Conditional Loops
CSIS 1595: Fundamentals of
Programming and Problem Solving 1
Guessing Game
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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:
Loop back to condition at end of loop
Condition true
while condition:
first statement inside loop
Condition
…
false
last statement inside loop
first statement after loop
• Statements in loop indicated with indentation
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 number
Prompt 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