Aim: to read a string in the form First\nSecond from a file and to print it as
First
Second
Problem: if the string is defined in the code, as in line = "First\nSecond";, then it is printed on two lines; if instead I read it from a file then is printed as
First\nSecond
Short program illustrating the problem:
#include "stdafx.h" // I'm using Visual Studio 2008
#include <fstream>
#include <string>
#include <iostream>
void main() {
std::ifstream ParameterFile( "parameters.par" ) ;
std::string line ;
getline (ParameterFile, line) ;
std::cout << line << std::endl ;
line = "First\nSecond";
std::cout << line << std::endl ;
return;
}
The parameters.par file contains only the line
First\nSecond
The Win32 console output is
C:\blabla>SOtest.exe
First\nSecond
First
Second
Any suggestion?
In C/C++ string literals ("...") the backslash is used to mark so called "escape sequences" for special characters. The compiler translates (replaces) the two characters '\' (ASCII code 92) followed by 'n' (ASCII code 110) by the new-line character (ASCII code 10). In a text file one would normally just hit the [RETURN] key to insert a newline character. If you really need to process input containing the two characters '\' and 'n' and want to handle them like a C/C++ compiler then you must explicitely replace them by the newline character:
replace(line, "\\n", "\n");
where you have to supply a replace function like this:
Replace part of a string with another string (Standard C++ does not supply such a replace function by itself.)
Other escape sequences supported by C/C++ and similar compilers:
\t -> [TAB]
\" -> " (to distinguish from a plain ", which marks the end of a string literal, but is not part of the string itself!)
\\ -> \ (to allow having a backslash in a string literal; a single backslash starts an escape sequence)
The character indicated in a string literal by the escape sequence \n is not the same as the sequence of characters that looks like \n!
When you think you're assigning First\nSecond, you're not. In your source code, \n in a string literal is a "shortcut" for the invisible newline character. The string does not contain \n - it contains the newline character. It's automatically converted for you.
Whereas what you're reading from your file is the actual characters \ and n.
Related
Take a look at the following example:
cout << "option 1:
\n option 2:
\n option 3";
I know,it's not the best way to output a string,but the question is why does this cause an error saying that a " character is missing?There is a single string that must go to stdout but it just consists of a lot of whitespace charcters.
What about this:
string x="
string_test";
One may interpret that string as: "\nxxxxxxxxxxxxstring_test" where x is a whitespace character.
Is it a convention?
That's called multiline string literal.
You need to escape the embedded newline. Otherwise, it will not compile:
std::cout << "Hello world \
and stackoverflow";
Note: Backslashes must be immediately before the line ends as they need to escape the newline in the source.
Also you can use the fun fact "Adjacent string literals are concatenated by the compiler" for your advantage by this:
std::cout << "Hello World"
"Stack overflow";
See this for raw string literals. In C++11, we have raw string literals. They are kind of like here-text.
Syntax:
prefix(optional) R"delimiter( raw_characters )delimiter"
It allows any character sequence, except that it must not contain the
closing sequence )delimiter". It is used to avoid escaping of any
character. Anything between the delimiters becomes part of the string.
const char* s1 = R"foo(
Hello
World
)foo";
Example taken from cppreference.
so there's a text file where I have 1. languages, a 2. text of a number written in the said language, 3. the base of the number and 4. the number written in digits. Here's a sample:
francais deux mille quatre cents 10 2400
How I went about it:
struct Nomen{
char langue[21], nomNombre [31], baseC[3], nombreC[21];
int base, nombre;
};
and in the main:
if(myfile.is_open()){
{
while(getline(myfile, line))
{
strcpy(Linguo[i].langue, strtok((char *)line.c_str(), " "));
strcpy(Linguo[i].nomNombre, strtok(NULL, " "));
strcpy(Linguo[i].baseC, strtok(NULL, " "));
strcpy(Linguo[i].nombreC, strtok(NULL, "\n"));
i++;
}
Difficulty: I'm trying to put two whitespaces as a delimiter, but it seems that strtok() counts it as if there were only one whitespace. The fact there are spaces in the text number, etc. is messing up the tokenization. How should I go about it?
strtok treats any single character in the provided string as a delimiter. It does not treat the string itself as a single delimiter. So " " (two spaces) is the same as " " (one space).
strtok will also treat multiple delimiters together as a single delimiter. So the input "t1 t2" will be tokenized as two tokens, "t1" and "t2".
As mentioned in comments, strtok is also writes the NUL character into the input to create the token strings. So, it is an error to pass the result of string::c_str() as input to the function. The fact that you need to cast the constant string should have been enough to dissuade you from this approach.
