# how to comfortably concatenate substitutions

Often I have to do perform a succession of substitutions on some expression, and in a notebook the way I know to do that is

((((expr /. subst1) /.subst2) /.subst3) ...


So everytime I want to add a substitution I have to go back at the beginning of the line and add a ( and then go at the end of the line and add a ) and then put the new substitution. Is there a more comfortable way to do that? Something like

expr // function1 // function2 // function3 ...


On the same line, when I have to both apply functions and substitution I have to use again the brackets, because

expr // function1 /. subst1 // function2...


mess up the order in which things are done: function2 is applied to subst1 which is then applied to function1 which is then applied to expr.

• ReplaceAll has an operator form. expr // ReplaceAll[expr -> 1] // ReplaceAll[1 -> 2]
– Kuba
Oct 24, 2018 at 12:03
• You don't need any of those parentheses, since /. is already left associative. Just use expr /. subst1 /.subst2 /.subst3. Oct 24, 2018 at 14:25
• you might find Fold convenient: e.g., substitutions = {a -> 1, b -> d, c -> 2, d -> 3}; Fold[ReplaceAll, {a, b, c, d}, substitutions] .
– kglr
Oct 24, 2018 at 16:50
• Note that replacement can take list arguments, so if the substitutions are independent you could write expr/.{subst1,subst2,subst3}. It they are not independent, expr//.{subst1,subst2,subst3} might give you the answer you need. Oct 24, 2018 at 17:12

Summing up, this is perfectly fine:

2 /. 2->{2,3} /. 2 -> 4


{4, 3}

And if you need to mix it with functions you can use an operator form of ReplaceAll:

4 // Sqrt // ReplaceAll[2 -> 5]


5