I'm using the following code which is similar to Stroustrup's C++ 4th Edition Page 127&128. Per output log below, it prints the first match, however not the match for the trailing -XXXX digits.
Does anyone know why the trailing digits are not matched and/or printed??
Thanks
#include <iostream>
#include <regex>
using namespace std;
int main(int argc, char *argv[])
{
// ZIP code pattern: XXddddd-dddd and variants
regex pat (R"(\w{2}\s*\d{5}(−\d{4})?)");
int lineno = 0;
for (string line; getline(cin,line);) {
++lineno;
smatch matches; // matched strings go here
if (regex_search(line, matches, pat)) // search for pat in line
for (auto p : matches) {
cout << p << " ";
}
cout << endl;
// cout << lineno << ": " << matches[0] << '\n';
}
return 0;
}
Output log:
$ ./a.out
AB00000-0000
AB00000
− is not -. That are two different symbols. You have − in the code and - in the input. Here I fixed the code:
#include <iostream>
#include <regex>
using namespace std;
int main(int argc, char *argv[])
{
// ZIP code pattern: XXddddd-dddd and variants
regex pat (R"(\w{2}\s*\d{5}(-\d{4})?)");
int lineno = 0;
for (string line; getline(cin,line);) {
++lineno;
smatch matches; // matched strings go here
if (regex_search(line, matches, pat)) // search for pat in line
for (auto p : matches) {
cout << p << " ";
}
cout << endl;
// cout << lineno << ": " << matches[0] << '\n';
}
return 0;
}
here is my code, I get messy code if I extract boost::regex_search into function #match
boost::smatch match() {
std::string s = "foobar";
std::string re_s = "f(oo)(b)ar";
boost::regex re(re_s);
boost::smatch what;
if (boost::regex_search(s, what, re)) {
return what;
}
}
int main(int argc, char **argv) {
boost::smatch what = match();
std::cout << what.size() << std::endl;
std::cout << what[0] << std::endl;
std::cout << what[1] << std::endl;
std::cout << what[2] << std::endl;
return (0);
};
the output is:
3
\000\000\000\000\000
\000\000
\000
how to make what[n] return real string
boost::smatch contains string::iterator values for tracking the matches internally. You are matching against a string object that is on the stack. When the match() function returns, that string is destructed and the iterators become invalid. Try moving the string s to the main() function and passing it into match() as a reference.
In Boost, the operator[](int index) of smatch returns a const_reference which is a typedef for sub_match<BidirectionalIterator>. sub_match<BidirectionalIterator> has a cast operator to a string, but you must cast the match to a string, otherwise it calls the operator<<(basic_ostream,sub_match) function which returns the distance from the last match. If you cast the what[0] to a std::string, it will print out. (I tested it on my machine.)
This is the code I used:
#include <iostream>
#include <string>
#include <boost/regex.hpp>
boost::smatch match() {
std::string s = "foobar";
std::string re_s = "f(oo)(b)ar";
boost::regex re(re_s);
boost::smatch what;
if (boost::regex_search(s, what, re)) {
return what;
}
}
int main(int argc, char **argv) {
boost::smatch what = match();
std::cout << what.size() << std::endl;
std::string what0 = what[0];
std::cout << what0 << std::endl;
std::cout << what[1] << std::endl;
std::cout << what[2] << std::endl;
return (0);
};
If only wanna use regex, use std::regex_search instead of boost::regex_search is good, following work well.
#include "boost/regex.hpp"
#include "iostream"
#include "regex"
std::smatch match() {
std::string s = "foobar";
std::string re_s = "f(oo)(b)ar";
std::regex re(re_s);
std::smatch what;
if (std::regex_search(s, what, re)) {
return what;
}
}
int main(int argc, char **argv) {
std::smatch what = match();
std::cout << what.size() << std::endl;
std::cout << what[0].str() << std::endl;
std::cout << what[1].str() << std::endl;
std::cout << what[2].str() << std::endl;
return (0);
};
How do I implement the following (Python pseudocode) in C++?
if argv[1].startswith('--foo='):
foo_value = int(argv[1][len('--foo='):])
(For example, if argv[1] is --foo=98, then foo_value is 98.)
