Strings

Topics to Cover

Reading and writing an individual letter in a string.
length() function of a string
Difference between strings and arrays

Reading and Writing a Letter In a String

Indexing with []

Recall 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

`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;

As in the above, 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

string array

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 Mandatory Practice Problem

Read firstname lastname
Prints out
```
lastname, firstname
```
In addition, all the letters should be capitalized.

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
{
  // empty string s
  string s;

  for(int i = 0; i < w.length(); i++)   // NOTE: .length() was used
  {
    char c = w[i];                      // NOTE: type of w[i] is char

    // if c is a lowercase letter, capitalize it
    if( 'a' <= c && c <= 'z')           
      c = 'A' + (c - 'a');

    // add c to s
    s += c;
  }
  return s;
}

Another version

We can write the same function without introducing a new string variable s.


string capitalize(string w)           
{
  for(int i = 0; i < w.length(); i++)   
  {
    // if w[i] is a lowercae letter
    if( 'a' <= w[i] && w[i] <= 'z')           
      w[i] += 'A' - 'a';    // capitalize w[i]
  }
  return w;
}

Array of strings

Important!

type of p: string*
type of p[2]: string

In general, if p is an array of X objects, p[2] is type X.
Here, X = string
type of p[2][4]: char

In general, if Y is a string, then Y[4] is a char.
Here, Y = p[2]

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