# How to replace the first occurence in an expression instead of replacing all?

Is there a function that replaces the first occurence of the expression instead of replacing all? For example, if I have HoldForm[x + 2 + 4 + x] /. x -> 4, is there a way to return 4 + 2 + 4 + x?

• $f_x^x$ Which one is the first x ? Sep 23 '14 at 22:34
• I'm not really sure, but I guess if you hit Ctrl+Shift+E, and the first occurence of x is what I meant Sep 23 '14 at 22:36
• What I mean is that the order of the symbols in a general math expression doesn't mean anything "in general". Perhaps if you restrict the domain of the expressions ... Sep 23 '14 at 22:38
• fun ClearAll[r]; r := (r = x; 4); HoldForm[x + 2 + 4 + x] /. x :> RuleCondition@r and for less fun take a look at Position 4th arg + ReplacePart.
– Kuba
Sep 23 '14 at 22:41

Another way:

hf = HoldForm[x + 2 + 4 + x]
i = 0
hf /. (x :> 4 /; i++ == 0)


4 + 2 + 4 + x

Well here's a way. Find the position of the first occurrence of x:

expr = HoldForm[x + 2 + 4 + x];

pos = Position[expr, x, -1, 1];


Then:

ReplacePart[expr, pos -> 4]


4 + 2 + 4 + x

• +1 With version 10, we can use FirstPosition, e.g. HoldForm[x + 2 + 4 + x] // ReplacePart[#, FirstPosition[#, x] -> 4] &. Sep 24 '14 at 5:22
• @WReach I was just updating the answer with that optimization. I considered using FirstPosition but it doesn't seem to bring much benefit here (a bit of clarity I guess) so I used the general equivalent. Sep 24 '14 at 5:24
• @Mr.Wizard, thanks for the update, certainly cleaner :) Sep 24 '14 at 5:38
• @WReach, good point, I forgot about FirstPosition Sep 24 '14 at 5:39

Edit

For order preserving as Jens says, I changed Attributes

ClearAttributes[Plus, Orderless]

HoldForm[7 + x + 2 + 4 + x + 5] /. f___ + x + l___ :> f + 4 + l


7 + 4 + 2 + 4 + x + 5

And you can revert by SetAttributes[Plus, Orderless]

Origin

HoldForm[x + 2 + 4 + x] /. x + a___ -> 4 + a


4 + 2 + 4 + x

• It won't work with this example: HoldForm[7 + x + 2 + 4 + x] /. x + a___ -> 4 + a because the order isn't preserved.
– Jens
Sep 24 '14 at 2:25
• @Jens : I got it, and I changed attributes. Sep 24 '14 at 4:37
• Yes, that works. But now you'd have to do the same if it were Times instead of Plus... anyway, you put your finger exactly on the reason why the original didn't work, and changing the attributes is certainly a way to make it work.
– Jens
Sep 24 '14 at 5:21
• Related: (30152) Sep 24 '14 at 5:26
• @Jens : I see. thank you. Sep 24 '14 at 5:31
expr = HoldForm[x + 2 + 4 + x];
expr[[##& @@ FirstPosition[expr, x]]] = 4; expr


4 + 2 + 4 + x