If you want to treat a double space as a delimiter, you will have to scan the string and search for them yourself. Given you are using C APIs, you can consider strstr. However, in C++, you can use string::find.
Here's an algorithm to parse your string manually:
Given an input string input:
language is the substring from the start of input to the first SPC character.
From where language ends, skip over all whitespace, changing input to begin at the first non-whitespace character.
text is the substring from the start of input to the first double SPC sequence.
From where text ends, skip over all whitespace, changing input to begin at the first non-whitespace character.
Parse base, and parse number.
I am trying to manipulate a string by finding the \ character in the string Find\inHere. However, I can't put that as an input in test.find('\', 0). It won't work and gives me the error "missing terminating character." Is there a way to fix test.find('\', 0)?
string test = "Find\inHere";
int x = test.find('\', 0); // error on this line
cout << x; // x should equal 4
\ is a character used to introduce special characters, for example \n newline, \xDB shows the ASCII character with hexadecimal number DB etc.
So, in order to search this special character, you have to escape it by adding another \, use:
test.find("\\",0);
EDIT : Also, in your first string, it is not written in it "Find\inHere" but "Find" and an error because \inHere isn't a special instruction. So, same way to avoid it, write "Find\\inHere".
I have written a c++ code for changing file formats. Part of the functionality is to add a configured line end character. For one of file conversions, the line end character required is "\r\n" i.e. CR+NL .
My code basically reads the configured value from DB and appends it to the end of each record. Something on the lines of
//read DB and store line end char in a string lets say lineEnd.
//code snippet for file writting
string record = "this is a record";
ofstream outFileStream;
string outputFileName = "myfile.txt";
outFileStream.open (outputFileName.c_str());
outFileStream<<record;
outFileStream<<lineEnd; // here line end contains "\r\n"
But this prints record followed by \r\n as it is, no translation to CR+NL takes place.
this is a record\r\n
While the following works (prints CR+LF in output file)
outFileStream<<record;
outFileStream<<"\r\n";
this is a record
But I can not hard code it. I am facing similar issues with "\n" also.
Any suggestions on how to do it.
The translation of \r into the ASCII character CR and of \n into the ASCII character LF is done by the compiler when parsing your source code, and in literals only. That is, the string literal "A\n" will be a 3-character array with values 65 10 0.
The output streams do not interpret escape sequences in any way. If you ask an output stream to write the characters \ and r after each other, it will do so (write characters with ASCII value 92 and 114). If you ask it to write the character CR (ASCII code 13), it will do so.
The reason std::cout << "\r"; writes the CR character is that the string literal already contains the character 13. So if your database includes the string \r\n (4 characters: \, \r, \, n, ASCII 92 114 92 110), that is also the string you will get on output. If it contained the string with ASCII 13 10, that's what you'd get.
Of course, if it's impractical for you to store 13 10 in the database, nothing prevents you from storing 92 114 92 110 (the string "\r\n") in there, and translating it at runtime. Something like this:
void translate(std::string &str, const std::string &from, const std:string &to)
{
std::size_t at = 0;
for (;;) {
at = str.find(from, at);
if (at == str.npos)
break;
str.replace(at, from.size(), to);
}
}
std::string lineEnd = getFromDatabase();
translate(lineEnd, "\\r", "\r");
translate(lineEnd, "\\n", "\n");
I've got two questions. I need to write a program that extracts all non-alphabetic characters and displays them, then removes them.
I am using isalpha which is working for symbols, but only if the input string has no spaces like "hello world"
but if it is more than one word like "hello! world!", it will only extract the first exclamation mark but not the second.
Second question which may be related, I want my program to detect the spaces between the words (I tried isspace but I must have used it wrong? and remove them and put them in a char variable
so for example
if the input is hello4 world! How3 are you today?
I want it to tell me
removed: 4
removed:
removed: !
removed:
removed: 3
removed:
removed:
removed:
long story short, if there is no other way, I'd like to detect spaces as !isalpha, or find something similar to isalpha for space between text.
Thanks
# include <iostream>
# include <string>
using namespace std;
void main()
{
string message;
cin >> message;
for (int i = 0; message[i]; i++)
if(!isalpha(message[i]))
cout << "deleted following character: " << message[i] <<endl;
else
cout <<"All is good! \n";
}
>> reads a single word, stopping when a whitespace character is found. To read a whole line, you want
std::getline(cout, message);
There is a better way by which you can get non-alphabetic characters,
You can check with asci value of each character and compare with alphabetic asci character if not in it & not a space (space asci val),
then you get your non-alphabetic character.
You can get all ascii codes over here :=> http://www.asciitable.com/
-Jayesh