From 010bd8c1459dd605d6294a0aa86f53eb4181eca0 Mon Sep 17 00:00:00 2001
From: Stuart Buchanan <stuart_d_buchanan@yahoo.co.uk>
Date: Thu, 19 Nov 2020 18:44:00 +0000
Subject: [PATCH] Fix props.condition

Previously props.condition threw an error if any of the
properties in the condition were not defines.  This is
contrary to the behaviour of SGCondition, which it seeks
to emulate, which considers such undefined properties as
having the value 0.0.

Now this is the case.

This function only appears to be used by tutorials.nas,
where this behaviour was seen as discrepancy between
the checklist <condition> behaviour and the tutorial
<condition> behaviour.

See https://sourceforge.net/p/flightgear/codetickets/2394/?page=1
---
 Nasal/props.nas | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/Nasal/props.nas b/Nasal/props.nas
index 31ce8108b..7578f4f11 100644
--- a/Nasal/props.nas
+++ b/Nasal/props.nas
@@ -452,11 +452,10 @@ var _cond_cmp = func(p, op) {
             return nil;
         }
     }
-    if(left == nil or right == nil) {
-        logprint(LOG_ALERT, "condition: comparing with nil");
-        dump(p);
-        return nil;
-    }
+
+    if (left == nil) left = 0.0;
+    if (right == nil) right = 0.0;
+
     if(op < 0) return left < right;
     if(op > 0) return left > right;
     return left == right;