments.
This is a problem that a lot of new comers seem to stumble into, as they can get some familiar mathematical concepts confused with programming concepts.
By your logic, the following would be impossible :
i = i + 1;
It's not an equation, it's an assignment.

Also, int main()... is not returning anything.