1 files changed, 41 insertions, 0 deletions
diff --git a/tests/general/prefix.icn b/tests/general/prefix.icn
new file mode 100644
index 0000000..0f6b0e0
--- /dev/null
+++ b/tests/general/prefix.icn
@@ -0,0 +1,41 @@
+#
+#          I N F I X - T O - P R E F I X   C O N V E R S I O N
+#
+
+#  This program accepts infix expressions from standard input and
+#  writes the corresponding prefix expressions to standard output.
+
+procedure main()
+   while write(prefix(read()))
+end
+
+procedure prefix(s)
+   s := strip(s)
+   return lassoc(s,'+-' | '*/') | rassoc(s,'^') | s
+end
+
+procedure strip(s)
+   while s ? (="(" & s <- tab(bal(')')) & pos(-1))
+   return s
+end
+
+procedure lassoc(s,c)
+   local j
+   s ? every j := bal(c)
+   return form(s,\j)
+end
+
+procedure rassoc(s,c)
+   local j
+   return form(s,s ? bal(c))
+end
+
+procedure form(s,k)
+   local a1, a2, op
+   s ? {
+      a1 := tab(k)
+      op := move(1)
+      a2 := tab(0)
+      }
+   return op || "(" || prefix(a1) || "," || prefix(a2) || ")"
+end