Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Loops in Python (part 1) Loops Loops Loops in Python (part 1) Loops while while Iteration Loops allow us to carry out one or more statements repeatedly. Loops in Python (part 1) L555 Syntax: Dept. of Linguistics, Indiana University Fall 2010 while < t e s t > : do . . . do . . . 1/9 While Loops in Python (part 1) 2/9 While Loops Example Loops while while mycount = 1 while mycount <= 100: p r i n t mycount mycount += 1 I the test is executed first, and if it is positive, the body of the loop is executed I if the test is false, the next statement after the loop is executed I I Loops in Python (part 1) Beware of infinite loops! It is easy to create infinite loops with while. Make sure that your while condition will return false at some point. the body of the loop is executed until the test is false if the test is false form the beginning, the body of the loop is not executed at all 3/9 Common Errors Loops in Python (part 1) 4/9 Common Errors Loops Loops in Python (part 1) Loops while while Wrong incrementing Forgetting to advance your control variable If you want to decrement, make sure that you do not automatically increment Make sure that the variable involved in the test changes in the body of the loop. Example Example mycount = 100 while mycount > 0 : p r i n t mycount mycount += 1 mycount = 1 while mycount <= 100: p r i n t mycount 5/9 6/9 Common Errors Loops in Python (part 1) Use #1: iteration Loops Loops in Python (part 1) Loops while while Off-by-1 errors Getting the beginning and the end of a loop right can be tricky: Should it start with 0 or 1? And remember that the control variable is incremented before the loop is finished. As we’ve just seen, while loops can be used to iterate over a sequence. Example p r i n t ’ m u l t i p l e s o f 33 ’ mycount = 0 while mycount <= 100: mycount += 33 p r i n t mycount p r i n t ’ T h i s i s t h e end : ’ + s t r ( mycount ) 7/9 Use #2: until Loops in Python (part 1) Loops while Another, subtly different use is to perform the same actions until a certain condition is reached. Example user input = ” ” while l e n ( u s e r i n p u t ) < 3 : u s e r i n p u t = r a w i n p u t ( ’ Please e n t e r l o n g s t r i n g : ’) p r i n t ” Thank you f o r e n t e r i n g a l o n g enough s t r i n g ! ” 9/9 I Commonly done by iterating over integers: integers easily count how many times you do something. I You can change the way you iterate: i += 2, i -= 1, ... 8/9