What is a loop in computer science?

Loops in programming In computer science and programming, loops are a central structure for executing repeated processes in an efficient and controlled manner. Whether iterating over data structures or performing recurring calculations, loops help us to make code lean and effective. A loop repeats a sequence of instructions until a certain condition is met.

What are loops used for in programming?

Programmers use loops to cycle through values, add or subtract sums of numbers, repeat functions and much more.

Loops are used to:

  • iterate over data structures such as arrays or lists,
  • Perform calculations multiple times,
  • to check certain conditions repeatedly,
  • and much more.

A loop is therefore a construct that allows a block of code to be executed repeatedly until a certain condition is met. This not only saves lines of code, but also improves the readabilityand maintainability of a program.

Every programming language supports loops

Loops are supported by all modern programming languages, although their implementations and syntax may differ. Two of the most common types of loops are the while loop and the for loop.

Types of loops and their areas of application

You should first know what types of loops there are. Then you will also quickly learn when you can use which type of loop most effectively if you know the respective advantages and disadvantages.

Which loops can I use most effectively for programming and when?

There are several types of loops in programming languages, including:

For loops

The For loop is ideal for iterating over a fixed number of passes, e.g. when processing arrays. It is often used when the number of iterations is known. A FOR loop is used to iterate over a fixed range of values, e.g. the elements of an array or the numbers from 1 to 100.

Principle of the For loop

for (initialization; condition; update)

Simple code example of a for-loop

        for (let i = 0; i < 10; i++) {
            console.log(i); // Gibt die Zahlen 0 bis 9 aus
        }



Strengths of a FOR loop


    

  • For loops are suitable for iterating over a certain number of iterations, which makes them ideal for operations that require a fixed number of iterations.
    • 

  • the For loop is efficient because it allows you to declare and initialize a loop variable, test a condition and update the loop variable in a single statement.
    • 

  • For loops are versatile and can be used to iterate over arrays, collections and other data structures.
    • 



Weaknesses of a FOR loop


    

  • For loops can be difficult to read and understand if the loop variable is not clearly named or the loop condition is too complex.
    • 

  • For loops are not suitable for iterating over an unknown number of iterations or for tasks that require a variable or dynamic number of iterations.
    • 



While loops


While loops are used when the number of iterations is not fixed from the outset and the loop is to be executed until a condition is met. A WHILE loop is used to execute a code block repeatedly as long as a certain condition is met.


Principle of the while loop


while condition: # Code to be executed


Simple code example of a while loop


let x = 0; 
while (x < 5) 


Strengths of the while loop


    

  • While loops are useful for iterating an unknown number of iterations or for tasks that require a variable or dynamic number of iterations.
    • 

  • The while loop can be more readable and easier to understand than for loops in certain situations.
    • 

  • While loops are often used to check input or to check the status of a program or a condition.
    • 



Weaknesses of the while loop


    

  • A while loop can lead to an infinite loop if the condition is not set up correctly or if the condition is not updated at each iteration.
    • 

  • While loops can be less efficient than for loops because the condition must be evaluated at the beginning of each iteration.
    • 



DO-WHILE loops


In contrast to the while loop, the do-while loop executes the code block at least once before the condition is checked. A do-while loop behaves in a similar way to while loops, but the code block is executed at least once before the condition is checked.


Principle of the do-while loop


do  while (condition);



Easy-to-understand code example of a do-while loop


let x = 0; 
do  while (x < 5);


 


Strengths of the do-while loop


    

  • Do-while loops are useful to ensure that the loop body is executed at least once, regardless of the condition.
    • 

  • The do-while loop is often used for input validation or to check the status of a program or a condition.
    • 



Weaknesses of the do-while loop


    

  • Do-while loops can lead to an infinite loop if the condition is not set correctly or if the condition is not updated at each iteration.
    • 

  • The do-while loop can be less efficient than for loops, as the condition must be evaluated at the end of each iteration.
    • 



For-each loops


The For-Each loop is particularly suitable for iterating over arrays and collections, as it runs through the individual elements of the collection without the need for an explicit index.


