How can I use substr() method of std::wstring? How can I get a substring of a string in a C++ app? Here are the answers with C++ examples.
Generally, as an introduction to C++, in addition to int, float, double there is another data type called string that we use for alphanumeric variables. In C++ there are several typedefs for common character types are provided: String types are defined in header
Strings are the string class for byte characters represented with string and alphanumeric characters are stored and displayed in string forms. In other words, string stores the alphanumeric text with 1-byte chars, called ASCII chars. Strings are the instantiation of the basic_string class template that uses char as the character type. Simply we can define a string as below,
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std::string str = "This is a String"; |
string has methods to assign, copy, align, replace or to operate with other strings. These methods can be used in all string methods with their appropriate syntax. We can retrieve a substring of a string by using substr() method. Let’s see this method.
How do I get a substring of a string with the substr() method in C++?
The substr() method of std::string, can be used to retrieve a substring of a string. As its parameters, you just need to indicate where to start to retrieve chars (start_position) and the count of chars (char_count). Here is syntax of a substr() method,
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basic_string substr( size_type start_position, size_type char_count) const; |
Here is a example how we can use substr() method with in this given syntax, see example below,
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#include #include int main() { std::string s1, s2; s1 = "my first string"; s2 = s1.substr(3,5); // copy substring of s1 from 3 to 3+5 to the string s2 std::cout << "String: '" << s1 << "'\n"; std::cout << "Sub String: '" << s2 << "'\n"; getchar(); return 0; } |
and the output will be,
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String: 'my first string' Sub String: 'first' |
Why do I get “Out of Range” when retrieving a substring of a string?
As given in this method we can directly copy, assign, replace or insert any substring of a string. When we use substr() method, we must be sure that the first parameter is lower than its size (s1.size()) and the second parameter is lower than size()-first_parameter, which means both should be inside the string. If second parameter exceeds the string size, it returns substring of string till the end of string without throwing an exception.
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#include #include int main() { std::string s1, s2; s1 = "my first string"; int start_pos = 3; try { std::string s2 = s1.substr( start_pos, 32); std::cout << "String: '" << s1 << "'\n"; std::cout << "Sub String: '" << s2 << "'\n"; } catch(const std::out_of_range& e) { std::cout << "out of range in substring, please check string size\n"; } getchar(); return 0; } |
for the start_pos=3 the output will be,
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String: 'my first string' Sub String: 'first string' |
in this example above, If the first parameter start_pos is equal to string size it will return empty string, for example for the start_pos=15 the output will be,
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String: 'my first string' Sub String: '' |
and in this example above, if the first parameter exceeds the size of string there will be out_of_range throw, for example for the start_pos=16 the output will be,
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out of range in substring, please check string size |
If you are checking range carefully you don’t need to use try-catch() method. You can also check these parameters by using its size(). As you see substr() is very easy to use to retrieve any substring of a string, and this substr() method can be used with wstrings as shown in the examples above.