Update: I'm hesitant to look into Boost, since I'm just looking at making a very small change to a simple little command-line tool (I'd rather not have to learn how to link in and use Boost for a minor change).
Use rfind overload that takes the search position pos parameter, and pass zero for it:
std::string s = "tititoto";
if (s.rfind("titi", 0) == 0) { // pos=0 limits the search to the prefix
// s starts with prefix
}
Who needs anything else? Pure STL!
Many have misread this to mean "search backwards through the whole string looking for the prefix". That would give the wrong result (e.g. string("tititito").rfind("titi") returns 2 so when compared against == 0 would return false) and it would be inefficient (looking through the whole string instead of just the start). But it does not do that because it passes the pos parameter as 0, which limits the search to only match at that position or earlier. For example:
std::string test = "0123123";
size_t match1 = test.rfind("123"); // returns 4 (rightmost match)
size_t match2 = test.rfind("123", 2); // returns 1 (skipped over later match)
size_t match3 = test.rfind("123", 0); // returns std::string::npos (i.e. not found)
You would do it like this:
std::string prefix("--foo=");
if (!arg.compare(0, prefix.size(), prefix))
foo_value = std::stoi(arg.substr(prefix.size()));
Looking for a lib such as Boost.ProgramOptions that does this for you is also a good idea.
Just for completeness, I will mention the C way to do it:
If str is your original string, substr is the substring you want to
check, then
strncmp(str, substr, strlen(substr))
will return 0 if str
starts with substr. The functions strncmp and strlen are in the C
header file <string.h>
(originally posted by Yaseen Rauf here, markup added)
For a case-insensitive comparison, use strnicmp instead of strncmp.
This is the C way to do it, for C++ strings you can use the same function like this:
strncmp(str.c_str(), substr.c_str(), substr.size())
If you're already using Boost, you can do it with boost string algorithms + boost lexical cast:
#include <boost/algorithm/string/predicate.hpp>
#include <boost/lexical_cast.hpp>
try {
if (boost::starts_with(argv[1], "--foo="))
foo_value = boost::lexical_cast<int>(argv[1]+6);
} catch (boost::bad_lexical_cast) {
// bad parameter
}
This kind of approach, like many of the other answers provided here is ok for very simple tasks, but in the long run you are usually better off using a command line parsing library. Boost has one (Boost.Program_options), which may make sense if you happen to be using Boost already.
Otherwise a search for "c++ command line parser" will yield a number of options.
Code I use myself:
std::string prefix = "-param=";
std::string argument = argv[1];
if(argument.substr(0, prefix.size()) == prefix) {
std::string argumentValue = argument.substr(prefix.size());
}
Nobody used the STL algorithm/mismatch function yet. If this returns true, prefix is a prefix of 'toCheck':
std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()
Full example prog:
#include <algorithm>
#include <string>
#include <iostream>
int main(int argc, char** argv) {
if (argc != 3) {
std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
<< "Will print true if 'prefix' is a prefix of string" << std::endl;
return -1;
}
std::string prefix(argv[1]);
std::string toCheck(argv[2]);
if (prefix.length() > toCheck.length()) {
std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
<< "'prefix' is longer than 'string'" << std::endl;
return 2;
}
if (std::mismatch(prefix.begin(), prefix.end(), toCheck.begin()).first == prefix.end()) {
std::cout << '"' << prefix << '"' << " is a prefix of " << '"' << toCheck << '"' << std::endl;
return 0;
} else {
std::cout << '"' << prefix << '"' << " is NOT a prefix of " << '"' << toCheck << '"' << std::endl;
return 1;
}
}
Edit:
As #James T. Huggett suggests, std::equal is a better fit for the question: Is A a prefix of B? and is slight shorter code:
std::equal(prefix.begin(), prefix.end(), toCheck.begin())
Full example prog:
#include <algorithm>
#include <string>
#include <iostream>
int main(int argc, char **argv) {
if (argc != 3) {
std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
<< "Will print true if 'prefix' is a prefix of string"
<< std::endl;
return -1;
}
std::string prefix(argv[1]);
std::string toCheck(argv[2]);
if (prefix.length() > toCheck.length()) {
std::cerr << "Usage: " << argv[0] << " prefix string" << std::endl
<< "'prefix' is longer than 'string'" << std::endl;
return 2;
}
if (std::equal(prefix.begin(), prefix.end(), toCheck.begin())) {
std::cout << '"' << prefix << '"' << " is a prefix of " << '"' << toCheck
<< '"' << std::endl;
return 0;
} else {
std::cout << '"' << prefix << '"' << " is NOT a prefix of " << '"'
<< toCheck << '"' << std::endl;
return 1;
}
}
With C++17 you can use std::basic_string_view & with C++20 std::basic_string::starts_with or std::basic_string_view::starts_with.
