X-Git-Url: https://scm.cri.ensmp.fr/git/linpy.git/blobdiff_plain/556abe7f3b2c7e3985560f3e3cfb6f66bacc4122..8fe086b5ae03df5fadf1ec7de1aa41c0ace4a3ae:/pypol/linexprs.py

diff --git a/pypol/linexprs.py b/pypol/linexprs.py
index ed68493..10daf9d 100644
--- a/pypol/linexprs.py
+++ b/pypol/linexprs.py
@@ -249,6 +249,29 @@ class Expression:
                 return left / right
         raise SyntaxError('invalid syntax')
 
+    def subs(self, symbol, expression=None):
+        if expression is None:
+            if isinstance(symbol, dict):
+                symbol = symbol.items()
+            substitutions = symbol
+        else:
+            substitutions = [(symbol, expression)]
+        result = self
+        for symbol, expression in substitutions:
+            symbol = symbolname(symbol)
+            result = result._subs(symbol, expression)
+        return result
+
+    def _subs(self, symbol, expression):
+        coefficients = {name: coefficient
+            for name, coefficient in self.coefficients()
+            if name != symbol}
+        constant = self.constant
+        coefficient = self.coefficient(symbol)
+        result = Expression(coefficients, self.constant)
+        result += coefficient * expression
+        return result
+
     _RE_NUM_VAR = re.compile(r'(\d+|\))\s*([^\W\d_]\w*|\()')
 
     @classmethod
@@ -418,7 +441,7 @@ def symbolname(symbol):
 def symbolnames(symbols):
     if isinstance(symbols, str):
         return symbols.replace(',', ' ').split()
-    return (symbolname(symbol) for symbol in symbols)
+    return tuple(symbolname(symbol) for symbol in symbols)
 
 
 class Constant(Expression):