Question

How do I iterate through a list of numbers, and how many different ways are there to do it?

What I thought would work:

#include <cstdlib>
#include <iostream>
#include <list>


using namespace std;


int main()

{
    int numbers[] = {2, 4, 6, 8};
    int i = 0;
    for(i=0; i< numbers.size();i++)
            cout << "the current number is " << numbers[i];


    system("pause");

    return 0;

}

I get an error on the for loop line:

request for member 'size' in 'numbers', which is of non-class type 'int[4]'

Answer 1

Unlike a lot of modern languages plain C++ arrays don’t have a .size() function. You have a number of options to iterate through a list depending on the storage type.

Some common options for storage include:

// used for fixed size storage. Requires #include <array>
std::array<type, size> collection;

// used for dynamic sized storage. Requires #include <vector>
std::vector<type> collection;

// Dynamic storage. In general: slower iteration, faster insert
// Requires #include <list>     
std::list<type> collection;  

// Old style C arrays
int myarray[size]; 

Your options for iteration will depend on the type you’re using. If you’re using a plain old C array you can either store the size somewhere else or calculate the size of the array based on the size of it’s types. Calculating the size of an array has a number of drawbacks outlined in this answer by DevSolar

// Store the value as a constant
int oldschool[10];
for(int i = 0; i < 10; ++i) {
    oldschool[i]; // Get
    oldschool[i] = 5; // Set
} 

// Calculate the size of the array
int size = sizeof(oldschool)/sizeof(int);
for(int i = 0; i < size; ++i) {
    oldschool[i]; // Get
    oldschool[i] = 5; // Set
}

If you’re using any type that provides a .begin() and .end() function you can use those to get an iterator which is considered good style in C++ compared to index based iteration:

// Could also be an array, list, or anything with begin()/end()
std::vector<int> newschool;     

// Regular iterator, non-C++11
for(std::vector<int>::iterator num = newschool.begin(); num != newschool.end(); ++num) {
    int current = *num; // * gets the number out of the iterator
    *num = 5; // Sets the number.
}

// Better syntax, use auto! automatically gets the right iterator type (C++11)
for(auto num = newschool.begin(); num != newschool.end(); ++num) {
    int current = *num; // As above
    *num = 5;
}

// std::for_each also available
std::for_each(newschool.begin(), newschool.end(), function_taking_int);

// std::for_each with lambdas (C++11)
std::for_each(newschool.begin(), newschool.end(), [](int i) {
    // Just use i, can't modify though.
});

Vectors are also special because they are designed to be drop-in replacements for arrays. You can iterate over a vector exactly how you would over an array with a .size() function. However this is considered bad practice in C++ and you should prefer to use iterators where possible:

std::vector<int> badpractice;
for(int i = 0; i < badpractice.size(); ++i) {
    badpractice[i]; // Get
    badpractice[i] = 5; // Set
}

C++11 (the new standard) also brings the new and fancy range based for that should work on any type that provides a .begin() and .end(). However: Compiler support can vary for this feature. You can also use begin(type) and end(type) as an alternative.

std::array<int, 10> fancy;
for(int i : fancy) {
    // Just use i, can't modify though.
}

// begin/end requires #include <iterator> also included in most container headers.
for(auto num = std::begin(fancy); num != std::end(fancy); ++num) {
    int current = *num; // Get
    *num = 131; // Set
}

std::begin also has another interesting property: it works on raw arrays. This means you can use the same iteration semantics between arrays and non-arrays (you should still prefer standard types over raw arrays):

int raw[10];
for(auto num = std::begin(raw); num != std::end(raw); ++num) {
    int current = *num; // Get
    *num = 131; // Set
}

You also need to be careful if you want to delete items from a collection while in a loop because calling container.erase() makes all existing iterators invalid:

std::vector<int> numbers;
for(auto num = numbers.begin(); num != numbers.end(); /* Intentionally empty */) {
    ...

    if(someDeleteCondition) {
        num = numbers.erase(num);
    } else {
        // No deletition, no problem
        ++num;
    }
}

This list is far from comprehensive but as you can see there’s a lot of ways of iterating over a collection. In general prefer iterators unless you have a good reason to do otherwise.