Matthew S. answered • 07/08/19
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Physics student teaching Math and Science
An assignment such as = sets a variable to that value.
For example if you write and evaluate in a cell " x = 3", anytime the x value appears it will "appear" as a 3.
the -> operator is a Rule and not an assignment. So to give an example if we have an expression in a cell such as x^2 + 3 and then apply the Rule that x -> 3 as,
x^2 + 3 /.x->3 will evaluate to 12.
In this case the x will be replaced by 3 but if you were to use the x variable somewhere else it will still be a free variable and show up as an x. Contrary to the previous example x would show up as a 3.
