what is char ctemp = ' '; and string stemp = ""; means? when they put ' ' and " " inside without writing anything inside? Help please! Will appreciate who answer it.
Single quotes (') indicate a character literal: a single character. Double quotes (") denote a string literal, i.e: an array of characters.
' ' is a single space character, while " " is a single space character followed by a null terminator, as is customary for C-style strings.
Character literals are directly assignable to char variables.
The type of a string literal is const char[N], where N is the length of the literal, including the null terminator. In C and C++, a static array decays to (is implicitly convertible to) a pointer to the first element, and std::string is constructible from a const char * pointer (see constructor (5)), which in C usually means a pointer to an array of characters terminated by a null terminator.
The char ctemp = ' ' will put the value ' ' (32 in ASCII decimal) inside the ctemp variable.
The string stemp = ""; will create an empty string in stemp.
Here
char ctemp = ' ';
you are assigning a whitespace character ' ' to ctemp.
Here
string stemp = "";
the initializer "" creates a empty string.
' ' is the space character. "" is an empty string. " " is a string that contains only the space character.
Note that a statement like string stemp = "" implicitly invokes the string(char const *) constructor to create a new string instance from a char const * pointer.
the first one means a "whitespace" like when you write something and need to divide the words with the space key. That empty space is still part of the string and so you can say your char is only that empty space.
The second one is of type string but it is even less than a white space. It is a completely empty string.
string is array(collection) of char
ctemp = ' '
mean whitespace character
stemp = ""
mean empty string no character in string
you can put ' ' to char variable.
you can put " " to array of char.
In C++ the single quote is used to identify the single character, and double quotes are used for string literals. A string literal “x” is a string, it is containing character ‘x’ and a null terminator ‘\0’. So “x” is two-character array in this case.
Some Examples:
string s = "" ; => empty string
char s =' ' ; => space (you should have only one character inside the single quotes)
string s = " " ; => space followed by '\0' character (two character array)
Whitespace character and empty string. You can see the string as a sequence of characters, but they are two different types
Related
I need to generate a string that can match another both containing special characters. I wrote what I thought would be a simple method, but so far nothing has given me a successful match.
I know that specials characters in c++ are preceded with a "\". Per example a single quote would be written as "\'".
string json_string(const string& incoming_str)
{
string str = "\\\"" + incoming_str + "\\\"";
return str;
}
And this is the string I have to compare to:
bool comp = json_string("hello world") == "\"hello world\"";
I can see in the cout stream that in fact I'm generating the string as needed but the comparison still gives a false value.
What am I missing? Any help would be appreciated.
One way is to filter one string and compare this filtered string. For example:
#include <iostream>
#include <algorithm>
using namespace std;
std::string filterBy(std::string unfiltered, std::string specialChars)
{
std::string filtered;
std::copy_if(unfiltered.begin(), unfiltered.end(),
std::back_inserter(filtered), [&specialChars](char c){return specialChars.find(c) == -1;});
return filtered;
}
int main() {
std::string specialChars = "\"";
std::string string1 = "test";
std::string string2 = "\"test\"";
std::cout << (string1 == filterBy(string2, specialChars) ? "match" : "no match");
return 0;
}
Output is match. This code also works if you add an arbitrary number of characters to specialChars.
If both strings contain special characters, you can also put string1 through the filterBy function. Then, something like:
"\"hello \" world \"" == "\"hello world "
will also match.
If the comparison is performance-critical, you might also have a comparison that uses two iterators, getting a comparison complexity of log(N+M), where N and M are the sizes of the two strings, respectively.
bool comp = json_string("hello world") == "\"hello world\"";
This will definitely yield false. You are creating string \"hello world\" by json_string("hello world") but comparing it to "hello world"
The problem is here:
string str = "\\\"" + incoming_str + "\\\"";
In your first string literal of str, the first character backlash that you’re assuming to be treated like escape character is not actually being treated an escape character, rather just a backslash in your string literal. You do the same in your last string literal.
