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Perform the test and exit on false

The first thing that happens following entry is that a condition is tested for true or false.

If the test returns false, control simply exits the structure without taking any action at all.

Perform some actions and repeat the test on true

If the test returns true:

  • One or more actions are performed in sequence.
  • The condition is tested again and the process is repeated.

During each iteration , if the test returns false, control exits the structure. If the test returns true, the entire process is repeated.

Each action element may be another structure

Each of the action elements may be implemented by another sequence, selection, or loop structure.

Eventually all of the actions will be completed and the condition will be tested again.

Need to avoid infinite loops

Generally speaking, unless something is done in one of the actions to cause the test to eventually return false, control will never exit the loop.

In this case, the program will be caught in what is commonly called an infinite loop.

Other possible structures

In some programming languages, there are structures other than sequence, selection, and loop that structured-programming experts are willing toaccept for convenience including:

  • The switch-case structure.
  • The do-until structure.
  • The for loop
  • The for-each loop

While sometimes more convenient than the three main structures, these structures are not required for the solution of programming logic problems.


In this module, I will present and explain the simplest example of a selection structure that I was able to write in Scratch without using variablesand without using relational or logical operators. (I will explain operators, including relational and logical operators in future modules.)

Programming interface for the program named IfSimple01

The program places a basketball and two beachballs on the Stage as shown in Image 4 .

Image 4. scratch 2.0 programming interface for the program named ifsimple01.

Missing image.
Image 4. Scratch 2.0 programming interface for the program named IfSimple01.

Click the green flag or the basketball

When the user clicks the green flag in the upper right corner of the stage, the three balls are placed in a horizontal line with the basketball in the center.

Scratch code, which can be paraphrased as shown in Image 5 , is executed each time the user clicks the basketball with the mouse.

Image 5. a paraphrased version of the scratch code.

when Basketball is clicked{ move basketball forward by 90 stepsif(Basketball is touching RightBeachball){ turn Basketball by 180 degrees}//end ifif(Basketball is touching LeftBeachball){ turn Basketball by 180degrees }//end if}
Image 5. A paraphrased version of the Scratch code.

In other words, if you repetitively click the basketball with the mouse, it will move back and forth from left to right bouncing off of the two beachballs.The basketball will keep bouncing back and forth between the two beachballs for as long as you continue clicking on the basketball.

Discussion and sample code

Let's walk through the steps required to develop this program. I will deal first with the code that defines the behavior of the program when the userclicks the green flag in the upper right corner of the stage in Image 4 .

Questions & Answers

what is math number
Tric Reply
x-2y+3z=-3 2x-y+z=7 -x+3y-z=6
Sidiki Reply
Need help solving this problem (2/7)^-2
Simone Reply
what is the coefficient of -4×
Mehri Reply
the operation * is x * y =x + y/ 1+(x × y) show if the operation is commutative if x × y is not equal to -1
Alfred Reply
An investment account was opened with an initial deposit of $9,600 and earns 7.4% interest, compounded continuously. How much will the account be worth after 15 years?
Kala Reply
lim x to infinity e^1-e^-1/log(1+x)
given eccentricity and a point find the equiation
Moses Reply
12, 17, 22.... 25th term
Alexandra Reply
12, 17, 22.... 25th term
College algebra is really hard?
Shirleen Reply
Absolutely, for me. My problems with math started in First grade...involving a nun Sister Anastasia, bad vision, talking & getting expelled from Catholic school. When it comes to math I just can't focus and all I can hear is our family silverware banging and clanging on the pink Formica table.
I'm 13 and I understand it great
I am 1 year old but I can do it! 1+1=2 proof very hard for me though.
Not really they are just easy concepts which can be understood if you have great basics. I am 14 I understood them easily.
find the 15th term of the geometric sequince whose first is 18 and last term of 387
Jerwin Reply
I know this work
The given of f(x=x-2. then what is the value of this f(3) 5f(x+1)
virgelyn Reply
hmm well what is the answer
If f(x) = x-2 then, f(3) when 5f(x+1) 5((3-2)+1) 5(1+1) 5(2) 10
how do they get the third part x = (32)5/4
kinnecy Reply
make 5/4 into a mixed number, make that a decimal, and then multiply 32 by the decimal 5/4 turns out to be
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
is it a question of log
I rally confuse this number And equations too I need exactly help
But this is not salma it's Faiza live in lousvile Ky I garbage this so I am going collage with JCTC that the of the collage thank you my friends
Commplementary angles
Idrissa Reply
im all ears I need to learn
right! what he said ⤴⤴⤴
greetings from Iran
salut. from Algeria
A soccer field is a rectangle 130 meters wide and 110 meters long. The coach asks players to run from one corner to the other corner diagonally across. What is that distance, to the nearest tenths place.
Kimberly Reply
Jeannette has $5 and $10 bills in her wallet. The number of fives is three more than six times the number of tens. Let t represent the number of tens. Write an expression for the number of fives.
August Reply
What is the expressiin for seven less than four times the number of nickels
Leonardo Reply
How do i figure this problem out.
how do you translate this in Algebraic Expressions
linda Reply
why surface tension is zero at critical temperature
I think if critical temperature denote high temperature then a liquid stats boils that time the water stats to evaporate so some moles of h2o to up and due to high temp the bonding break they have low density so it can be a reason
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
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Source:  OpenStax, Teaching beginners to code. OpenStax CNX. May 27, 2013 Download for free at http://cnx.org/content/col11498/1.20
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