# \#variables

**URL:** https://discuss.ampl.com/tag/variables/7.md

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## [Is it necessary to apply normalization to unknown parameters with different scales?](https://discuss.ampl.com/t/is-it-necessary-to-apply-normalization-to-unknown-parameters-with-different-scales/1947)

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**Author:** [@ae.naderi](https://discuss.ampl.com/u/ae.naderi)\
**Replies:** 3\
**Last updated:** [October 22, 2024, 11:52pm UTC](https://discuss.ampl.com/t/is-it-necessary-to-apply-normalization-to-unknown-parameters-with-different-scales/1947 "2024-10-22T23:52:33Z")

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Hello everybody! I have a model built in AMPL. The scales of my unknown values are different. Two of them are coordinates, for the another; one has a scale between 2 and 9, and the other is between -150 and 50. I want …

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## [No variables declared](https://discuss.ampl.com/t/no-variables-declared/629)

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**Author:** [@sta6779](https://discuss.ampl.com/u/sta6779)\
**Replies:** 1\
**Last updated:** [June 30, 2023, 9:43pm UTC](https://discuss.ampl.com/t/no-variables-declared/629 "2023-06-30T21:43:48Z")

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define the variables var wrenches \>=0; #number of wrenches to produce var pliers \>=0; objective function maximize earnings: 0.13 \* wrenches + 0.10 \* pliers; contraints subject to steel: 1.5 \* wrenches + 1.0 \* pliers; …

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## [When I set option substout 1, why doesn’t AMPL make the substitutions that I expect?](https://discuss.ampl.com/t/when-i-set-option-substout-1-why-doesn-t-ampl-make-the-substitutions-that-i-expect/58)

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**Author:** [@megrobert](https://discuss.ampl.com/u/megrobert)\
**Replies:** 0\
**Last updated:** [December 15, 2021, 5:33pm UTC](https://discuss.ampl.com/t/when-i-set-option-substout-1-why-doesn-t-ampl-make-the-substitutions-that-i-expect/58 "2021-12-15T17:33:35Z")

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A variable is ineligible to be substituted out of a problem if (a) it is subject to any bounds or integrality conditions, or (b) it has already appeared on the right-hand side of a constraint that was used to make a subs…

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## [Why does AMPL treat my linear program as nonlinear?](https://discuss.ampl.com/t/why-does-ampl-treat-my-linear-program-as-nonlinear/57)

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**Author:** [@megrobert](https://discuss.ampl.com/u/megrobert)\
**Replies:** 0\
**Last updated:** [December 15, 2021, 5:31pm UTC](https://discuss.ampl.com/t/why-does-ampl-treat-my-linear-program-as-nonlinear/57 "2021-12-15T17:31:29Z")

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Symptoms of this problem are an option show\_stats 1 listing that refers to nonlinear variables and constraints, and rejection by the solver with a “contains nonlinear constraints” message. The cure may be (a) to reformul…

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## [When certain variables are fixed, the nonlinear expressions in my model all become linear. Does AMPL consider my model to be linear in this case?](https://discuss.ampl.com/t/when-certain-variables-are-fixed-the-nonlinear-expressions-in-my-model-all-become-linear-does-ampl-consider-my-model-to-be-linear-in-this-case/56)

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**Author:** [@megrobert](https://discuss.ampl.com/u/megrobert)\
**Replies:** 0\
**Last updated:** [December 15, 2021, 5:30pm UTC](https://discuss.ampl.com/t/when-certain-variables-are-fixed-the-nonlinear-expressions-in-my-model-all-become-linear-does-ampl-consider-my-model-to-be-linear-in-this-case/56 "2021-12-15T17:30:55Z")

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Yes, AMPL fixes the variables first, and then analyzes the model to determine whether it is linear. So if you make your model linear by fixing certain variables, you can direct AMPL to send the resulting problem instance…