i'm trying to make a function that outputs n amount of colored spaces or " ". I specifically need it to be a quote string thing (not s) because I'm sending as an argument to the system() function.
is there a way that I can use a char variable inside a string.
I need it to function like this(I know it won't work):
system("echo -e \"\e[45m _myCharHere_ \"");
this way I can make the spaces (" ") any size I choose by multiplying the char by what ever integer I choose.
This is probably a stupid question, but I don't know all the technical programming terms for these operations etc; so I can't google it.
Thanks
You may use something like:
// Assuming mychar doesn't need escape sequence.
std::string command = std::string("echo -e \"\e[45m ") + myChar + " \"";
system(command.c_str());
Or as mentioned in comment, write directly (and here myChar can be special char):
std::cout << "\033[45m " << myChar;
Related
I'm writing a larger program that takes arguments from the command line after the executable. Some of the arguments are expected to be passed after the equals sign of an option. For instance, the output to the log is a comma separated vector by default, but if the user wants to change the separator to a period or something else instead of a comma, they might give the argument as:
./main --separator="."
This works fine, but if a user wants the delimiter be a special character (for example: tab), they might expect to pass the escape sequence in one of the following ways:
./main --separator="\t"
./main --separator='\t'
./main --separator=\t
It doesn't behave the way I want it to (to interpret \t as a tab) and instead prints out the string as written (sans quotes, and with no quotes it just prints 't'). I've tried using double slashes, but I think I might just be approaching this incorrectly and I'm not sure how to even ask the question properly (I tried searching).
I've recreated the issue in a dummy example here:
#include <string>
#include <iostream>
#include <cstdio>
// Pull the string value after the equals sign
std::string get_option( std::string input );
// Verify that the input is a valid option
bool is_valid_option( std::string input );
int main ( int argc, char** argv )
{
if ( argc != 2 )
{
std::cerr << "Takes exactly two arguments. You gave " << argc << "." << std::endl;
exit( -1 );
}
// Convert from char* to string
std::string arg ( argv[1] );
if ( !is_valid_option( arg ) )
{
std::cerr << "Argument " << arg << " is not a valid option of the form --<argument>=<option>." << std::endl;
exit( -2 );
}
std::cout << "You entered: " << arg << std::endl;
std::cout << "The option you wanted to use is: " << get_option( arg ) << "." << std::endl;
return 0;
}
std::string get_option( std::string input )
{
int index = input.find( '=' );
std::string opt = input.substr( index + 1 ); // We want everything after the '='
return opt;
}
bool is_valid_option( std::string input )
{
int equals_index = input.find('=');
return ( equals_index != std::string::npos && equals_index < input.length() - 1 );
}
I compile like this:
g++ -std=c++11 dummy.cpp -o dummy
With the following commands, it produces the following outputs.
With double quotes:
/dummy --option="\t"
You entered: --option=\t
The option you wanted to use is: \t.
With single quotes:
./dummy --option='\t'
You entered: --option=\t
The option you wanted to use is: \t.
With no quotes:
./dummy --option=\t
You entered: --option=t
The option you wanted to use is: t.
My question is: Is there a way to specify that it should interpret the substring \t as a tab character (or other escape sequences) rather than the string literal "\t"? I could parse it manually, but I'm trying to avoid re-inventing the wheel when I might just be missing something small.
Thank you very much for your time and answers. This is something so simple that it's been driving me crazy that I'm not sure how to fix it quickly and simply.
The escape sequences are already parsed from the shell you use, and are passed to your command line parameters array argv accordingly.
As you noticed only the quoted versions will enable you to detect that a "\\t" string was parsed and passed to your main().
Since most shells may just skip a real TAB character as a whitespace, you'll never see it in your command line arguments.
But as mentioned it's mainly a problem of how the shell interprets the command line, and what's left going to your program call arguments, than how to handle it with c++ or c.
My question is: Is there a way to specify that it should interpret the substring \t as a tab character (or other escape sequences) rather than the string literal "\t"? I could parse it manually, but I'm trying to avoid re-inventing the wheel when I might just be missing something small.
You actually need to scan for a string literal
"\\t"
within the c++ code.
Currently I only know of two methods to insert values into a C++ string or C string.
The first method I know of is to use std::sprintf() and a C-string buffer (char array).
The second method is to use something like "value of i: " + to_string(value) + "\n".
However, the first one needs the creation of a buffer, which leads to more code if you just want to pass a string to a function. The second one produces long lines of code, where a string gets interrupted every time a value is inserted, which makes the code harder to read.
From Python I know the format() function, which is used like this:
"Value of i: {}\n".format(i)
The braces are replaced by the value in format, and further .format()'s can be appended.
I really like Python's approach on this, because the string stays readable, and no extra buffer needs to be created. Is there any similar way of doing this in C++?
Idiomatic way of formatting data in C++ is with output streams (std::ostream reference). If you want the formatted output to end up in a std::string, use an output string stream:
ostringstream res;
res << "Value of i: " << i << "\n";
Use str() member function to harvest the resultant string:
std::string s = res.str();
This matches the approach of formatting data for output:
cout << "Value of i: " << i << "\n";
I'm struggling with the following: I'm reading from an XML file the following std::stringstream
"sigma=0\nreset"
Which after some copying&processing is written to a text-file. And I was hoping for the following
sigma=0
reset
But sadly I only get
sigma=0\nreset
but when I directly stream
out << "sigma=0\nreset"
I get:
sigma=0
reset
I currently suspect that some qualifier of the "\n" is lost during the "copy&processing"... is this possible? How to track down a "\n" in the stream which isn't a linefeed anymore?
