* Variable fails to increment with lvalue error
@ 2023-10-23 15:27 Evan Clearfield
2023-10-23 15:33 ` Roman Perepelitsa
2023-10-23 16:34 ` Dominik Vogt
0 siblings, 2 replies; 6+ messages in thread
From: Evan Clearfield @ 2023-10-23 15:27 UTC (permalink / raw)
To: zsh-users
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 “(($count++))”, and when I run the script, I receive the error “bad math expression: lvalue required”
Any idea what I’m doing wrong? I’ve searched high and low, tried multiple variations of the “((count++))” expression, changed my initial number, etc. Nothing seems to work.
HELP
Sent from my iPhone
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Variable fails to increment with lvalue error
2023-10-23 15:27 Variable fails to increment with lvalue error Evan Clearfield
@ 2023-10-23 15:33 ` Roman Perepelitsa
2023-10-23 16:43 ` Peter Stephenson
2023-10-23 16:34 ` Dominik Vogt
1 sibling, 1 reply; 6+ messages in thread
From: Roman Perepelitsa @ 2023-10-23 15:33 UTC (permalink / raw)
To: Evan Clearfield; +Cc: zsh-users
On Mon, Oct 23, 2023 at 5:28 PM Evan Clearfield <efclear@gmail.com> wrote:
>
> I then increment it with a simple call “(($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.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Variable fails to increment with lvalue error
2023-10-23 15:27 Variable fails to increment with lvalue error Evan Clearfield
2023-10-23 15:33 ` Roman Perepelitsa
@ 2023-10-23 16:34 ` Dominik Vogt
1 sibling, 0 replies; 6+ messages in thread
From: Dominik Vogt @ 2023-10-23 16:34 UTC (permalink / raw)
To: zsh-users
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
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Variable fails to increment with lvalue error
2023-10-23 15:33 ` Roman Perepelitsa
@ 2023-10-23 16:43 ` Peter Stephenson
2023-10-23 17:03 ` Roman Perepelitsa
0 siblings, 1 reply; 6+ messages in thread
From: Peter Stephenson @ 2023-10-23 16:43 UTC (permalink / raw)
To: zsh-users
> 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.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Variable fails to increment with lvalue error
2023-10-23 16:43 ` Peter Stephenson
@ 2023-10-23 17:03 ` Roman Perepelitsa
2023-10-23 18:21 ` Ray Andrews
0 siblings, 1 reply; 6+ messages in thread
From: Roman Perepelitsa @ 2023-10-23 17:03 UTC (permalink / raw)
To: Peter Stephenson, Evan Clearfield; +Cc: zsh-users
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.
^ permalink raw reply [flat|nested] 6+ messages in thread
* Re: Variable fails to increment with lvalue error
2023-10-23 17:03 ` Roman Perepelitsa
@ 2023-10-23 18:21 ` Ray Andrews
0 siblings, 0 replies; 6+ messages in thread
From: Ray Andrews @ 2023-10-23 18:21 UTC (permalink / raw)
To: zsh-users
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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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^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2023-10-23 18:22 UTC | newest]
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2023-10-23 15:27 Variable fails to increment with lvalue error Evan Clearfield
2023-10-23 15:33 ` Roman Perepelitsa
2023-10-23 16:43 ` Peter Stephenson
2023-10-23 17:03 ` Roman Perepelitsa
2023-10-23 18:21 ` Ray Andrews
2023-10-23 16:34 ` Dominik Vogt
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