Coding #108: For Loops

Good weekend everyone! I really hope you are enjoying your free time!

In case you are feeling bored, feel free to deep dive into this new episode of Learn2Code.

Last time, we had a general overview on what iterations are and how they work. Today, our side quest in the world of coding brings us to deep dive on for loops and on its different configurations.

Let’s dive in!

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Overview

This type of loop iterates through a collection of values, for example and array or a vector. It is based on the keyword for and it is used when the exact number of iterations to be made on a certain object is known a priori.

The simplest example you can make is the following. We declare an array with length 3 and we use a for loop to print every component of it.

int array[3] = { 1, 5, 7};

// For loop initialization
for(int i = 0; i < 3; i++) {

     // This is the code block
     std::cout << “Array value: “ << array[i] << std::endl;
} 

The console output would read:

Array value: 1
Array value: 5
Array value: 7

Let’s see more in details how it works!

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Simple For Loop

Let’s take a look at the initialization of a for loop.

for(int i = 0; i < 3; i++)

Other than the for keyword, we can see that the declaration is composed of three different parts:

• int i = 0 → This first statement is executed once before the execution of the code block. It initializes the variable that will be used to count the number of iterations.

• i < 3 → The second statement is the stopping condition. The loop will continue to iterate while the variable i remains lower than a certain amount (3 in this case).

• i++ → The last statement is run every time the code block is executed. It increases the value of the i variable.

The following image visually summarizes the previous point.

Inside the execution code, i can be used in different ways, like keeping track of iterations or accessing a certain position in an array.

// Accessing i-th position in the array
std::cout << “Array value: “ << array[i] << std::endl;

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Range-based For Loops

This particular type of for loop was introduced in C++11 and is used as a more readable equivalent to the traditional one. It operates over a range of values, such as all elements in a container. Here is the basic syntax:

for (declaration: range) {
    // statements
}

Here is a quick example. We declare a vector of integers, a container that holds N different values.

std::vector<int> v = {1, 10, 17, 25, 31};

// Iterating through vector
for (int i : v) {
    std::cout << i << “ “;
}

The variable i, declared as an int, is assigned one of the values inside the vector v at each iteration.

First iteration -> i = v[0] = 1 // Value at position 0
Second iteration -> i = v[1] = 10 // Value at position 1
.
.
N (fifth) iteration -> i = v[4] = 31 // Value at position N-1

The loop stops right after the last value is assigned.

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