How to see backslashes in string

Question

For a function I am making, I take a string in as a parameter and do things with it. However I treat characters in the string specially if there is a backslash before it. However I am having problems even seeing the blackslash!

std::string s = "01234\6";
std::cout << s << std::endl;
std::cout << s.at(5) << std::endl;
if(s.at(5)== '\\')
    std::cout << "It's a backslash" << std::endl;
else
    std::cout << "It's not a backslash" << std::endl;

outputs

01234

It's not a backslash

How am I supposed to check if mystring.at(i) == '\\' if it isn't showing up at all?

The input will be coming from another file (which I can't modify) like

myfunc("% \%  %");

If I read the string I count 3 '%' characters (so its not ignored by the backslash), and 0 '\' characters

edit: Code how I count

char percent = '%';
int current_index = 0;
int percent_count = 0;

int ret = str.find(percent, current_index);
while(ret != std::string::npos)
{
  percent_count++;
  current_index = ret +1;
  ret = str.find(percent, current_index);
 }

 return percent_count;

`myfunc("% \% %");` has invalid conversion specifiera so the behavior is undefined. You get what you get. — Captain Obvlious, May 18 '16 at 03:38
When hard-coding a string, the compiler gives special meaning to "\". And the hard-coded string will be translated into something else. Luckily (well by design), one of its special meanings is that "\\" will be treated as "\". So wherever you want a hard-coded string to contain "\", write "\\". **NOTE** That this applies to compilation only. When your program reads text from another source, it will read the text as it receives it. — Disillusioned, May 18 '16 at 11:36

R Sahu · Answer 1 · 2016-05-18T03:57:34.393

C++ supports three kinds of escape sequences:

simple-escape-sequence. It is one of:
```
\’ \" \? \\
\a \b \f \n \r \t \v
```
octal-escape-sequence. It is one of:
```
\ octal-digit
\ octal-digit octal-digit
\ octal-digit octal-digit octal-digit
```
\0 is the most well known octal escape sequence that represents the null character.

hexadecimal-escape-sequence. It is one of:

\x hexadecimal-digit
hexadecimal-escape-sequence hexadecimal-digit

When you use:

std::string s = "01234\6";

the \6 part represents an octal escape sequence. It does not represent two characters.

It is the same as

std::string s = "01234?";

where ? is the character represented by the octal number 6.

In order to have \ as an element of the string, you'll need to use:

std::string s = "01234\\6";

It's worth noting that when the input comes from a file instead of a string literal (as mentioned in the question), no escaping is needed. Backslash will be read as a backslash. — ColBeseder, May 18 '16 at 03:56

score 1 · Answer 2 · answered May 18 '16 at 03:17

1

The checking method is right, but \ escape 6, so \6 is counted once, you can check sizeof("12345\6"), which 7, or strlen("12345\6"), which is 6.

Change "12345\6" to "12345\\6".

answered May 18 '16 at 03:17

delta

3,778
15
22

@user2770808 don't worry. You don't really *edit* the string in a sense, you let the compiler knows that what you actually mean is to read a backslash. – Ian May 18 '16 at 03:20
@user2770808 compiler knows backslash if you read from file, which is not the same as the literal string representation in c/c++. – delta May 18 '16 at 03:25

score 0 · Answer 3 · answered May 18 '16 at 03:17

0

The C++ compiler would have already treated it specially if you have backslash in the string:

std::string s = "01234\6"; //\6 is treated differently already, as unicode character \6, not as backslash + 6

Unless what you mean is you want to have a text with backslash (say, from I/O). In that case, you should put \\ to make your compiler understand that you mean it as real backslash not a unicode character:

std::string s = "01234\\6"; //double backslash here

Then you can test your program.

answered May 18 '16 at 03:17

Ian

30,182
19
69
107

How come when I do myfunc("% \% %"), I still count 3 percents? Shouldn't it only be 2 then? – user2770808 May 18 '16 at 03:21
Your `count` does not seem to have `if-else` distinguishing backslash. Is that full code? – Ian May 18 '16 at 03:28
I took out the if(str.at(ret-1) == '\\') because it was not working. – user2770808 May 18 '16 at 03:30
What I suggest would be to detect the `str` `char` one by one and if you find `\\`, then you treat your *next* character differently. – Ian May 18 '16 at 03:31
@user2770808 `\%` is `%` itself. – delta May 18 '16 at 03:32
@delta actually it is implementation-defined (and may be unsupported entirely) – M.M May 18 '16 at 03:49

Ani Menon · Answer 4 · 2016-05-18T14:13:06.240

0

No compiler C++ will interpret \ as a backslash, since its the escape character. You will have to use \\ to denote a backslash in a string.

edited May 18 '16 at 14:13

answered May 18 '16 at 03:20

Ani Menon

27,209
16
105
126

Not quite true. You might want to broaden your horizons to other languages. ;) Or at least qualify what compilers you're describing. – Disillusioned May 18 '16 at 11:41

How to see backslashes in string

4 Answers4