Do this:
string str = "\"" + incoming_str + "\"";
In C++ string literals are delimited by quotes.
Then the problem arises: How can I define a string literal that does itself contain quotes? In Python (for comparison), this can get easy (but there are other drawbacks with this approach not of interest here): 'a string with " (quote)'.
C++ doesn't have this alternative string representation1, instead, you are limited to using escape sequences (which are available in Python, too – just for completeness...): Within a string (or character) literal (but nowhere else!), the sequence \" will be replaced by a single quote in the resulting string.
So "\"hello world\"" defined as character array would be:
{ '"', 'h', 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd', '"', 0 };
Note that now the escape character is not necessary...
Within your json_string function, you append additional backslashes, though:
"\\\""
{ '\', '"', 0 }
//^^^
Note that I wrote '\' just for illustration! How would you define single quote? By escaping again! '\'' – but now you need to escape the escape character, too, so a single backslash actually needs to be written as '\\' here (wheras in comparison, you don't have to escape the single quote in a string literal: "i am 'singly quoted'" – just as you didn't have to escape the double quote in the character literal).
As JSON uses double quotes for strings, too, you'd most likely want to change your function:
return "\"" + incoming_str + "\"";
or even much simpler:
return '"' + incoming_str + '"';
Now
json_string("hello world") == "\"hello world\""
would yield true...
1 Side note (stolen from answer deleted in the meanwhile): Since C++11, there are raw string literals, too. Using these, you don't have to escape either.
I am trying to convert #"\n", a Char, to "\n", a String. I used
Char.toString(#"\n");
and it gives
val it = "\\n" : string
Why does not it return "\n"?
Char.toString from the documentation.
returns a printable string representation of the character, using, if
necessary, SML escape sequences.
It also specifies that some control characters are converted to two-character escape sequences and \n is one of it.
To return a string of size one, use String.str.
- String.str(#"\n");
val it = "\n" : string
I am trying to read data from a text file and split the read line based on quotes. For example
"Hi how" "are you" "thanks"
Expected output
Hi how
are you
thanks
My code:
getline(infile, line);
ch = strdup(line.c_str());
ch1 = strtok(ch, " ");
while (ch1 != NULL)
{
a3[i] = ch1;
ch1 = strtok(NULL, " ");
i++;
}
I don't know what to specify as delimiter string. I am using strtok() to split, but it failed. Can any one help me?
Please have a look at the example code here. You should provide "\"" as delimiter string to strtok.
For example,
ch1 = strtok (ch,"\"");
Probably your problem is related with representing escape sequences. Please have a look here for a list of escape sequences for characters.
Given your input: "Hi how" "are you" "thanks", if you use strtok with "\"" as the delimiter, it'll treat the spaces between the quoted strings as if they were also strings, so if (for example) you printed out the result strings, one per line, surrounded by square brackets, you'd get:
[Hi how]
[ ]
[are you]
[ ]
[thanks]
I.e., the blank character between each quoted string is, itself, being treated as a string. If the delimiter you supplied to strtok was " \"" (i.e., included both a quote and a space) that wouldn't happen, but then it would also break on the spaces inside the quoted strings.
Assuming you can depend on every item you care about being quoted, you want to skip anything until you get to a quote, ignore the quote, then read data into your input string until you get to another quote, then repeat the whole process.
I am trying to modify someone's code which uses this line:
out.write(&vecBuffer[0], x.length());
However, I want to modify the buffer beforehand so it removes any bad characters I don't want to be output. For example if the buffer is "Test%string" and I want to get rid of %, I want to change the buffer to "Test string" or "Teststring" whichever is easier.
std::replace will allow replacing one specific character with
another, e.g. '%' with ' '. Just call it normally:
std::replace( vecBuffer.begin(), vecBuffer.end(), '%', ' ' );
Replace the '%' with a predicate object, call replace_if,
and you can replace any character for which the predicate
object returns true. But always with the same character. For
more flexibility, there's std::transform, which you pass
a function which takes a char, and returns a char; it will
be called on each character in the buffer.
