       Re: ReplacePart except first element?

• To: mathgroup at smc.vnet.net
• Subject: [mg102085] Re: ReplacePart except first element?
• From: Ray Koopman <koopman at sfu.ca>
• Date: Wed, 29 Jul 2009 05:08:40 -0400 (EDT)
• References: <h4m4ar\$eah\$1@smc.vnet.net>

```On Jul 27, 11:03 pm, mokambo <alexandrepassosalme... at gmail.com> wrote:
> I was trying to modify a list by adding 1 to a certain element.
> I can't understand the following result:
>
> h[x_List, a_Integer] := ReplacePart[x, a -> x[[a]] + 1]
>
> h[{1, 2, 3}, 2] gives {,3,3} (what happened to the first element?) but
> h[{1, 2, 3}, 1] gives {2,2,3} as expected.
>
> Eventually I changed to:
>
> h[x_List, a_Integer] := MapAt[Function[y, y + 1], x, a]
>
> but the ReplacePart version still puzzles me.
> Is it because ReplacePart creates a copy instead of altering inline
> like x[[a]] = val does?

I don't know why

h[x_,a_] := ReplacePart[x, a -> x[[a]] + 1]

gave you what you posted. I get an error message
and then the input with the Rule evaluated:

h[{1,2,3}, 1]

ReplacePart::argt:
ReplacePart called with 2 arguments; 3 or 4 arguments are expected.

ReplacePart[{1,2,3}, 1 -> 2]

Here are three versions, all of which do what you want:

h[x_,a_] := MapAt[#+1&, x, a]

h[x_,a_] := ReplacePart[x, x[[a]]+1, a]

h[x_,a_] := x /. (x[[a]] -> x[[a]] + 1)

```

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