Thank you!
It's because the output functions doesn't handle the escape sequences like '\n', it's the compiler that does and then only for literals. The compiler knows nothing of the contents of strings, and so can not do the translation "\n" to newline when inside a string.
You have to parse the string itself, and write out newlines when appropriate.
Assuming that the std::stringstream actually contains what is equivalent to the literal "sigma=0\\nreset" (length = 14 characters) and not "sigma=0\nreset" (length = 13 characters), you'll have to replace it yourself. Doing so is not very difficult, either use boost's replace_all (http://www.boost.org/doc/libs/1_53_0/doc/html/boost/algorithm/replace_all.html), or std::string::find and std::string::replace:
std::stringstream inStream;
inStream.str ("sigma=0\\nreset");
std::string content = inStream.str();
size_t index = content.find("\\n",0);
while(index != std::string::npos)
{
content.replace(index, 2, "\n");
index = content.find("\\n",index);
}
std::cout << content << '\n';
Note: you may want to consider cases when the system end-of-line is something other than "\n"
If the std::stringstream actually contains "sigma=0\nreset", then please post the code that does the copying/processing and the writing to the text file.
I am having problem formatting a string which contains quotationmarks.
For example, I got this std::string: server/register?json={"id"="monkey"}
This string needs to have the four quotation marks replaced by \", because it will be used as a c_str() for another function.
How does one do this the best way on this string?
{"id"="monkey"}
EDIT: I need a solution which uses STL libraries only, preferably only with String.h. I have confirmed I need to replace " with \".
EDIT2: Nvm, found the bug in the framework
it is perfectly legal to have the '"' char in a C-string. So the short answer is that you need to do nothing. Escaping the quotes is only required when typing in the source code
std::string str("server/register?json={\"id\"=\"monkey\"}")
my_c_function(str.c_str());// Nothing to do here
However, in general if you want to replace a substring by an other, use boost string algorithms.
#include <boost/algorithm/string/replace.hpp>
#include <iostream>
int main(int, char**)
{
std::string str = "Hello world";
boost::algorithm::replace_all(str, "o", "a"); //modifies str
std::string str2 = boost::algorithm::replace_all_copy(str, "ll", "xy"); //doesn't modify str
std::cout << str << " - " << str2 << std::endl;
}
// Displays : Hella warld - Hexya warld
If you std::string contains server/register?json={"id"="monkey"}, there's no need to replace anything, as it will already be correctly formatted.
The only place you would need this is if you hard-coded the string and assigned it manually. But then, you can just replace the quotes manually.
How print format string passed as argument ?
example.cpp:
#include <iostream>
int main(int ac, char* av[])
{
printf(av[1],"anything");
return 0;
}
try:
example.exe "print this\non newline"
output is:
print this\non newline
instead I want:
print this
on newline
No, do not do that! That is a very severe vulnerability. You should never accept format strings as input. If you would like to print a newline whenever you see a "\n", a better approach would be:
#include <iostream>
#include <cstdlib>
int main(int argc, char* argv[])
{
if ( argc != 2 ){
std::cerr << "Exactly one parameter required!" << std::endl;
return 1;
}
int idx = 0;
const char* str = argv[1];
while ( str[idx] != '\0' ){
if ( (str[idx]=='\\') && (str[idx+1]=='n') ){
std::cout << std::endl;
idx+=2;
}else{
std::cout << str[idx];
idx++;
}
}
return 0;
}
Or, if you are including the Boost C++ Libraries in your project, you can use the boost::replace_all function to replace instances of "\\n" with "\n", as suggested by Pukku.
At least if I understand correctly, you question is really about converting the "\n" escape sequence into a new-line character. That happens at compile time, so if (for example) you enter the "\n" on the command line, it gets printed out as "\n" instead of being converted to a new-line character.
I wrote some code years ago to convert escape sequences when you want it done. Please don't pass it as the first argument to printf though. If you want to print a string entered by the user, use fputs, or the "%s" conversion format:
int main(int argc, char **argv) {
if (argc > 1)
printf("%s", translate(argv[1]));
return 0;
}
You can't do that because \n and the like are parsed by the C compiler. In the generated code, the actual numerical value is written.
What this means is that your input string will have to actually contain the character value 13 (or 10 or both) to be considered a new line because the C functions do not know how to handle these special characters since the C compiler does it for them.
Alternatively you can just replace every instance of \\n with \n in your string before sending it to printf.
passing user arguments directly to printf causes a exploit called "String format attack"
See Wikipedia and Much more details
There's no way to automatically have the string contain a newline. You'll have to do some kind of string replace on your own before you use the parameter.
It is only the compiler that converts \n etc to the actual ASCII character when it finds that sequence in a string.
If you want to do it for a string that you get from somewhere, you need to manipulate the string directly and replace the string "\n" with a CR/LF etc. etc.
If you do that, don't forget that "\\" becomes '\' too.
Please never ever use char* buffers in C++, there is a nice std::string class that's safer and more elegant.
I know the answer but is this thread is active ?
btw
you can try
example.exe "print this$(echo -e "\n ")on newline".
I tried and executed
Regards,
Shahid nx