Alternatively, you can do something like:
vecBuffer.erase(
std::remove( vecBuffer.begin(), vecBuffer.end(), '%' ).
vecBuffer.end() );
To remove the characters. Here too, you can replace remove
with remove_if, and use a predicate, which may match many
different characters.
The simplest library you can use is probably the Boost String Algorithms library.
boost::replace_all(buffer, "%", "");
will replace all occurrences of % by nothing, in place. You could specify " " as a replacement, or even "REPLACEMENT", as suits you.
std::string str("Test string");
std::replace_if(str.begin(), str.end(), boost::is_any_of(" "), '');
std::cout << str << '\n';
You do not need to use the boost library. The easiest way is to replace the % character with a space, using std::replace() from the <algorithm> header:
std::replace(vecBuffer.begin(), vecBuffer.end(), '%', ' ');
I assume that vecBuffer, as its name implies, is an std::vector. If it's actually a plain array (or pointer), then you would do:
std::replace(vecBuffer, vecBuffer + SIZE_OF_BUFFER, '%', ' ');
SIZE_OF_BUFFER should be the size of the array (or the amount of characters in the array you want to process, if you don't want to convert the whole buffer.)
Assuming you have a function
bool goodChar( char c );
That returns true for characters you are approved of and false otherwise,
then how about
void fixBuf( char* buf, unsigned int len ) {
unsigned int co = 0;
for ( unsigned int cb = 0 ; cb < len ; cb++ ) {
if goodChar( buf[cb] ) {
buf[co] = buf[cb];
co++;
}
}
}
I need to transform a literal filepath (C:/example.txt) to one that is compatible with the various WinAPI Registry functions (C://example.txt) and I have no idea on how to go about doing it.
I've broken it down to having to add a backslash after a certain character (/ in this case) but i'm completely stuck after that.
Guidance and Code Examples will be greatly appreciated.
I'm using C++ and VS2012.
In C++, strings are made up of individual characters, like "foo". Strings can be composed of printable characters, such as the letters of the alphabet, or non-printable characters, such as the enter key or other control characters.
You cannot type one of these non-printable characters in the normal way when populating a string. For example, if you want a string that contains "foo" then a tab, and then "bar", you can't create this by typing:
fooTABbar
because this will simply insert that many spaces -- it won't actually insert the TAB character.
You can specify these non-printable characters by "escaping" them out. This is done by inserting a back slash character (\) followed by the character's code. In the case of the string above TAB is represented by the escape sequence \t, so you would write: "foo\tbar".
The character \ is not itself a non-printable character, but C++ (and C) recognize it to be special -- it always denotes the beginning of an escape sequence. To include the character "\" in a string, it has to itself be escaped, with \\.
So in C++ if you want a string that contains:
c:\windows\foo\bar
You code this using escape sequences:
string s = "c:\\windows\\foo\\bar"
\\ is not two chars, is one char:
for(size_t i = 0, sz = sPath.size() ; i < sz ; i++)
if(sPath[i]=='/') sPath[i] = '\\';
But be aware that some APIs work with \ and some with /, so you need to check in which cases to use this replacement.
If replacing every occurrence of a forward slash with two backslashes is really what you want, then this should do the job:
size_t i = str.find('/');
while (i != string::npos)
{
string part1 = str.substr(0, i);
string part2 = str.substr(i + 1);
str = part1 + R"(\\)" + part2; // Use "\\\\" instead of R"(\\)" if your compiler doesn't support C++11's raw string literals
i = str.find('/', i + 1);
}
EDIT:
P.S. If I misunderstood the question and your intention is actually to replace every occurrence of a forward slash with just one backslash, then there is a simpler and more efficient solution (as #RemyLebeau points out in a comment):
size_t i = str.find('/');
while (i != string::npos)
{
str[i] = '\\';
i = str.find('/', i + 1);
}
Or, even better:
std::replace_if(str.begin(), str.end(), [] (char c) { return (c == '/'); }, '\\');