Why does one of my Manipulate integer variables get turned into a real?

Question

Bug introduced in 9.0 or earlier and persisting through 11.0.1 or later

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I'm developing code for manipulating graphs. Spot the difference in the 2 Manipulates below:

Manipulate[
 {n, m},
 {{n, 10, "no of vertices"}, 1, 18, 1},
 {{m, n, "number of edges"}, n, n (n - 1)/2 , 1}
 (* using the +/-/slider for the number of edges does not generate Reals 
    but only Integers as expected *)]

this first example works as expected but

Manipulate[
 {n, m},
 {{n, 10, "no of vertices"}, 1, 18, 1},
 {{m, n, "number of edges"}, n - 1, n (n - 1) /2, 1}
 (* using the +/-/slider for the number of edges now generates Reals 
    not Integers: what gives? *)]

turns my nice integers into reals as soon as I change the number of edges. I can turn this back into an integer when that's what I need, but I'd prefer not to. Naturally I have a more complex task to complete but these examples are simple enough to illustrate the issue.

Version: Mathematica 9.0.1.0

Platform: Mac

Answer 1

This is an example of unintended behavior in Mathematica that the developers are aware of. There are a number of workarounds. The simplest is to wrap a function around the appropriate bounds; instead of

Manipulate[m, {n, {10}}, {m, n - 1, 2 n, 1}]

which illustrates the problem, try

Manipulate[Floor@m, {n, {10}}, {m, n - 1, 2 n, 1}]

A solution can also be pushed into the controller itself:

Manipulate[m, {n, {10}}, {{m, 9}, Dynamic[Slider[#1, {n - 1, 2 n, 1}]] &}]

Thanks for this feedback and suggestions are due to Karl Isensee.