Get first char from std::string

get character from string c++
get first character of string c++
c++ substring
how to read each character of a string in c++
char to string c++
c++ get first n characters string
c++ get string until character
std::string example

I need to get the first character of an std::string with a minimum amount of code.

It would be great if it would be possible to get the first char in one line of code, from an STL std::map<std::string, std::string> map_of_strings. Is the following code correct:

map_of_strings["type"][0]

EDIT Currently, I am trying to use this piece of code. Is this code correct?

if ( !map_of_strings["type"].empty() )
    ptr->set_type_nomutex( map_of_strings["type"][0] );

The prototype of the set_type function is:

void set_type_nomutex(const char type);

It's not exactly clear from your question what your problem is, but the thing likely to go wrong with map_settings["type"][0] is that the returned string may be empty, resulting in undefined behavior when you do [0]. You have to decide what you want to do if there is no first character. Here's a possibility that works in a single line.

ptr->set_type_nomutex( map_settings["type"].empty() ? '\0' : map_settings["type"][0]);

It gets the first character or a default character.

std::string - string::at, std::string::at Note: The first character in a string is denoted by a value of 0 (not 1). string::operator[]: Get character of string (public member function ). Position of the first character in the string to be considered in the search. If this is greater than the string length , the function never finds matches. Note: The first character is denoted by a value of 0 (not 1 ): A value of 0 means that the entire string is searched.

That should work if you've put a non-empty string into map_of_strings["type"]. Otherwise, you'll get an empty string back, and accessing its contents will probably cause a crash.

If you can't be sure whether the string exists, you can test:

std::string const & type = map["type"];
if (!type.empty()) {
    // do something with type[0]
}

Or, if you want to avoid adding an empty string to the map:

std::map<std::string,std::string>::const_iterator found = map.find("type");
if (found != map.end()) {
    std::string const & type = found->second;
    if (!type.empty()) {
        // do something with type[0]
    }
}

Or you could use at to do a range check and throw an exception if the string is empty:

char type = map["type"].at(0);

Or in C++11, the map also has a similar at which you can use to avoid inserting an empty string:

char type = map.at("type").at(0);

string::front - C++ Reference, std::string::front Returns a reference to the first character of the string. Unlike member string::begin, which returns an iterator to this same character, this function returns a direct reference. So we can get a pointer to the underlying array behind std::string by invoking either &str[0] or &*str.begin() method. This works in constant time as no copying is involved. Please note that any change made to the char* will now be reflected in the string object and vice versa.

The c_str() method will return a pointer to the internal data. If the string is empty, then a pointer to a NULL-termination is returned, so a simple one-liner is safe and easy:

std::string s = "Hello";
char c = *s.c_str();

string at() in C++, std::string::at can be used to extract characters by characters from a given string. index number Both forms return the character that has the index idx (the first character has index 0). Don't stop now and take your learning to the next level. Access first character Returns a reference to the first character of the string . Unlike member string::begin , which returns an iterator to this same character, this function returns a direct reference.

string s("type");
char c = s.at(0);

Substring in C++, In C++, std::substr() is a predefined function used for string handling. string.h is the pos: Position of the first character to be copied. len: Length of the sub-string . In this a string and a character is given and you have to print the sub-string´┐Ż Value with the position of a character within the string. Note: The first character in a string is denoted by a value of 0 (not 1). If it is not the position of a character, an out_of_range exception is thrown. size_t is an unsigned integral type (the same as member type string::size_type). Return value The character at the specified position in the string.

std::basic_string<CharT,Traits,Allocator>::erase , basic_string::find hash<std::string>hash<std::wstring>hash<std::u32string> hash<std::u16string>hash< 3) Removes the characters in the range [first, last) . Note: The first character is denoted by a value of 0 (not 1). len Number of characters to include in the substring (if the string is shorter, as many characters as possible are used). A value of string::npos indicates all characters until the end of the string. size_t is an unsigned integral type (the same as member type string::size_type

substr - Manual, For instance, in the string 'abcdef', the character at position 0 is 'a', the character at position 2 is 'c', and so forth. I wanted to work out the fastest way to get the first few characters from a string (With standard deviations 0.01, 0.02 and 0.04) char ch = myStr.back(); In C++03, std::string::back is not available due to an oversight, but you can get around this by dereferencing the reverse_iterator you get back from rbegin: char ch = *myStr.rbegin(); In both cases, be careful to make sure the string actually has at least one character in it!

Strings and string manipulation in C++, using namespace std; // Or using std::string; Since we can only convert characters to upper case, and not strings, we have to handle the For example, we could convert the first character of a string to upper case with the following code:. In this example, we first stored the size ( 8 ) of the original value of the string s in a variable size. s.resize( size+2, '+' ); - This statement resized the string 's' to 'size + 2' i.e. 10 and assigned the character ' + ' to the two extra character places in the resized string.

Comments
  • "Does not work" is not a problem description.
  • What do you mean "does not work correctly"? What happened? What did you expect to happen?
  • Are you sure that prototype is correct? If you're using type as the key to the map you should have gotten a compile error.
  • map_of_strings is probably something like, std::map<std::string,std::string>
  • @abrahab: You can surround code with backticks (```) to make it appear as written. Unless the code itself is a backtick, apparently.
  • Note .at(0) will throw an out_of_range exception for an empty string. Otherwise it's the same behavior as operator[]