The benefit of std::string_view in comparison to std::string - regarding memory management - is that it only holds a pointer to a "string" (contiguous sequence of char-like objects) and knows its size. Example without moving/copying the source strings just to get the integer value:
#include <exception>
#include <iostream>
#include <string>
#include <string_view>
int main()
{
constexpr auto argument = "--foo=42"; // Emulating command argument.
constexpr auto prefix = "--foo=";
auto inputValue = 0;
constexpr auto argumentView = std::string_view(argument);
if (argumentView.starts_with(prefix))
{
constexpr auto prefixSize = std::string_view(prefix).size();
try
{
// The underlying data of argumentView is nul-terminated, therefore we can use data().
inputValue = std::stoi(argumentView.substr(prefixSize).data());
}
catch (std::exception & e)
{
std::cerr << e.what();
}
}
std::cout << inputValue; // 42
}
Given that both strings — argv[1] and "--foo" — are C strings, #FelixDombek's answer is hands-down the best solution.
Seeing the other answers, however, I thought it worth noting that, if your text is already available as a std::string, then a simple, zero-copy, maximally efficient solution exists that hasn't been mentioned so far:
const char * foo = "--foo";
if (text.rfind(foo, 0) == 0)
foo_value = text.substr(strlen(foo));
And if foo is already a string:
std::string foo("--foo");
if (text.rfind(foo, 0) == 0)
foo_value = text.substr(foo.length());
Starting with C++20, you can use the starts_with method.
std::string s = "abcd";
if (s.starts_with("abc")) {
...
}
text.substr(0, start.length()) == start
Using STL this could look like:
std::string prefix = "--foo=";
std::string arg = argv[1];
if (prefix.size()<=arg.size() && std::equal(prefix.begin(), prefix.end(), arg.begin())) {
std::istringstream iss(arg.substr(prefix.size()));
iss >> foo_value;
}
At the risk of being flamed for using C constructs, I do think this sscanf example is more elegant than most Boost solutions. And you don't have to worry about linkage if you're running anywhere that has a Python interpreter!
#include <stdio.h>
#include <string.h>
int main(int argc, char **argv)
{
for (int i = 1; i != argc; ++i) {
int number = 0;
int size = 0;
sscanf(argv[i], "--foo=%d%n", &number, &size);
if (size == strlen(argv[i])) {
printf("number: %d\n", number);
}
else {
printf("not-a-number\n");
}
}
return 0;
}
Here's some example output that demonstrates the solution handles leading/trailing garbage as correctly as the equivalent Python code, and more correctly than anything using atoi (which will erroneously ignore a non-numeric suffix).
