通常当我尝试用 IPOPT 解决某事时，即使问题不可行，IPOPT 也会显示运行过程，例如显示问题有多少约束以及问题的其他一般信息，但这次我遇到了一个奇怪的问题。我第一次解决问题时它什么也没显示，但第二次解决它时，IPOPT 返回最佳解决方案。像这样：
-> ampl.eval('option solver ipopt ;');
-> ampl.solve()
（它在这里什么都不返回！！！！）
-> solve_result = ampl.getValue('solve_result_num')
solve_result = -1
所以在这里，我再次运行代码：
-> ampl.solve()
Ipopt 3.12.4:
******************************************************************************
This program contains Ipopt, a library for large-scale nonlinear optimization.
Ipopt is released as open source code under the Eclipse Public License (EPL).
For more information visit http://projects.coin-or.org/Ipopt
******************************************************************************
This is Ipopt version 3.12.4, running with linear solver mumps.
NOTE: Other linear solvers might be more efficient (see Ipopt documentation).
Number of nonzeros in equality constraint Jacobian...: 4439
Number of nonzeros in inequality constraint Jacobian.: 7888
Number of nonzeros in Lagrangian Hessian.............: 50
Total number of variables............................: 1610
variables with only lower bounds: 0
variables with lower and upper bounds: 70
variables with only upper bounds: 0
Total number of equality constraints.................: 1360
Total number of inequality constraints...............: 2585
inequality constraints with only lower bounds: 0
inequality constraints with lower and upper bounds: 2585
inequality constraints with only upper bounds: 0
iter objective inf_pr inf_du lg(mu) ||d|| lg(rg) alpha_du alpha_pr ls
0 5.0001000e+03 1.31e+00 8.32e-02 -1.0 0.00e+00 - 0.00e+00 0.00e+00 0
1 5.0079602e+03 1.31e+00 2.71e+01 -1.0 1.68e+01 - 1.05e-02 7.73e-04f 1
2 3.9278729e+03 1.17e+00 9.17e+00 -1.0 1.24e+00 - 3.65e-02 9.32e-02f 1
3 2.7624370e+03 9.13e-01 2.49e+01 -1.0 4.52e-01 - 7.04e-01 1.99e-01f 1
4 1.6350590e+03 7.87e-02 2.17e+01 -1.0 6.17e-01 - 2.98e-01 7.82e-01f 1
5 8.6271513e+02 3.73e-13 2.34e-01 -1.0 1.53e-01 - 9.90e-01 1.00e+00f 1
6 1.0292941e+03 1.98e-13 8.49e-13 -1.0 1.03e-01 - 1.00e+00 1.00e+00f 1
7 1.0259140e+03 1.86e-13 6.85e-13 -2.5 6.57e-04 - 1.00e+00 1.00e+00f 1
8 9.1964915e+02 2.02e-13 7.11e-04 -3.8 1.84e-02 - 9.96e-01 1.00e+00f 1
9 3.5639825e+02 2.30e-13 4.23e-02 -5.7 9.45e-02 - 7.68e-01 1.00e+00f 1
iter objective inf_pr inf_du lg(mu) ||d|| lg(rg) alpha_du alpha_pr ls
10 7.9479322e+01 2.23e-13 9.26e-03 -5.7 1.85e-01 - 8.95e-01 1.00e+00f 1
11 1.9247063e+01 2.72e-13 2.80e-14 -5.7 1.19e-01 - 1.00e+00 1.00e+00f 1
12 6.9889052e+00 3.55e-13 9.99e-05 -8.6 4.59e-02 - 9.33e-01 1.00e+00f 1
13 1.2789711e+00 4.20e-13 4.22e-04 -8.6 2.96e-01 - 4.85e-01 8.98e-01f 1
14 2.9354384e-01 2.00e-13 6.84e-06 -8.6 4.49e-02 - 9.92e-01 1.00e+00f 1
15 1.0626050e-01 2.41e-13 7.17e-14 -8.6 9.55e-03 - 1.00e+00 1.00e+00h 1
16 6.3028082e-02 3.77e-13 5.27e-14 -8.6 1.37e-03 - 1.00e+00 1.00e+00h 1
17 4.3933480e-03 3.27e-13 6.90e-08 -11.0 2.32e-03 - 9.99e-01 1.00e+00h 1
18 1.1280540e-03 2.81e-13 8.08e-14 -11.0 1.02e-04 - 1.00e+00 1.00e+00h 1
19 3.9735544e-04 3.93e-13 5.46e-14 -11.0 5.59e-05 - 1.00e+00 1.00e+00h 1
iter objective inf_pr inf_du lg(mu) ||d|| lg(rg) alpha_du alpha_pr ls
20 2.2989758e-04 3.19e-13 1.68e-14 -11.0 2.72e-05 - 1.00e+00 1.00e+00h 1
Number of Iterations....: 20
(scaled) (unscaled)
Objective...............: 2.4678359201693643e-10 2.2989757642660348e-04
Dual infeasibility......: 1.6794654637445761e-14 1.5645490717250029e-08
Constraint violation....: 2.7491418973006631e-15 3.1883523599063324e-13
Complementarity.........: 5.3838886983188674e-11 5.0154994235157414e-05
Overall NLP error.......: 5.3838886983188674e-11 5.0154994235157414e-05
Number of objective function evaluations = 21
Number of objective gradient evaluations = 21
Number of equality constraint evaluations = 21
Number of inequality constraint evaluations = 21
Number of equality constraint Jacobian evaluations = 21
Number of inequality constraint Jacobian evaluations = 21
Number of Lagrangian Hessian evaluations = 20
Total CPU secs in IPOPT (w/o function evaluations) = 0.295
Total CPU secs in NLP function evaluations = 0.004
EXIT: Optimal So
Ipopt 3.12.4: Optimal Solution Found
suffix ipopt_zU_out OUT;
suffix ipopt_zL_out OUT;
-> solve_result = ampl.getValue('solve_result_num')
solve_result = 0
所以谁能知道为什么会发生这种情况，这是什么意思？