K and R exercise 1-24

Question

I am doing programs in The C Programming Language by Kernighan and Ritchie.

I am currently at exercise 1-24 that says:

Write a program to check a C Program for rudimentary syntax errors like unbalanced parentheses, brackets and braces. Don't forget about quotes, both single and double, escape sequences, and comments.

I have done everything well... But I am not getting how escape sequences would affect these parentheses, brackets and braces?

Why did they warned about escape sequences?

Don't count `\"` as part of the syntax? – Alex K. Feb 13 '13 at 17:00 — Alex K., Feb 13 '13 at 17:00
`char excerpt[] = "\"Aha!\" he said.";` – pmg Feb 13 '13 at 17:00 — pmg, Feb 13 '13 at 17:00

Fred Foo · Accepted Answer · 2013-02-13T19:40:06.047

4

In "\"", there are three double quote characters, but still it's a valid string literal. The middle " is escaped, meaning the outer two balance each other. Similarly, '\'' is a valid character literal.

Parentheses, brackets and braces are not affected, unless of course they appear in a string literal that you don't parse correctly because of an escaped quote.

edited Feb 13 '13 at 19:40

answered Feb 13 '13 at 17:00

Fred Foo

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`Parentheses, brackets and braces are not affected.` -That is what I wanted! Thanks!!! – Sam Feb 13 '13 at 17:05
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@SAM: `Parentheses, brackets and braces are not affected.` Except when they are within a pair of quotes, in which case, they should be ignored for syntax checking purposes. – RobH Feb 13 '13 at 17:15

simonc · Answer 2 · 2013-02-13T17:05:59.157

3

I'd guess they mean that you need to differentiate between " (which starts or ends a string) and \" (which is a " character, possibly inside a string)

This is important if you're to avoid reporting e.g. strlen("\")"); as having unbalanced parentheses.

edited Feb 13 '13 at 17:05

answered Feb 13 '13 at 17:00

simonc

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score 2 · Answer 3 · answered Feb 13 '13 at 17:00

The obvious possibility would be an escaped quote inside a string. If you don't take the escape into account, you might think the string ended there. For example: "\")\"". The ) is part of the string literal, so it doesn't count as a mis-matched parenthesis.

K and R exercise 1-24

3 Answers3