watch out for negative args of log in evalf

Krastanov · Apr 27, 2014 · 04aa71c · 04aa71c
1 parent 79e5d90
commit 04aa71c
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Showing 2 changed files with 3 additions and 1 deletion.
diff --git a/sympy/core/evalf.py b/sympy/core/evalf.py
@@ -783,7 +783,8 @@ def evalf_log(expr, prec, options):
         arg = C.Add(S.NegativeOne, arg, evaluate=False)
         xre, xim, _, _ = evalf_add(arg, prec, options)
         prec2 = workprec - fastlog(xre)
-        re = mpf_log(mpf_add(xre, fone, prec2), prec, rnd)
+        # xre is now x - 1 so we add 1 back here to calculate x
+        re = mpf_log(mpf_abs(mpf_add(xre, fone, prec2)), prec, rnd)
 
     re_acc = prec
 

diff --git a/sympy/core/tests/test_evalf.py b/sympy/core/tests/test_evalf.py
@@ -125,6 +125,7 @@ def test_evalf_complex_cancellation():
 def test_evalf_logs():
     assert NS("log(3+pi*I)", 15) == '1.46877619736226 + 0.808448792630022*I'
     assert NS("log(pi*I)", 15) == '1.14472988584940 + 1.57079632679490*I'
+    assert NS('log(-1 + 0.00001)', 2) == '-1.0e-5 + 3.1*I'
 
 
 def test_evalf_trig():