Variable fails to increment with lvalue error

Variable fails to increment with lvalue error

[Evan Clearfield]

I’m in the process of creating a shell script on macOS Sonoma. I’m just implementing a simple counter in a for loop. I initialized my variable outside the loop, such that “count=0”. I then increment it with a simple call “(($count++))”, and when I run the script, I receive the error “bad math expression: lvalue required”

Any idea what I’m doing wrong? I’ve searched high and low, tried multiple variations of the “((count++))” expression, changed my initial number, etc. Nothing seems to work.

HELP
Sent from my iPhone

Re: Variable fails to increment with lvalue error

[Roman Perepelitsa]

On Mon, Oct 23, 2023 at 5:28 PM Evan Clearfield <efclear@gmail.com> wrote:
>
> I then increment it with a simple call “(($count++))”, and when I run the script, I receive the error “bad math expression: lvalue required”

`$count` expands to `0`, so `(($count++))` is equivalent to `((0++))`.
The post-increment operator (++) requires something that can be
incremented (an lvalue). `0` is not an lvalue, so you get an error.
What you want instead is `((count++))`. This will increment `count` as
you expect.

Roman.

Re: Variable fails to increment with lvalue error

[Peter Stephenson]

> On 23/10/2023 16:33 Roman Perepelitsa <roman.perepelitsa@gmail.com> wrote:
> On Mon, Oct 23, 2023 at 5:28 PM Evan Clearfield <efclear@gmail.com> wrote:
> >
> > I then increment it with a simple call “(($count++))”, and when I run the script, I receive the error “bad math expression: lvalue required”
> 
> `$count` expands to `0`, so `(($count++))` is equivalent to `((0++))`.

Is it worth drawing attention to this, or is this just excess verbiage?

pws

diff --git a/Doc/Zsh/arith.yo b/Doc/Zsh/arith.yo
index bc3e35ad5..e6380b2cb 100644
--- a/Doc/Zsh/arith.yo
+++ b/Doc/Zsh/arith.yo
@@ -23,8 +23,10 @@ command which begins with a `tt(LPAR()LPAR())', all the characters until a
 matching `tt(RPAR()RPAR())' are treated as a double-quoted expression and
 arithmetic expansion performed as for an argument of tt(let).  More
 precisely, `tt(LPAR()LPAR())var(...)tt(RPAR()RPAR())' is equivalent to
-`tt(let ")var(...)tt(")'.  The return status is 0 if the arithmetic value
-of the expression is non-zero, 1 if it is zero, and 2 if an error occurred.
+`tt(let ")var(...)tt(")': note the presence of double quotes, so that variable
+expansion etc. takes place before arithmetic evaluation.  The return
+status is 0 if the arithmetic value of the expression is non-zero, 1 if
+it is zero, and 2 if an error occurred.
 
 For example, the following statement
 
@@ -37,6 +39,10 @@ example(let "val = 2 + 1")
 both assigning the value 3 to the shell variable tt(val) and returning a
 zero status.
 
+Note further that it would be incorrect to replace tt(val) with tt($val)
+as the latter is affected by double quote substitution, so that
+arithmetic evaluation does not see the name of the variable.
+
 cindex(arithmetic base)
 cindex(bases, in arithmetic)
 Integers can be in bases other than 10.

Re: Variable fails to increment with lvalue error

[Roman Perepelitsa]

On Mon, Oct 23, 2023 at 6:44 PM Peter Stephenson
<p.w.stephenson@ntlworld.com> wrote:
>
> > On 23/10/2023 16:33 Roman Perepelitsa <roman.perepelitsa@gmail.com> wrote:
> > On Mon, Oct 23, 2023 at 5:28 PM Evan Clearfield <efclear@gmail.com> wrote:
> > >
> > > I then increment it with a simple call “(($count++))”, and when I run the script, I receive the error “bad math expression: lvalue required”
> >
> > `$count` expands to `0`, so `(($count++))` is equivalent to `((0++))`.
>
> Is it worth drawing attention to this, or is this just excess verbiage?
>
> pws
>
> diff --git a/Doc/Zsh/arith.yo b/Doc/Zsh/arith.yo
> index bc3e35ad5..e6380b2cb 100644
> --- a/Doc/Zsh/arith.yo
> +++ b/Doc/Zsh/arith.yo
> @@ -23,8 +23,10 @@ command which begins with a `tt(LPAR()LPAR())', all the characters until a
>  matching `tt(RPAR()RPAR())' are treated as a double-quoted expression and
>  arithmetic expansion performed as for an argument of tt(let).  More
>  precisely, `tt(LPAR()LPAR())var(...)tt(RPAR()RPAR())' is equivalent to
> -`tt(let ")var(...)tt(")'.  The return status is 0 if the arithmetic value
> -of the expression is non-zero, 1 if it is zero, and 2 if an error occurred.
> +`tt(let ")var(...)tt(")': note the presence of double quotes, so that variable
> +expansion etc. takes place before arithmetic evaluation.  The return
> +status is 0 if the arithmetic value of the expression is non-zero, 1 if
> +it is zero, and 2 if an error occurred.
>
>  For example, the following statement
>
> @@ -37,6 +39,10 @@ example(let "val = 2 + 1")
>  both assigning the value 3 to the shell variable tt(val) and returning a
>  zero status.
>
> +Note further that it would be incorrect to replace tt(val) with tt($val)
> +as the latter is affected by double quote substitution, so that
> +arithmetic evaluation does not see the name of the variable.
> +
>  cindex(arithmetic base)
>  cindex(bases, in arithmetic)
>  Integers can be in bases other than 10.
>

Let's ask the OP if this would've been helpful.

Evan, have you read this documentation page? It's available at `man
zshmisc` and online at
https://zsh.sourceforge.io/Doc/Release/Arithmetic-Evaluation.html. If
you have, would it have helped if the docs were amended as per Peter's
patch?

Roman.

Re: Variable fails to increment with lvalue error

[Ray Andrews]

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On 2023-10-23 10:03, Roman Perepelitsa wrote:
> Let's ask the OP if this would've been helpful.
>
> Evan, have you read this documentation page? It's available at

Beeeutiful!  Only the learner can say if a doc was useful, so ask him :-)

My two cents would be:

Note further that it would be incorrect to replace tt(val) with tt($val) as the latter is affected by the built-in double quote substitution, so that the arithmetic evaluation does not see the name of the variable but rather it's expanded value.  Thus if $val = 0, the latter expression would attempt: 0 = 2 + 1 ... which is obviously nonsense.

... maybe too much hand-holding there? ...

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Re: Variable fails to increment with lvalue error

[Dominik Vogt]

On Mon, Oct 23, 2023 at 11:27:50AM -0400, Evan Clearfield wrote:
> initialized my variable outside the loop, such that ???count=0???.
> I then increment it with a simple call ???(($count++))???, and
> when I run the script, I receive the error ???bad math expression:
> lvalue required???

Use ((count++)) instead of (($count++)).

Ciao

Dominik ^_^  ^_^

--

Dominik Vogt