Day 3 – Lesson 10 Iteration: Loops

1. Day 3 – Lesson 10Iteration: Loops Python Mini-Course University of Oklahoma Department of Psychology Python Mini-Course: Day 3 - Lesson 10

2. Lesson objectives • Use arithmetic and binary operators • Use the increment and decrement operators • Create loops using the for and while statements • Control loop execution using the break and continue statements Python Mini-Course: Day 3 - Lesson 10

3. Arithmetic operators add subtract multiply divide x+y x–y x*y x/y exponent floor divide modulus x**y x//y x%y negation absolute value -x abs(x) Python Mini-Course: Day 3 - Lesson 10

4. Binary (bitwise) operators AND OR XOR a&b a|b a^b Left shift Right shift Complement a<<x a>>x ~a Python Mini-Course: Day 3 - Lesson 10

5. Incrementing • Updating the value of a variable by adding some value to the current value • Example: x = 4 x = x + 3 Python Mini-Course: Day 3 - Lesson 10

6. Decrementing • Updating the value of a variable by subtracting some value from the current value • Example: x = 4 x = x - 3 Python Mini-Course: Day 3 - Lesson 10

7. Updating operators • Python uses the augmented assignment operators: += Increment -= Decrement *= Update by multiplication /= Update by division Python Mini-Course: Day 3 - Lesson 10

8. Examples x = 5 print x x += 4 print x x -= 3 print x x *= 15 print x x /= 5 print x Python Mini-Course: Day 3 - Lesson 10

9. Implicit type conversion • Just like the arithmetic operators, the update operators perform automatic type conversion x = 3 print x, type(x) x += 1.5 print x, type(x) Python Mini-Course: Day 3 - Lesson 10

10. The while statement • Syntax while conditional: do_something • A general loop that executes code as long as the conditional statement is true • Exits loop when conditional is false Python Mini-Course: Day 3 - Lesson 10

11. Example 1: blastoff2.py def countdown(n): while n > 0: print n n = n-1 print 'Blastoff!' countdown(10) Python Mini-Course: Day 3 - Lesson 10

12. Example 2: sequence.py def sequence(n): while n != 1: print n, if n%2 == 0: # n is even n = n/2 else: # n is odd n = n*3+1 sequence(15) Python Mini-Course: Day 3 - Lesson 10

13. Important notes • The while statement uses negative logic to express the stop condition: • "keep going until that happens" • Also, the loop can only terminate at the beginning of each iteration • Often we want positive logic: • "stop when this happens" Python Mini-Course: Day 3 - Lesson 10

14. The break statement • Used to break out of a loop (for or while loop) early • Loop stops immediately and execution picks up at the next line outside the loop Python Mini-Course: Day 3 - Lesson 10

15. Example: break.py while True: line = raw_input('> ') if line == 'done': break print line print 'Done!' Python Mini-Course: Day 3 - Lesson 10

16. The continue statement • Used to restart a loop early • Execution immediately goes back to the loop header Python Mini-Course: Day 3 - Lesson 10

17. Example: print_odd.py def print_odd(start=1, stop=1): for x in range(start, stop): if x % 2 == 0: continue print x print_odd(1,7) Python Mini-Course: Day 3 - Lesson 10

18. Example: print_odd2.py def print_odd(start=1, stop=1): if start % 2 == 0: start += 1 for x in range(start, stop, 2): print x print_odd(4,10) Python Mini-Course: Day 3 - Lesson 10