$ ./scan --foo=2 --foo=2d --foo='2 ' ' --foo=2'
number: 2
not-a-number
not-a-number
not-a-number
I use std::string::compare wrapped in utility method like below:
static bool startsWith(const string& s, const string& prefix) {
return s.size() >= prefix.size() && s.compare(0, prefix.size(), prefix) == 0;
}
C++20 update :
Use std::string::starts_with
https://en.cppreference.com/w/cpp/string/basic_string/starts_with
std::string str_value = /* smthg */;
const auto starts_with_foo = str_value.starts_with(std::string_view{"foo"});
In C++20 now there is starts_with available as a member function of std::string defined as:
constexpr bool starts_with(string_view sv) const noexcept;
constexpr bool starts_with(CharT c) const noexcept;
constexpr bool starts_with(const CharT* s) const;
So your code could be something like this:
std::string s{argv[1]};
if (s.starts_with("--foo="))
In case you need C++11 compatibility and cannot use boost, here is a boost-compatible drop-in with an example of usage:
#include <iostream>
#include <string>
static bool starts_with(const std::string str, const std::string prefix)
{
return ((prefix.size() <= str.size()) && std::equal(prefix.begin(), prefix.end(), str.begin()));
}
int main(int argc, char* argv[])
{
bool usage = false;
unsigned int foos = 0; // default number of foos if no parameter was supplied
if (argc > 1)
{
const std::string fParamPrefix = "-f="; // shorthand for foo
const std::string fooParamPrefix = "--foo=";
for (unsigned int i = 1; i < argc; ++i)
{
const std::string arg = argv[i];
try
{
if ((arg == "-h") || (arg == "--help"))
{
usage = true;
} else if (starts_with(arg, fParamPrefix)) {
foos = std::stoul(arg.substr(fParamPrefix.size()));
} else if (starts_with(arg, fooParamPrefix)) {
foos = std::stoul(arg.substr(fooParamPrefix.size()));
}
} catch (std::exception& e) {
std::cerr << "Invalid parameter: " << argv[i] << std::endl << std::endl;
usage = true;
}
}
}
if (usage)
{
std::cerr << "Usage: " << argv[0] << " [OPTION]..." << std::endl;
std::cerr << "Example program for parameter parsing." << std::endl << std::endl;
std::cerr << " -f, --foo=N use N foos (optional)" << std::endl;
return 1;
}
std::cerr << "number of foos given: " << foos << std::endl;
}
Why not use gnu getopts? Here's a basic example (without safety checks):
#include <getopt.h>
#include <stdio.h>
int main(int argc, char** argv)
{
option long_options[] = {
{"foo", required_argument, 0, 0},
{0,0,0,0}
};
getopt_long(argc, argv, "f:", long_options, 0);
printf("%s\n", optarg);
}
For the following command:
$ ./a.out --foo=33
You will get
33
Ok why the complicated use of libraries and stuff? C++ String objects overload the [] operator, so you can just compare chars.. Like what I just did, because I want to list all files in a directory and ignore invisible files and the .. and . pseudofiles.
while ((ep = readdir(dp)))
{
string s(ep->d_name);
if (!(s[0] == '.')) // Omit invisible files and .. or .
files.push_back(s);
}
It's that simple..
You can also use strstr:
if (strstr(str, substr) == substr) {
// 'str' starts with 'substr'
}
but I think it's good only for short strings because it has to loop through the whole string when the string doesn't actually start with 'substr'.
With C++11 or higher you can use find() and find_first_of()
Example using find to find a single char:
#include <string>
std::string name = "Aaah";
size_t found_index = name.find('a');
if (found_index != std::string::npos) {
// Found string containing 'a'
}
Example using find to find a full string & starting from position 5:
std::string name = "Aaah";
size_t found_index = name.find('h', 3);
if (found_index != std::string::npos) {
// Found string containing 'h'
}
Example using the find_first_of() and only the first char, to search at the start only:
std::string name = ".hidden._di.r";
size_t found_index = name.find_first_of('.');
if (found_index == 0) {
// Found '.' at first position in string
}
More about find
More about find_first_of
Good luck!
std::string text = "--foo=98";
std::string start = "--foo=";
if (text.find(start) == 0)
{
int n = stoi(text.substr(start.length()));
std::cout << n << std::endl;
}
Since C++11 std::regex_search can also be used to provide even more complex expressions matching. The following example handles also floating numbers thorugh std::stof and a subsequent cast to int.
