ch. 4 loops

while loop

compound interest algorithm

planning:

• add 1 to years

• calculate interest

• add interest to balance

report the final year value

the loop execute instructions repeatedly while a condition is True

type i: count-controlled loops

• called definite loops

• a while loop that is controlled by a counter variable

type ii: event-controlled loops

• indefinite loops

• a while loop is controlled by a loop variable

processing sentinel values

• sentinel values are often used when taking input and

  • you don’t know how many input items are there

  • use a ‘special’ character or value to signal the end of input

  • for numeric input of positive numbers it is common to use the value -1

sentinel value denotes the end of a data set, but it’s not part of the data

priming read

some programmers don’t like the trick of initializing the input variable with a value other than a sentinel

an alternative is to change the variable with a read before the loop

modification read

the input operation at the bottom of the loop is used to obtain the next input

generating random numbers

random() returns a floating number that is >= 0 and <1

randint() function

a random integer in a given range

loop algorithm: sum values

initialize total to 0

use while loop with sentinel

loop algorithm: average values

keep a running total

initialize count to 0

increment total and count per input

check for count 0 before divide!

loop algorithm: counting matches

initialize negatives to 0

use a while loop

add to negatives per match

loop algorithm: prompt until a match is found

initialize boolean flag to false

test sentinel in while loop

• get input and compare to valid range

• if input is in range change flag to true

• loop will stop executing when flag changes

loop algorithm: maximum/minimum

get first input value

• the largest/smallest

get another input

loop as long as the input is valid (non-sentinel):

• compare new input to largest/smallest and update largest/smallest if needed

• get another input value

loop algorithm: comparing adjacent values

get first input and store it in value

get another input

loop as long as the input is valid (non-sentinel):

• copy value to previous variable

• store the new input in value variable

• compare value to previous and output message if same

• get another input