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C++ Data Types

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Accessing and Manipulating String Elements

Indexing a string

We can access the elements of a string (which are essentially char characters) using square bracket [] indexing, similar to arrays. This also allows us to replace a character at a specific position.

h

indexing

copy
string str = "Cpdefinity"; // Cpdefinity
str[1] = 'o'; // Codefinity

              
12

            
string str = "Cpdefinity"; // Cpdefinity
str[1] = 'o'; // Codefinity

Note

Indexing in a string starts from zero, just like in arrays.

The length of the string

With that indexing, you can try to access a wrong index (that is out of range), and the compiler will tell you nothing about that.

cpp

main

copy
#include <iostream>

int main()
{
std::string word = "Codefinity";
std::cout << word[12]; // Word's length is 10
}

              
1234567

            
#include <iostream>

int main()
{
    std::string word = "Codefinity";
    std::cout << word[12]; // Word's length is 10
}

As you can see, the output is unexpected, and C++ does not provide a warning about the issue. To prevent this, you can use the .length() method to determine the length of a string and ensure the index is within the valid range.

cpp

main

copy
#include <iostream>

int main()
{
std::string word = "Codefinity";
int index = 12;
if (index >= word.length())
std::cout << "The output is unexpected!" << std::endl;
else
std::cout << word[index]; // Word's length is 10
}

              
123456789101112

            
#include <iostream>

int main()
{
    std::string word = "Codefinity";
    int index = 12;
    
    if (index >= word.length()) 
	    std::cout << "The output is unexpected!" << std::endl;
    else
        std::cout << word[index]; // Word's length is 10
}

Indexing using method

Another way of indexing is using the .at() method. To get the character with index n, we can use the following syntax:

h

at

copy
str.at(n);

              
1

            
str.at(n);

It works the same as str[n] but will raise an error if you specify an index that is out of range.

cpp

main

copy
#include <iostream>

int main()
{
std::string word = "Codefinity";
std::cout << word.at(12); // word's length is 10
// Try outputting the character with the index 5
}

              
12345678

            
#include <iostream>

int main()
{
    std::string word = "Codefinity";
    std::cout << word.at(12); // word's length is 10
    // Try outputting the character with the index 5
}

Practice

As was said before, we can replace the characters inside a string using indexing (both with [] and .at()). Here is an example:

cpp

main

copy
#include <iostream>

int main()
{
std::string word = "Cpdefinitu";
word[1] = 'o';
word.at(word.length() - 1) = 'y'; // length - 1 accesses the last element
std::cout << word;
}

              
123456789

            
#include <iostream>

int main()
{
    std::string word = "Cpdefinitu";
    word[1] = 'o';
    word.at(word.length() - 1) = 'y'; // length - 1 accesses the last element
    std::cout << word;
}
Compito

Swipe to start coding

Your task is to check if we can add characters to the end of a string that way.

  1. Figure the index of the last character in a string.
  2. Assign 'y' to the index index of last character + 1 of str.

Soluzione

cpp

solution

#include <iostream>

int main()
{
std::string str = "Codefinit";
str[str.length()] = 'y';
std::cout << str << std::endl;
}

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Sezione 3. Capitolo 3
#include <iostream>

int main()
{
std::string str = "Codefinit";
___ = ___;
std::cout << str << std::endl;
}

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