Conditional Loops

CSIS 1595: Fundamentals of Programming and Problem Solving 1

Guessing Game

• Generate random int between 1 and 10• Prompt user for guess• Keep prompting as long as guess incorrect• Tell user guess correct

While Loop

• Execute list of statements while some condition is true– Keep executing those statements until condition is false

• Syntax:

while condition:first statement inside loop…last statement inside loop

first statement after loop

• Statements in loop indicated with indentation

Condition true

Loop back to condition at end of loop

Condition false

While Loop Design

• What is condition to be tested each time loop starts?• What must be done before the while loop?– Usually something to get initial value to be tested

in condition• What must be done inside the while loop?– What code should be repeated as long as

condition true?– Must be something that can eventually make the

condition false!

Guessing Game Design

• What is condition to be tested each time loop starts?– User guess not correct– guess != random number generated

• What must be done before the while loop?– Need a value for guess and random number– Generate random number

• random.randint(1, 10) generates random int from 1 to 10

– Prompt user for their first guess• What must be done inside the while loop?– Tell user guess incorrect– Prompt user for next guess

• Without this, will never exit loop!

Guessing Game Code

Guessing Game Trace

Computer generates 9 as numberPrompt user for first number

5 != 9 true so enter loop

1 != 9 true so enter loop again

8 != 9 true so enter loop again

2 != 9 true so enter loop again

6 != 9 true so enter loop again

9 != 9 false so exit loop

Infinite Loops

• What if loop never exits?• Usual cause: Nothing in loop body can change condition to false

• Can exit program manually with Ctrl-C

Branches and Loops

• Can nest complex code inside a loop

• Example: Hints in guessing game– If guess less than number, print “too low”– If guess greater than number, print “too high”

• Must be placed inside loop, since done for each wrong guess• Must be executed before prompting for next guess

Improved Guessing Game Code