Apprendre Accessing Elements and Indexing Rules

When you work with arrays in C++, you use indices to access individual elements. C++ uses zero-based indexing. This means the first element of an array is at index 0, the second at index 1, and so on. If an array has n elements, the valid indices go from 0 up to n - 1. Trying to access an index outside this range can cause problems in your program.

main.cpp


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#include <iostream>

int main()
{
    int numbers[5] = {10, 20, 30, 40, 50};
    std::cout << "Element at index 0: " << numbers[0] << std::endl;
    std::cout << "Element at index 2: " << numbers[2] << std::endl;
    std::cout << "Element at index 4: " << numbers[4] << std::endl;

    // WARNING: The following line accesses out of bounds!
    std::cout << "Element at index 5 (out of bounds): " << numbers[5] << std::endl;
}

In the code above, you see how to access elements at different positions in the array. Accessing numbers[0] gets the first element, numbers[2] gets the third, and numbers[4] gets the fifth. However, when the code tries to access numbers[5], it goes beyond the valid range of indices for a 5-element array. While the compiler might not give an error, this is unsafe: accessing an invalid index can lead to unpredictable results or even crash your program.

main.cpp


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// File: main.cpp
#include <iostream>

int main()
{
    int data[4] = {7, 14, 21, 28};
    int lastIndex = 4 - 1; // Calculate the last valid index
    std::cout << "Last element: " << data[lastIndex] << std::endl;
}

A common mistake when working with arrays is the off-by-one error. This happens when you accidentally use an index that is one too high or too low, such as using the array's size as an index instead of size - 1 for the last element. To avoid these errors, always remember that array indices start at 0 and end at size - 1. Double-check your index calculations, especially when looping through arrays or accessing the last element.

Note

Accessing an array out of bounds leads to undefined behavior in C++. This means the program may run incorrectly, crash, or appear to work but with hidden bugs. Always stay within the valid index range.

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Section 2. Chapitre 1

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main.cpp


              123456789101112
            
#include <iostream>

int main()
{
    int numbers[5] = {10, 20, 30, 40, 50};
    std::cout << "Element at index 0: " << numbers[0] << std::endl;
    std::cout << "Element at index 2: " << numbers[2] << std::endl;
    std::cout << "Element at index 4: " << numbers[4] << std::endl;

    // WARNING: The following line accesses out of bounds!
    std::cout << "Element at index 5 (out of bounds): " << numbers[5] << std::endl;
}

main.cpp


              123456789
            
// File: main.cpp
#include <iostream>

int main()
{
    int data[4] = {7, 14, 21, 28};
    int lastIndex = 4 - 1; // Calculate the last valid index
    std::cout << "Last element: " << data[lastIndex] << std::endl;
}

Note

Tout était clair ?

Merci pour vos commentaires !

Section 2. Chapitre 1