Strings and Characters

ASCII Characters

The char data type represents a single character as its value. It is represented with single quotes '.

These values are has internal code as integers ranging from -128-127 in the computer (for signed char type), and takes up 8 bits in memory. This means char variables can do calculation just as normal integers within its range.

The mapping relationship between value and character represented is according to the ASCII Table. Some important values:

Char	Value
0-9	091-100
A-Z	065-090
a-z	097-122

To cast a char value into int, use int(...) function.

#include <iostream>
using namespace std;

int main() {
   for (char ch = 'a'; ch <= 'z'; ch++) {
      // typecast!
      cout << ch << " (" << int(ch) << ")" << endl;
   }

   return 0;
}

Avoiding Magic Numbers

If you want to represent some value in character calculation, do not use the so called magic numbers. That is, to represent the index of lowercase letters, avoid this:

int(ch) - 96

Instead, use this:

int(ch) - ('a' - 1)

Strings in C++

String values are represented with double quotes ". Data type string

• Get automatically initialized with "" (empty string)

1. Strings are arrays of characters
   • That is, a string is a block of characters that are contiguous in memory that are indexed from 0 through (n - 1), where n is the overall number of characters in the string.
2. Strings are objects
   • Strings have methods (member functions of a class) that can be called on, using the dot . notation.
   • C++ strings are mutable

string a = "String1";
string b = 
  "String that "
  "span multiple lines.";
string c = 
  "String that \
escapes the new line.";

A list of string class methods

Member Function	Description
s.append(str)	add text to the end of a string
s.compare(str)	return -1, 0, or 1 depending on relative ordering
s.erase(index, length)	delete text from a string starting at given index
s.find(str) s.rfind(str)	return first or last index where the start of str appears in this string (returns string::npos if not found)
s.insert(index, str)	add text into string at a given index
s.length() s.size()	return number of characters in the string
s.replace(index, len, str)	replace len chars at given index with new text
s.substr(start, length) s.subsr(start)	return a new string with the next length chars beginning at start (inclusive); if length is omitted, grabs till end of string

String Mutation

Strings in C++ are objects, therefore, they are mutable values.

Mutating Individual Characters

#include <iostream>
using namespace std;

int main() {
   string s = "hello";
   cout << s << endl;

   // This changes the character at index 0. I read "s[0]" out loud as "s-sub-zero."
   s[0] = 'Y';
   cout << s << endl;

   // We can concatenate a single character to a string using the + operator. We know
   // the w below is just a character (not a string) because it's in single quotes.
   s += 'w';
   cout << s << endl;

   // We can also concatenate an entire string.
   s += " squashes";
   cout << s << endl;

   return 0;
}

Iteration Through Strings

1. for-loop
2. for-each loop

#include <iostream>
using namespace std;

int main() {
   string s = "hello";

   // We can loop through a string using a for-loop.
   for (int i = 0; i < s.length(); i++) {
      cout << i << ": " << s[i] << endl;
   }

   // We can also use a for-each loop (aka "range-based loop"), but this approach
   // doesn't give us a handy variable to tell us what index we're at in the string.
   cout << endl;
   for (char ch : s) {
      cout << ch << endl;
   }

   return 0;
}

Character Processing

Characters are as integers, they can be evaluated in an expression, compared, etc.

Example: Check if a character is lowercase

#include <iostream>
using namespace std;

bool isCharLower(char a) {
  return a >= 'a' && a <= 'z';
}

int main() {
   string password = "p4ssw0rd";
   string alphaPortion;

   for (int i = 0; i < password.length(); i++) {
      // checks whether password[i] is a lowercase char
      if (isCharLower(password[i])) {
         alphaPortion += password[i];
      }
   }

   cout << alphaPortion << endl;

   return 0;
}

Character Processing Library cctype

Member Function	Description
isalnum(ch)	checks if a character is alphanumeric
isalpha(ch)	checks if a character is alphabetic
islower(ch)	checks if a character is a lowercase alphabetic character
isupper(ch)	checks if a character is an uppercase alphabetic character
isdigit(ch)	checks if a character is a digit
isxdigit(ch)	checks if a character is a hexadecimal character
iscntrl(ch)	checks if a character is a control character
isgraph(ch)	checks if a character is a graphical (i.e., visible) character
isspace(ch)	checks if a character is a space character (typically tab or space)
isblank(ch)	checks if a character is a blank character
isprint(ch)	checks if a character is a printing character according to locale
ispunct(ch)	checks if a character is punctuation (visible non-alnum/non-space)
toupper(ch)	converts a character to uppercase (pass-by-value!)
tolower(ch)	converts a character to lowercase (pass-by-value!)

Strings and String Literals

C++ has two types of strings

• String objects (with string type)
• String literals

String literals are C-styled hard-coded strings in double quotes, they are literals, immutable direct values rather than objects.

Performing operation on string literals leads to compiling errors! While performing operation between a C++ string and a string literal triggers type-conversion, comverting the string literal to C++ string (informal to formal), and therefore get compiled without error.

Do the following:

• Implicit type conversion from C string literals to C++ strings
• Forced type conversion

string s1 = "Hello " + "world!" // Error!
string hl = "Hello"
// string + literal
string s2 = hl + "world" // Implicit type cast
// literal + literal to string
string s3 = "Hello " + (string)"world!" // Forced type cast

Extra: C-style String Processing

snprintf

Converting other types into a character array.

[!!TODO:TODO]