Arrays in C++

Passing Arrays in Functions

// Function Definition :
void func(int arr[5])  // ❌ size is ignored; treated as int*
void func(int arr[])  ✅
void func(int* arr) ✅

// Function Call :
func(arr[]);  // ❌ Syntax Error
func(arr);  // ✅

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Can’t declare array without size or initializer list

`int arr[]; // ❌ This will cause a compilation error.`

compiler doesn’t know the size of the array, You must either specify the size of the array explicitly or provide an initializer list.

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Default Value of Elements when initialize size

int arr[5]; // elments value are undefined
int arr[5]={}; // all element initialisez `0`
int arr[5]={1}; // first elmenent initialize `1`, rest `0`
int arr[5]={1, 2}; // arr[0] = 1, arr[1] = 2, and the rest are 0.

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Initializing Size of array should be constant

• int arr[a]; ❌ : if int a=5 -> variable
• int arr[b]; ✅ : if const int b=5 -> constant

Note: In C++, the code int arr[n]; is valid in some compilers (likegcc) due to Variable Length Array (VLA) support, but it is not part of the C++ standard, which requires array sizes to be known at compile time. A better and standard-compliant approach is to use std::vector for dynamic arrays, as it provides safer memory management and flexibility when the array size is determined at runtime.

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Assignment of array should include []

• int arr = { 1, 2, 3}; invalid in C++ ❌ : without [] compiler will think arr as Scaler Object
• int arr[] = { 1, 2, 3}; ✅

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Assigning all elements to -1 (without iteration)

#include <algorithm>
int n = 5; // Example size
int arr[n]; // in some compiler it is valid to use variable size but not safe
std::fill(arr, arr + n, -1); 🆕

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Method to find Size of array

• int n = arr.size(); not valid ❌ size() is part of C++ Standard Library such as std::vector, std::array
• int n = sizeof(arr) / sizeof(arr[0]); ✅

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Array should contain initialization object should be inside {} list initializer:

• int arr[] = [3, 1, 2] ❌
• int arr[] = {3, 1, 2} ✅

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String can be Stored in Array :

• char str1[] = {'H', 'e', 'l', 'l', 'o', '\0'}; ‘\0’ is the null terminator
• char str2[] = "Hello"; str2=str1 -> are string
• sizeof(str1)/sizeof(str1[0]) = sizeof(str2)/sizeof(str2[0]) = 6

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Sort an array

#include <algorithm>
int arr[] = {5, 2, 9, 1, 5, 6};
int n = sizeof(arr)/sizeof(arr[0]);
sort(arr, arr + n); ⭐

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In build Swap Func.

#include <algorithm>
    std::swap(arr[1], arr[3]);

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Passed array as Pointer and Reference in Function ? ⭐⭐

C++ requires the size of the array to be known at compile time when using array references.

Pass as reference If know the size at compile time

void modifyArray(int (&arr)[5]){...}

In C++, if you don’t know the size of the array at compile time, you cannot pass the array as a reference with a fixed size directly. but you can use Pointer. Pass as pointer if size not known at compile time (for similar functionality)

void modifyArray(int* arr, int size){...}
// We can't find size of pointer so need to pass run-time/compile-time size as additional parameter

#Revision Done Upto it (10 Nov 2024)✅