Add horizonlength

Author: bjack205

docs/src/problem.md

@@ -20,6 +20,7 @@ Problem
 cost(::Problem)
 states(::Problem)
 controls(::Problem)
+horizonlength
 get_objective
 get_constraints
 get_model

src/TrajectoryOptimization.jl

@@ -55,6 +55,7 @@ export  # methods
 	get_objective,
 	get_constraints,
 	get_model,
+	horizonlength,
 	state_dim,    # from RobotDynamics
 	control_dim   # from RobotDynamics
 

src/problem.jl

@@ -143,6 +143,14 @@ RD.dims(prob::Problem, i::Integer) = RD.dims(prob.model[i])..., prob.N
 RD.state_dim(prob::Problem, k::Integer) = state_dim(prob.model[k])
 RD.control_dim(prob::Problem, k::Integer) = control_dim(prob.model[k])
 
+"""
+    horizonlength(prob::Problem)
+
+Number of knot points in the time horizon, i.e. the length of the sampled 
+state and control trajectories.
+"""
+horizonlength(prob::Problem) = prob.N
+
 import Base.size
 @deprecate size(prob::Problem) dims(prob)