Minor cleanup to double_pendulum example. (#12986)

anntzer · timhoffm · commit 20bdd9678747 · 2018-12-13T22:36:24.000+01:00
The formulas are of the kind where I think that putting +/- at the
beginning of the lines instead of at the end really improves
readability.
diff --git a/examples/animation/double_pendulum_sgskip.py b/examples/animation/double_pendulum_sgskip.py
@@ -27,26 +27,28 @@ def derivs(state, t):
     dydx = np.zeros_like(state)
     dydx[0] = state[1]
 
-    del_ = state[2] - state[0]
-    den1 = (M1 + M2)*L1 - M2*L1*cos(del_)*cos(del_)
-    dydx[1] = (M2*L1*state[1]*state[1]*sin(del_)*cos(del_) +
-               M2*G*sin(state[2])*cos(del_) +
-               M2*L2*state[3]*state[3]*sin(del_) -
-               (M1 + M2)*G*sin(state[0]))/den1
+    delta = state[2] - state[0]
+    den1 = (M1+M2) * L1 - M2 * L1 * cos(delta) * cos(delta)
+    dydx[1] = ((M2 * L1 * state[1] * state[1] * sin(delta) * cos(delta)
+                + M2 * G * sin(state[2]) * cos(delta)
+                + M2 * L2 * state[3] * state[3] * sin(delta)
+                - (M1+M2) * G * sin(state[0]))
+               / den1)
 
     dydx[2] = state[3]
 
-    den2 = (L2/L1)*den1
-    dydx[3] = (-M2*L2*state[3]*state[3]*sin(del_)*cos(del_) +
-               (M1 + M2)*G*sin(state[0])*cos(del_) -
-               (M1 + M2)*L1*state[1]*state[1]*sin(del_) -
-               (M1 + M2)*G*sin(state[2]))/den2
+    den2 = (L2/L1) * den1
+    dydx[3] = ((- M2 * L2 * state[3] * state[3] * sin(delta) * cos(delta)
+                + (M1+M2) * G * sin(state[0]) * cos(delta)
+                - (M1+M2) * L1 * state[1] * state[1] * sin(delta)
+                - (M1+M2) * G * sin(state[2]))
+               / den2)
 
     return dydx
 
 # create a time array from 0..100 sampled at 0.05 second steps
 dt = 0.05
-t = np.arange(0.0, 20, dt)
+t = np.arange(0, 20, dt)
 
 # th1 and th2 are the initial angles (degrees)
 # w10 and w20 are the initial angular velocities (degrees per second)
@@ -91,7 +93,7 @@ def animate(i):
     time_text.set_text(time_template % (i*dt))
     return line, time_text
 
-ani = animation.FuncAnimation(fig, animate, np.arange(1, len(y)),
-                              interval=25, blit=True, init_func=init)
 
+ani = animation.FuncAnimation(fig, animate, range(1, len(y)),
+                              interval=25, blit=True, init_func=init)
 plt.show()
