In csh, why does 4 - 3 + 1 == 0?

Question

#!/bin/csh

@ cows = 4 - 3 + 1
echo $cows

This simple csh script when run produces "0" for output when I'd expect "2".

~root: csh simple.1
0

I did a bunch of looking and the only thing I could think of was that the "-" was being read as a unary negation rather than subtraction, therefore changing operator precedence and ending up with 4 - 4 rather than 2 + 1. Is this correct? If so, any reason why? If not...help!

Edit: So they're right associative! These operators are NOT right associative in C, are they? Is C-Shell that different from C?

Here's a better question... why does csh use right-associative parse trees and not left? — Joe Phillips, Jun 17 '09 at 23:56
I wasn't actually using it. It was a question I was trying to answer for a friend. Here's the new discussion: http://stackoverflow.com/questions/1010119/ — Instantsoup, Jun 18 '09 at 00:06
Just to add to the frivolity, in tcsh (to which /bin/csh is often a symlink), `4 - 3 + 1` evaluates to `2` — Keith Thompson, Mar 20 '13 at 23:14

score 33 · Accepted Answer · edited Oct 14 '12 at 17:11

33

While you are expecting the operators to be left associative, they are right associative in csh, so it's evaluated as 4-(3+1)

edited Oct 14 '12 at 17:11

alanc

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answered Jun 17 '09 at 23:51

Paul Dixon

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That is not a parse tree. It is not an AST. Look at the BNF again and see how both operands and operators are child nodes of an expression node. Compilers have to do extra work to convert parse trees to expression trees. – Windows programmer Jun 18 '09 at 00:24

score 21 · Answer 2 · answered Jun 17 '09 at 23:46

21

The + and - operators are right-associative in csh. This means that '4 - 3 + 1' is evaluated as '4 - (3 + 1)'.

answered Jun 17 '09 at 23:46

Brandon E Taylor

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score 3 · Answer 3 · answered Jun 17 '09 at 23:45

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Operator grouping. It's reading the operation as 4 - (3 + 1), as opposed to (4 - 3) + 1.

answered Jun 17 '09 at 23:45

Paul Sonier

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In csh, why does 4 - 3 + 1 == 0?

3 Answers3

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