Principle of the FOR-Each loop


foreach (var item in collection) 



Easy-to-understand code example of a FOR-Each-Loop


const array = [1, 2, 3, 4]; 
array.forEach(item => );



Strengths of the For-Each loop


    

  • Foreach loops are useful for iterating over arrays, collections and other data structures.
    • 

  • A Foreach loop is more readable and easier to understand than for loops when iterating over a collection.
    • 

  • Foreach loops can improve the performance of the code, as no index variable needs to be declared and managed.
    • 



Weaknesses of the for-each loop


    

  • Foreach loops are not suitable for iterating over an unknown number of iterations or for tasks that require a variable or dynamic number of iterations.
    • 

  • The Foreach loop is less flexible than for loops because you cannot access the loop index or change the collection during the loop.
    • 



Nested loops


A loop within another loop that is used to iterate over multiple dimensions or nested collections. Nested loops allow you to iterate through multidimensional arrays or repeat operations within an outer loop.


Code example and principle of the nested loop


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


Strengths of the nested loop


    

  • A nested loop is very useful for iterating over multiple dimensions or nested collections.
    • 

  • Nested loops are versatile and can be used with any type of loop construct.
    • 



Weaknesses of nested loops


    

  • Nested loops can be difficult to read and understand if the code is not indented correctly or if the loop variables are not clearly named.
    • 

  • Nested loops can be less efficient than single loops because the inner loop must be executed at each iteration of the outer loop.
    • 



Endless loop / Infinite loops


A loop that runs forever or until a certain condition is met, e.g. a user input. Endless loops run without a fixed end and are useful for continuous processes. It is important to define a condition for exiting the loop so as not to overload system resources.


Principle of an infinite loop


while (true) 


Strengths of the endless loop


    

  • Infinite loops are useful for executing background tasks or waiting for user input.
    • 

  • Endless loops can be used to create a continuous process that runs until the program is terminated.
    • 



Weaknesses of endless loops


    

  • Endless loops can lead to a program crash if they are not carefully controlled.
    • 

  • An infinite loop can consume a lot of system resources if they are not properly optimized.
    • 



Recursion


Recursion is a technique in which a function calls itself. It can replace loops in certain situations, but is more resource-intensive and carries the risk of stack overflow if there is no termination condition. These types of loops are a fundamental construct of programming and are used extensively in many programming languages.


Principle of a recursion loop


def factorial(n):
    if n == 0:
         return 1
    else:
         return n * factorial(n-1)



Easy to understand code example of a recursion


function factorial(n) 


Strengths of the recursion loop


    

  • Recursion is useful for solving problems that can be broken down into smaller sub-problems.
    • 

  • Recursion can be more elegant and efficient than iterative solutions in some cases
    • 



Weaknesses of recursion as a loop construct


    

  • Recursive functions can consume a lot of system resources if they are not properly optimized.
    • 



Is this loop list complete?


This list of loops is not exhaustive, as there can be other types of loops in different programming languages, and new types of loops can also be developed. However, the types of loops listed are the most commonand most frequently used in programming languages.


Loop syntax, variations and extensions


Different languages may have their own variations and extensions of these loops, so it is always a good idea to consult the documentation of the specific programming language you are working with. Note that this list is not exhaustive and that there may be additional loop constructs in each programming language as well as variations or differences in implementation between different versions or libraries.




Loops Examples in programming


for loop


FOR I = 1 TO N — do some stuff NEXT I


What are for loops useful for?


You use the for loop if the number of loop passes is known. For loops are so-called counter-controlled loops. We therefore always need a variable that we use to count the number of passes. These counting variables – also known as loop variables – are usually named starting with the alphabet letter i.


while loop


WHILE (boolean condition) THEN — do some stuff LOOP The while loop is usually linked to a boolean condition. We use the loop to check whether something is true or false, as it suits us best in programming. As long as the condition is true, the loop is executed. Conversely, we could also set up our While loop in such a way that we run through our loop until something is false, i.e. not true; the loop exit then takes place on true.


What are while loops useful for?


While loops – just like for loops –  are so-called head-controlled program loops. For us as programmers, this means that the checkpoint of our program loop is executed first before each run. If the loop block is to be executed first and then the condition for a new run is to be checked, we use the do while loop. These do while loops are therefore foot-controlled loops.


How do I learn to program loops?


The best way to learn programming is by programming. Game programming is a good way to get started because it’s simply a lot of fun. Python is a mature, object-oriented programming language that we can highly recommend.




Loop syntax and variations


The syntax for loops can vary depending on the programming language. Extensions such as the insertion of break and continue also allow flexible control over loop structures.


Tips for learning loops


The best way to master loops is by practicing regularly. Try writing small projects, such as simple games or tools, where loops play an essential role. Python is an excellent language for beginners as it offers clear and simple looping constructs.




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