Encodings and Strings

Like everything else in the computer, characters must be represented by a sequence of 0s and 1s. A catalogue of these representations is usually called an encoding.

The basic character set used by C++ is ASCII, the American Standard Code for Information Interchange. Internationally, it is sometimes known as US-ASCII. Originally, 7 bits were used to represent data, resulting in a total of 128 (2⁷) available characters. The first 32 are non-printing characters that were mainly needed by the mechanical devices for which the encoding was developed. Only a few non-printing characters are still used, among them line feed, carriage return, and even “bell.”

#include <iostream>

int main() {
/* Ring the bell
*/
    
    std::cout<<char(7);
    
    return 0;
}

ASCII now uses 8 bits (extended ASCII) which doubles the number of available characters, but the first 128 character codes are the same as 7-bit ASCII. It was not as well standardized as ASCII, though standards exist for Latin alphabets (ISO 8859-1, ISO Latin 1) and Cyrillic (ISO 8859-5). Nearly all programming languages continue to restrict the characters allowed for statements to the original ASCII, even when larger character sets are the default and may be used for comments and output.

Strings

A string is a sequence of characters of variable length. Using strings requires a header:

#define <string.h>

The string is a class , which is a little beyond our scope right now. But we can still use basic functions without understanding the class.

string str, str1, str2;

str.size();  // length of string
str1+str2; // concatenate two strings
str.substr(2,5); // substring (counts from 0)
str[n];  // nth character (counts from 0), no checking
str.at(n);  // nth character (counts from 0), checks bounds for error

Many useful operations are available to work with strings. Most of them have more options than presented here; see documentation for more details.

Example:

#include <iostream>
#include <string>

int main() {

    std::string title="This is a string";
    std::string subtitle="Another string";

    std::cout<<title.size()<<"\n";

    std::string newtitle=title+":"+subtitle;

    std::cout<<newtitle<<"\n";
    std::cout<<newtitle.substr(1,3)<<"\n";
    std::cout<<newtitle[4]<<"\n";
    std::cout<<newtitle.at(5)<<"\n";

    subtitle.clear();
    subtitle="The Sequel";
    subtitle.append("!");

    std::cout<<subtitle<<"\n";

    int start=newtitle.find("Another");
    int end=newtitle.size();
    int n=end-start;
    newtitle.replace(start,end,subtitle);
    std::cout<<newtitle<<"\n";
    int pos=newtitle.find(":");
    newtitle.insert(pos+1," ");
    std::cout<<newtitle<<"\n";
}

 
    

Exercise In the above code, change “This” to “That” in newtitle.

#include <iostream>
#include <string>

int main() {

    std::string title="This is a string";
    std::string subtitle="Another string";

    std::cout<<title.size()<<"\n";

    std::string newtitle=title+":"+subtitle;

    std::cout<<newtitle<<"\n";
    std::cout<<newtitle.substr(1,3)<<"\n";
    std::cout<<newtitle[4]<<"\n";
    std::cout<<newtitle.at(5)<<"\n";

    subtitle.clear();
    subtitle="The Sequel";
    subtitle.append("!");

    std::cout<<subtitle<<"\n";

    int start=newtitle.find("Another");
    int end=newtitle.size();
    int n=end-start;
    newtitle.replace(start,end,subtitle);
    std::cout<<newtitle<<"\n";
    int pos=newtitle.find(":");
    newtitle.insert(pos+1," ");
    std::cout<<newtitle<<"\n";

    newtitle="That"+title.substr(4)+":"+subtitle;
    std::cout<<newtitle<<"\n";

}

 
    

Strings, Wide Strings, and Unicode

Even extended ASCII accommodates far too few characters to accommodate more than a handful of alphabets, much less other writing systems. Unicode was created to address this. The first 128 codes are still the 7-bit ASCII codes even with the millions available through Unicode. The characters must still be encoded and there are multiple standards for Unicode. One of the most widely used is UTF-8, which has become a Web standard. It is variable-width; characters are encoded in one to four bytes, with the first 128 characters one byte in size and corresponding exactly to ASCII. The string class can support UTF-8 directly. However, some software, especially on Windows, may use the UTF-16 encoding. Windows uses the “wide string” or wstring_t class for much of its character support; this makes cross-platform coding awkward. We will not go deeply into Unicode support but here is an example:

#include <iostream>
#include <string>

int main() {

    std::string ChineseText = "\xe4\xb8\xad\xe6\x96\x87";
    std::cout<<ChineseText<<"\n";

}

Interested students can find discussions online of Unicode support. Those who wish to write for Windows in particular may need to examine Microsoft-specific documentation.