Is a sentinel always going to be an integer?
No, probably not.
Sometimes the user needs to be explicitly asked whether the loop should continue. The user will enter "yes" or "no" (or maybe "y" or "n"). The sentinel will be of type String or char. The next example illustrates this:
Say that you were interested in the value of the polynomial
7x3- 3x2 + 4x - 12
for various values of x. The value x will be a double precision value. For example, when x == 2.0 the polynomial is equal to:
7*23- 3*22 + 4*2 - 12 = 7*8 - 3*4 + 8 - 12 = 40
You want the user to be able to enter various values for x and see the result.