<< Chapter < Page Chapter >> Page >
An introduction to the for control structure with examples in the C++ programming language.

Introduction to test before loops

There are two commonly used test before loops in the iteration (or repetition) category of control structures. They are: while and for. This module covers the: for.

Understanding iteration in general – for

In most programming languages the for loop is used exclusively for counting; that is to repeat a loop action as it either counts up or counts down. There is a starting value and a stopping value. The question that controls the loop is a test expression that compares the starting value to the stopping value. This expression is a Boolean expression and is usually using the relational operators of either less than (for counting up) or greater than (for counting down). The term loop comes from the circular looping motion that occurs when using flowcharting. The basic form of the for loop (counting up) is as follows:

for initialization of the starting valuestarting value is less than the stopping value some statements or actionsome statements or action some statements or actionincrement the starting value

It might be best to understand the for loop by understanding a while loop acting like a counting loop. Let's consider;

initialization of the starting value while the starting value is less than the stopping valuesome statements or action some statements or actionsome statements or action increment the starting value

Within the for control structure there are four attributes to a properly working loop. They are:

  • Initializing the flag – done once
  • Test expression
  • Action or actions
  • Update of the flag

The initialization of the flag is not technically part of the while control structure, but it is usually part of the for control structure. The English phrasing is, "For x is 1; x less than 3; do the following actions; increment x; loop back to the test expression". This is doing the action on the true. When the test expression is false, you stop the loop and go on with the next item in the program. Notice, because this is a test before loop the action might not happen . It is called a test before loop because the test comes before the action. It is also sometimes called a pre-test loop, meaning the test is pre (or Latin for before) the action and update.

The for structure within c++

Syntax

The syntax of the for loop control structure within the C++ programming language is:

for (initializations; expression; updates) {statement; statement;statement; }

The initializations, test expression and updates are within the parentheses (each separated by a semi-colon), but this is not a function call. The parentheses are part of the control structure. Additionally, there is not a semicolon after the parenthesis following the expression.

An example

C++ source code: for

for (counter = 0; counter<5; counter++) {cout<<"\nI love ice cream!"; }

Got questions? Get instant answers now!

The four attributes of a test before loop (remember the for loop is one example of a test before loop) are present.

  • The initialization of the flag to a value of 0.
  • The test is the less than relational comparison of the value in the flag variable to the constant value of 5.
  • The action part consists of the 1 line of output.
  • The update of the flag is done with the increment operator.

Using indentation with the alignment of the loop actions is normal industry practice within the C++ community.

Infinite loops

At this point it's worth mentioning that good programming always provides for a method to insure that the loop question will eventually be false so that the loop will stop executing and the program continues with the next line of code. However, if this does not happen then the program is in an infinite loop. Infinite loops are a bad thing. Consider the following code:

C++ source code: infinite loop

for (counter = 0; counter<5;) {cout<<"\nI love ice cream!"; }

Got questions? Get instant answers now!

The programmer assigned a value to the flag during the initialization step which is correct. However, he forgot to update the flag (the update step is missing). Every time the test expression is asked it will always be true. Thus, an infinite loop because the programmer did not provide a way to exit the loop (he forgot to update the flag).

Multiple items in the initialization and update

The following shows the use of the sequence operator to separate the multiple initializations and multiple updates. This is not available in most languages, thus is more unique to the C++ programming language.

C++ source code: for with multiple initializations and updates

for (x = 0, y = 10; x<10; x++, y--) {cout<<x * y<<endl; }

Got questions? Get instant answers now!

Counting loop conversion – a while into a for

Below is a color coded the conversion of a while loop that displays a message exactly three times (which is a counting loop) into a for loop using C++ programming language syntax. The four loop attributes are color highlighted as follows:

Miscellaneous information about the for structure

Many languages (Pascal, FORTRAN, and other) have a for loop structure that is used exclusively for counting. The for loop in the C++ programming language is much more versatile and can be used (and generally is used) in place of the while loop structure. In reality a counting loop is just a particular use of a while loop.

The name for comes from mathematics’ method of writing an iteration (or repetition). In math we would say: “For the variable i starts at a given value and repeats an action increasing the value of i until i is executed for the stopping value”. Usually written in math as:

for i = 1 to 5 do some action

Note: here the = means equals not assignment. Another way to say it is that i varies from 1 to 5.

Definitions

for
A test before iteration control structure typically used for counting.

Questions & Answers

what is mutation
Janga Reply
what is a cell
Sifune Reply
how is urine form
Sifune
what is antagonism?
mahase Reply
classification of plants, gymnosperm features.
Linsy Reply
what is the features of gymnosperm
Linsy
how many types of solid did we have
Samuel Reply
what is an ionic bond
Samuel
What is Atoms
Daprince Reply
what is fallopian tube
Merolyn
what is bladder
Merolyn
what's bulbourethral gland
Eduek Reply
urine is formed in the nephron of the renal medulla in the kidney. It starts from filtration, then selective reabsorption and finally secretion
onuoha Reply
State the evolution relation and relevance between endoplasmic reticulum and cytoskeleton as it relates to cell.
Jeremiah
what is heart
Konadu Reply
how is urine formed in human
Konadu
how is urine formed in human
Rahma
what is the diference between a cavity and a canal
Pelagie Reply
what is the causative agent of malaria
Diamond
malaria is caused by an insect called mosquito.
Naomi
Malaria is cause by female anopheles mosquito
Isaac
Malaria is caused by plasmodium Female anopheles mosquitoe is d carrier
Olalekan
a canal is more needed in a root but a cavity is a bad effect
Commander
what are pathogens
Don Reply
In biology, a pathogen (Greek: πάθος pathos "suffering", "passion" and -γενής -genēs "producer of") in the oldest and broadest sense, is anything that can produce disease. A pathogen may also be referred to as an infectious agent, or simply a germ. The term pathogen came into use in the 1880s.[1][2
Zainab
A virus
Commander
Definition of respiration
Muhsin Reply
respiration is the process in which we breath in oxygen and breath out carbon dioxide
Achor
how are lungs work
Commander
where does digestion begins
Achiri Reply
in the mouth
EZEKIEL
what are the functions of follicle stimulating harmones?
Rashima Reply
stimulates the follicle to release the mature ovum into the oviduct
Davonte
what are the functions of Endocrine and pituitary gland
Chinaza
endocrine secrete hormone and regulate body process
Achor
while pituitary gland is an example of endocrine system and it's found in the Brain
Achor
what's biology?
Egbodo Reply
Biology is the study of living organisms, divided into many specialized field that cover their morphology, physiology,anatomy, behaviour,origin and distribution.
Lisah
biology is the study of life.
Alfreda
Biology is the study of how living organisms live and survive in a specific environment
Sifune
Got questions? Join the online conversation and get instant answers!
Jobilize.com Reply

Get Jobilize Job Search Mobile App in your pocket Now!

Get it on Google Play Download on the App Store Now




Source:  OpenStax, Programming fundamentals - a modular structured approach using c++. OpenStax CNX. Jan 10, 2013 Download for free at http://cnx.org/content/col10621/1.22
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Programming fundamentals - a modular structured approach using c++' conversation and receive update notifications?

Ask