The string Type

Chapter 7

Introduction

• To use the data type, string, a program must include the header file string.

• A string is a sequence of 0 or more characters

  • The first character is in position 0
  • The second character is in position 1, etc.

• Binary operator + performs the string concatenation operation

• The statement:

  string str1 = "Hello There";

stores the string "Hello There" in str1.

• The statement:

  string str2 = str1;

  copies the value of str1 into str2.

• If str1 == "Sunny", the statement:

  str2 = str1 + " Day";

  stores the string "Sunny Day" into str2.

• Suppose str1 == "Hello" and str2 == "There". The statement:

  str3 = str1 + " " + str2;

  stores "Hello There" into str3. This statement is equivalent to the statement:

  str3 = str1 + ' ' + str2;

• Also, the statement:

  str3 += ", Mickey";

  updates the value of str3 by appending the string ", Mickey" to its old value. Therefore, the new value of str3 is "Hello There, Mickey".

The Array-Subscript Operator

Array-subscript operator [] allows access to an individual character in a string.

string str1 = "know";

str1[0] == 'k'; // true
str1[3] == 'w'; // true

str1[0] = 's';
cout << str1 << endl; // snow

The number inside of the array subscript operator [] is called the index of the character.

• A string's size (the number of characters it can hold) is larger than the maximum value of an int or unsigned int. Therefore, a new data type, string::size_type, should be used.

  • string::size_type An unsigned integer (data) type. This is like an int but guaranteed to be big enough to hold a string of any size. (Very useful for looping over the length of a string!) Unsigned means it cannot hold negative numbers.

  • string::npos The maximum value of the (data) type string::size_type. This number is 4,294,967,295 (i.e., ) on many machines.

  string::size_type index = 0;
  string str1 = "know";
  str1[index] == 'k'; // true

• Example of iterating over each character in a string:

  // Display a string with a space between each character.
  for (string::size_type index = 0; index < str1.length(); ++index)
  {
      cout << str1[index] << ' ';
  }

Range-Based Loops

An easier way to iterate!

The string is a data type that contains a range of values (unlike the other types discussed, which each hold a single value). We can use a new type of loop, the range-based for loop, to iterate over each char in a string.

The following example code displays, H e l l o , w o r l d!.

const string MESSAGE = "Hello, world!\n";

for (char letter : MESSAGE) {
    cout << letter << ' ';
}

For each iteration of the loop in the example above, letter gets a copy of the next character in MESSAGE.

You may also decare the loop variable as a reference if you want to edit the string.

The following example changes all of the values to be the next letter in the alphabet.

string greeting = "Hiya";

for (char& letter : greeting) {
    ++letter;
}

cout << greeting << endl; // Displays, "Ijzb"

TIP

Use a range-based for loop to iterate over all strings if the following conditions are met:

1. You don't need the index (we don't need to know where the character is in the string).
2. You are iterating over the whole string from the first to last character.

Additional string Operations

• Swap Function

  string str1 = "Warm";
  string str2 = "Cold";
  str1.swap(str2);

  Now, str1 is "Cold" and str2 is "Warm".

• The substr() member function the returns portion of a string from a starting index. See the following table for details.

  string sentence = "Houston, we have a problem.";
  cout << sentence.substr(0, 7) << endl; // Houston
  cout << sentence.substr(9, 2) << endl; // we

• The table below has many other useful string member functions.

• The find() member function returns the position (index) of the parameter value within the string. If the value doesn't exist within the string, the constant string::npos is returned.

  const string PHIL_4_13 = "I can do all things through him who strengthens me."; // ESV
  auto loc = PHIL_4_13.find("all"); // 9
  cout << PHIL_4_13.substr(loc, 2) << endl; // do

Some useful string functions where str1 and str2 are string variables.

Expression	Effect
str1.at(index)	Returns the element at the position specified by index.
str1[index]	Returns the element at the position specified by index.
str1.append(num, ch)	Appends num copies of ch to str1, in which ch is a char variable or a char constant.
str1.append(str2)	Appends str2 to str1.
str1.clear()	Deletes all the characters in str1.
str1.compare(str2)	returns 1 if str1 > str2; returns 0 if str1 == str2; returns -1 if str1 < str2.
str1.empty()	Returns true if str1 is empty; otherwise, it returns false.
str1.erase()	Deletes all the characters in str1.
str1.erase(pos, num)	Deletes num characters from str1 starting at position pos.
str1.find(str2)	Returns the index of the first occurrence of str2 in str1. If str2 is not found, the special value string::npos is returned.
str1.find(str2, pos)	Returns the index of the first occurrence at or after pos where str2 is found in str1.
str1.find_first_of(str2, pos)	Returns the index of the first occurrence of any character of str1 in str2. The search starts at pos.
str1.find_first_not_of(str2, pos)	Returns the index of the first occurrence of any character of str2 not in str1. The search starts at pos.
str1.insert(pos, num, ch);	Inserts num occurrences of the character ch at index pos into str1; pos and num are of type string::size_type; ch is a character.
str1.insert(pos, str2);	Inserts all the characters of str2 at index pos into str1.
str1.length()	Returns a value of type string::size_type giving the number of characters str1. Equivalent to str1.size()
str1.replace(pos, num, str2);	Starting at index pos, replaces the next num characters of str1 with all the characters of str2. If num > the length of str1, all the characters until the end of str1 are replaced.
str1.substr(pos, len)	Returns a substring of str1 starting at pos. The length of the substring is at most len characters. If len is too large, it means "to the end" of the string in str1.
str1.size()	Returns a value of type string::size_type giving the number of characters str1. Equivalent to str1.length() .
str1.swap(str2);	Swaps the contents of str1 and str2.