C-style string: array of chars.

As you know, C++ was designed as an extension of the C language. That means that (for the most part) any C program is also a C++ program!

In C, quite naturally, a string is simply an array of chars. There is a small catch, though:

The final char of an array must be the null character (value 0 in the ASCII table).
So, the following program works nicely. 

#include <iostream>
using namespace std;
int main()
{
  char* hi = new char[3];
  hi[0] = 'h';
  hi[1] = 'i';
  hi[2] = '\0'; // alternatively, 0 (i.e., ASCII code for '\0')

  cout << hi << endl;
  delete [] hi;

  return 0;
}

C-style strings are not so convenient. Use C++ string objects!

However, it's quite inconvenient to deal with strings using arrays of chars directly: Fortunately, we can use string objects! As you already know, using strings is much easier and more convenient!

Conversion: from char* to string and vice versa.

The conversion from char* to string is very simple: 
 
  char* hi = new char[3];
  hi[0] = 'h';
  hi[1] = 'i';
  hi[2] = '\0'; 

  string s = hi;  // simple conversion!
The other direction is also quite simple: 
 
  string s = "hi";
  const char* hi = s.c_str(); // call c_str() function!
  cout << hi << endl;
Here, const char* is a pointer to a constant char (that is the type returned by the c_str() function), meaning the char in question can't be modified. This means that variable hi doesn't allow modification of its content, e.g., by hi[0] = 'p';. However, any change in s also changes hi

C++ string objects: They work like arrays

Indexing with []

A string is a sequence of characters. As with an array, you can access each character in a string using an index. For example: 

string s = "hello";
cout << s[0] << endl;
cout << s[4] << endl;
s[0] = 'x';
cout << s << endl;
The above code will print out the following (drag your mouse for the answer): 
h
o
xello

C++ string is not an array

The type string is not a basic built-in type in C++. It is a type that is defined in the string library, as we will eventually learn to define new types in our programs.

Creating a string (vs an array)

string array
You can simply declare a string: 


string s;
If you don't initialize s with any specific string, s is ensured to be a string with length 0 (an empty string).

You can change its contents (even its length) freely. 


s = "need a break!";
You need to use new to create an array: 

int* p = new int[5];
You can change its contents, but unfortunately, you cannot change its size: 

for(int i=0; i < 5; i++)
  p[i] = i;
If you really want to change both the contents and size of an array, you need to destroy it and create a new one: 

delete [] p;
p = new int[10];
for(int i=0; i < 10; i++)
  p[i] = i;

Length of a string

stringarray
It's easy with strings: Put a .length() at the end of it and the resulting expression returns the length of the string. For example: 

string s = "hello";
cout << s.length() << endl;

The code will print 5.

Built-in arrays don't provide a .length() function. For an array, it is you who should store its size in an int variable like n. Then, you can refer to n if you want to know the size of an array later.

A Problem

Applying the stepwise refinement, we can write an initial abstract version as follows: 

int main()
{
  string first, last;

  // read
  cin >> first >> last;

  // write: we will create a function capitalize() that capitalizes a string
  cout << capitalize(last)  << ", " << capitalize(first) << endl; 
}
In the above, we assumed that we have a function string capitalize(string) that capitalizes a string. Now, it is time to write it! 

// prototype -- it should be placed before main()
string capitalize(string w);            

// definition
string capitalize(string w)             // NOTE: remember the principle of pass by value
{
  for(int i = 0; i < w.length(); i++)   // NOTE: .length() was used
  {
    char c = w[i];                      // NOTE: type of w[i] is char 
    if( 'a' <= c && c <= 'z')
      w[i] = 'A' + (c - 'a');           // NOTE: you can modify w[i]
  }
  return w;
}

Array of strings

Important!
You may run into a situation where you want to bundle up many strings together into an array. Here is a toy example: 

string* p = new string[3];  // p: string*
p[0] = "hello";
p[1] = "world"; 
p[2] = "array of strings";
cout << p[2] << endl;       // p[2]: string
cout << p[2][4] << endl;    // p[2][4]: char
delete [] p;
The code will print out the following (drag your mouse for the answer): 
array of strings
y

Practice Problems

  1. Print in reverse
  2. A predicate that tests whether two strings are reverses of each other.
  3. palindromes
  4. A predicate that takes a string s and a char c, and tests whether or not c appears in s.