3.1.1 Types of Numbers
Four underlying types of numbers are built into Mathematica.
Rational numbers always consist of a ratio of two integers, reduced to lowest terms.
Intrinsic types of numbers in Mathematica.
Approximate real numbers are distinguished by the presence of an explicit decimal point.
An approximate real number can have any number of digits.
Complex numbers can have integer or rational components.
In:= 4 + 7/8 I
They can also have approximate real number components.
In:= 4 + 5.6 I
Several versions of the number 123.
You can distinguish different types of numbers in Mathematica by looking at their heads. (Although numbers in Mathematica have heads like other expressions, they do not have explicit elements which you can extract.)
The object 123 is taken to be an exact integer, with head Integer.
The presence of an explicit decimal point makes Mathematica treat 123. as an approximate real number, with head Real.
NumberQ[x] tests for any kind of number.
Tests for different types of numbers.
5. is treated as a Real, so IntegerQ gives False.
If you use complex numbers extensively, there is one subtlety you should be aware of. When you enter a number like 123., Mathematica treats it as an approximate real number, but assumes that its imaginary part is exactly zero. Sometimes you may want to enter approximate complex numbers with imaginary parts that are zero, but only to a certain precision.
When the imaginary part is the exact integer 0, Mathematica simplifies complex numbers to real ones.
In:= Head[ 123 + 0 I ]
Here the imaginary part is only zero to a certain precision, so Mathematica retains the complex number form.
In:= Head[ 123. + 0. I ]
The distinction between complex numbers whose imaginary parts are exactly zero, or are only zero to a certain precision, may seem like a pedantic one. However, when we discuss, for example, the interpretation of powers and roots of complex numbers in Section 3.2.7, the distinction will become significant.
x does not explicitly have head Integer, so IntegerQ returns False.
One way to find out the type of a number in Mathematica is just to pick out its head using Head[expr]. For many purposes, however, it is better to use functions like IntegerQ which explicitly test for particular types. Functions like this are set up to return True if their argument is manifestly of the required type, and to return False otherwise. As a result, IntegerQ[x] will give False, unless you have assigned x an explicit integer value.
In doing symbolic computations, however, you may sometimes want to treat x as an integer, even though you have not assigned an explicit integer value to it. You can override the assumption that the symbol x is not an integer by explicitly making an assignment of the form x/:IntegerQ[x]=True. This assignment specifies that whenever you specifically test x with IntegerQ, it will give the result True. You should realize, however, that the assignment does not actually change the head of x, so that, for example, x will still not match n_Integer. Mathematica also does not automatically make inferences based on the assignment. Thus, for example, it cannot determine solely on the basis of this assignment that IntegerQ[x^2] is also True.
This specifies that x is in fact an integer. The x/: specifies that the rule is associated with x, not IntegerQ.
In:= x/: IntegerQ[x] = True
The definition overrides the default assumption that x is not an integer.
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