However the parseInt method shown below could throw a std::invalid_argument exception if the prefix is not matched; this can be easily adapted depending on the given application:
#include <iostream>
#include <regex>
int parseInt(const std::string &str, const std::string &prefix) {
std::smatch match;
std::regex_search(str, match, std::regex("^" + prefix + "([+-]?(?=\\.?\\d)\\d*(?:\\.\\d*)?(?:[Ee][+-]?\\d+)?)$"));
return std::stof(match[1]);
}
int main() {
std::cout << parseInt("foo=13.3", "foo=") << std::endl;
std::cout << parseInt("foo=-.9", "foo=") << std::endl;
std::cout << parseInt("foo=+13.3", "foo=") << std::endl;
std::cout << parseInt("foo=-0.133", "foo=") << std::endl;
std::cout << parseInt("foo=+00123456", "foo=") << std::endl;
std::cout << parseInt("foo=-06.12e+3", "foo=") << std::endl;
// throw std::invalid_argument
// std::cout << parseInt("foo=1", "bar=") << std::endl;
return 0;
}
The kind of magic of the regex pattern is well detailed in the following answer.
EDIT: the previous answer did not performed the conversion to integer.
if(boost::starts_with(string_to_search, string_to_look_for))
intval = boost::lexical_cast<int>(string_to_search.substr(string_to_look_for.length()));
This is completely untested. The principle is the same as the Python one. Requires Boost.StringAlgo and Boost.LexicalCast.
Check if the string starts with the other string, and then get the substring ('slice') of the first string and convert it using lexical cast.
From the following code, I expect to get this output from the corresponding input:
Input: FOO Output: Match
Input: FOOBAR Output: Match
Input: BAR Output: No Match
Input: fOOBar Output: No Match
But why it gives "No Match" for input FOOBAR?
#include <iostream>
#include <vector>
#include <fstream>
#include <sstream>
#include <boost/regex.hpp>
using namespace std;
using namespace boost;
int main ( int arg_count, char *arg_vec[] ) {
if (arg_count !=2 ) {
cerr << "expected one argument" << endl;
return EXIT_FAILURE;
}
string InputString = arg_vec[1];
string toMatch = "FOO";
const regex e(toMatch);
if (regex_match(InputString, e,match_partial)) {
cout << "Match" << endl;
} else {
cout << "No Match" << endl;
}
return 0;
}
Update:
Finally it works with the following approach:
#include <iostream>
#include <vector>
#include <fstream>
#include <sstream>
#include <boost/regex.hpp>
using namespace std;
using namespace boost;
bool testSearchBool(const boost::regex &ex, const string st) {
cout << "Searching " << st << endl;
string::const_iterator start, end;
start = st.begin();
end = st.end();
boost::match_results<std::string::const_iterator> what;
boost::match_flag_type flags = boost::match_default;
return boost::regex_search(start, end, what, ex, flags);
}
int main ( int arg_count, char *arg_vec[] ) {
if (arg_count !=2 ) {
cerr << "expected one argument" << endl;
return EXIT_FAILURE;
}
string InputString = arg_vec[1];
string toMatch = "FOO*";
static const regex e(toMatch);
if ( testSearchBool(e,InputString) ) {
cout << "MATCH" << endl;
}
else {
cout << "NOMATCH" << endl;
}
return 0;
}
Use regex_search instead of regex_match.
Your regular expression has to account for characters at the beginning and end of the sub-string "FOO".
I'm not sure but "FOO*" might do the trick
match_partial would only return true if the partial string was found at the end of the text input, not the beginning.
A partial match is one that matched
one or more characters at the end of
the text input, but did not match all
of the regular expression (although it
may have done so had more input been
available)
So FOOBAR matched with "FOO" would return false.
As the other answer suggests, using regex.search would allow you to search for sub-strings more effectively.