CH 4 Repetition Structures

What is the purpose of repetition structures?

makes computer repeat included code as necessary

Disadvantages of writing code that performs the same task multiple times.

• makes program large

• time consuming 

• may need to be corrected in many places

What are the two broad categories of loops

Condition controlled and Count controlled

What type of loop is a while loop?

condition controlled loop

What type of loop is a for loop?

count controlled loop

What is a condition controlled loop/ while loop?

uses a true/false condition to control the number of times the loop iterates 

ex:

count = 0

while count > 0:

what is a count controlled loop/ for loop?

repeats a specific number of times

ex: for days in range (0,5) 

In a while loop, what are the two parts?

1. test if the condition is true or false 

2. statements are repeated as may times as the condition is true 

This is the general format for what type of loop?

Condition controlled/while loop

In a flowchart for a condition controlled loop the line goes

back to the previous part

In a while loop what do we have to do for the loop to stop executing?

make a condition inside the loop false 

define iteration

one execution of the body of the loop

the while loop is known as a

pretest loop

What is a pretest loop?

a test condition before performing an iteration meaning it will never execute if condition is false to start with

• it requires performing some steps prior to the loop

What is an infinite loop?

loop that does not have a way of stopping

• repeated until program is interrupted 

• normally occurs when programmer forgets a stopping code in loop

Infinite loops in while loops

loop must contain within themselves a way to terminate

• something inside a while loop must eventually make the condition false

Using the while loop as a count controlled loop by pairing it with a 

counter variable 

What is a counter variable?

assigned a unique value during each iteration of a loop

• used to count the number of times the loop iterates 

What are the three actions a count controlled while loop must perform?

1. initialization

2. comparison 

3. update  

Initialization

the counter variable must be initialized to a suitable starting value before the loop begins

comparison

the loop must compare the counter variable to a suitable ending value, to determine whether the loop should iterate or not 

update

during each iteration, the loop must update the counter variable to a new value

If there is only one statement in the body of a while loop you can

write the entire. loop on one line

what is the general format for a single line while loop

while condition: statement

example of single line while loop

n = 0

while n<10: n+=1 

count controlled loop definition

iterates a specific number of times

What is a for loop designed for?

work with a sequence of data items

• iterates once for each item in the seqeunce 

What is the general format for a for loop?

for variable in [val1,val2,val3]:

tab—> statements

What is a target variable?

the variable which is the target of the assignment at the beginning of each iteration

What function simplifies the process of writing a for loop?

range

Range will

return a iterable object

what is iterable?

contains a sequence of values that can be iterated over

range characters when there is one argument

ending limit

range characters when there is two arguments

start value and ending limit

range characters when there is three arguments

starting value, ending value, multiple

What is the purpose of a target variable?

reference each item in the sequence as the loop iterates

• used in calculations or tasks in the body of the loop

When the programmer does not know how many times to loop we can 

ask the user and receive range inputs from the user and place them in a variable 

When descending order in a range function, make sure the starting number is

larger than end limit, and the step value is negative

ex: range(10,0,-1) 

When calculating a running total make sure to include

1. a loop that reads each number in series 

2. include a accumulator variable 

known as program that keeps a running total means

accumulate total and reads in series, where the end of the loop, accumulator will reference the total 

augmented assignment operators 

special set of operators designed for this type of job, shorthand operators 

Define sentinel

special value that marks the end of a sequence of items

• program will know that the end of the sequence of items was reaches and loop terminates 

• must be distinctive from regular value in sequence 

Can computer tell the difference between good and bad data?

no, so design a program that bad input is never accepted

GIGO garbage in garbage out 

input validation definition

inspecting input before it is processed by the program, accomplished by a while loop

Walrus operator can be used to

create an assignment expression that combines the priming read with the input validation loop

nested loop

loop that is contained inside another loop

• inner loops goes through all iterations for each iteration of outer loop 

• inner loop completes iterations faster 

• total number of iterations in nested loop is number of outer loops x inner loops 

break statement causes a loop to 

terminate 

continue statement causes the current iteration of a loop to

end early

• all statements in the body of the loop that appear after it are ignores, and loop begins its next iteration 

else clause in a loop is useful only

when the loop contains a break statement

block statement appears after the

else clause executes only when the loop terminates normally, without encountering a break statement

the else statement will not execute its block of statements if the

loop terminates because of a break statement