Python Logical Types and Sorting Algorithms

Intro to Python Tufts University Instructor: Joel Grodstein joel.grodstein@tufts.edu if then, logical types Python Joel Grodstein 1

Demo a bubble sort https://www.youtube.com/watch?v=xli_FI7CuzA https://www.youtube.com/watch?v=k4RRi_ntQc8 Python Joel Grodstein 2

Card sort with if then The code below is for a card sort. It already has an "if" And hopefully it even makes sense Let s just fix the format a bit n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): if card #i and #(i+1) are backwards then swap them Python Joel Grodstein 3

Follow the bouncing ball n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): swap them 10 6 6 8 10 [0] [1] [2] 10>6 is True 3 U 0 U 0 U n_cards pass i card Python Joel Grodstein 4

Follow the bouncing ball n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): swap them 8 10 8 10 6 [0] [1] [2] 10>8 is True 0 0 1 3 The for i loop is now done But the for pass loop is not done n_cards pass i card Python Joel Grodstein 5

Follow the bouncing ball Starting this loop all over again from the top n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): swap them 8 10 6 [0] [1] [2] 6>8 is False skip over the if code 1 0 1 3 The for i loop is not done n_cards pass i card Python Joel Grodstein 6

Follow the bouncing ball n_cards = 3 for pass in range(n_cards-1): for i in range(n_cards-1): if (card[i] > card[i+1]): swap them 8 10 6 [0] [1] [2] 8>10 is False skip over the if code 1 0 1 3 The for i= loop is done The pass= loop is done n_cards pass i card Finally finished!!! Python Joel Grodstein 7

if then details The format is simple: if (condition): statement The usual Python note: if there are more than 1 statement s, they must all have the same indentation As usual, there are several variations: We can make the condition fairly complex There is an else Let s look at both of these Python Joel Grodstein 8

if then else Here s some code with an obvious use: if (it s Sunday evening): do your homework if (it s not Sunday evening): go to sleep early We can rewrite it: if (it s Sunday evening): do your homework else: go to sleep early Else replaces the inverted condition Gain in clarity Python Joel Grodstein 9

More complex conditions We don t have homework due every week if (it s Sunday evening and there s homework due) do your homework else: go to sleep early The condition can have and and or, or really be any arbitrary expression But how arbitrary can an expression get, anyway? In fact, very arbitrary Without the else , this would have been if ((it s not Sunday evening) or (there s no homework due)) . That s a bit of a mess! Python Joel Grodstein 10

Logical expressions False and True are the simplest expressions if (False) do not execute the statements if (True) do execute the statements <, >, <=, >= work as expected 3<4 evaluates to True. 3>=4 evaluates to False And then see the case above So if (3<4) becomes if (True) , which executes the subsequent statements You can replace 3 with any variable that evaluates to 3 a=3 if (a<4): statements happen, since 3<4 These statements do Python Joel Grodstein 11

Testing equality & inequality Testing for equality is a bit weird if (3=4) error if (3==4) evaluates to false Single = is used only for assignment to a variable Testing for equality is ==; test for inequality is != Example: a=3 b=4 if (a==b): statements These statements do not happen, since 3 is not equal to 4 Python Joel Grodstein 12

AND, OR expression and expression is an expression Ditto for or And you can do this recursively Example: a=3; b=4; c=5; d=6; e=7 if ((a<4) and (b==c) and !((d==5) or (e==6))): So in the end, this big expression just evaluates to 'False' if ((3<4) and (4==5) and !((6==5)or(7==6))): if (True and False and !(False or False)): if (True and False and !False): if (True and False and True): if (False): Python Joel Grodstein 13

elif Sometimes you have a sequence of conditions: if it s midnight-10am, keep the house at 55 if 10-11am, 70 11am-10pm, 55 10pm-midnight, 71 if (time <= 10): temp=55 if (time>10 && time<=11): temp=70 if (time>11 && time<=22): temp=55 if (time>22 && time<=24): temp=70 The program on the right works, but it s inefficient, since it tests the last 3 if s even when the first is true it s easy to mis-type the conditions Python Joel Grodstein 14

Nested if How about this? Pros and cons? More obvious that it works More efficient But the indentation is ugly if (time <= 10): temp=55 else: if (time<=11): temp=70 else: if (time<=22): temp=55 These must line up else: temp=70 These too And these Python Joel Grodstein 15

Elif The solution is elif Many (not all) languages have it Code is more obvious and elegant We won t need this until the Manduca homework if (time <= 10): temp=55 elif (time<=11): temp=70 elif (time<=22): temp=55 else: temp=70 Python Joel Grodstein 16

Group activity What do each of these evaluate to? a=3 b=2 (a==3) and (b==2) (a==4) or (b==2) (a*b==6) and (a+b==6) (a*b==6) and ((a+b==6) or (1==1)) True True False True Python Joel Grodstein 17

Group activity What do each of these print? Assume a=3 and b=2 if (a==3) and (b==2): print ('yes') if (a==4): if (b==2): yes print ('42') else: print ('4 not 2') nothing if (a==4) or (b==2): print ('yes') else: print ('no') yes for i in range(2:6): if (i>4): print (i, '*', i, '=', i*i) 5*5=25 if (a==3): if (b==2): print ('32') else: print ('4 not 2') 32 Python Joel Grodstein 18

What does this do? def muscle_report (musc1_on, changed): green = pyb.LED(2) red = pyb.LED(1) if musc1_on: red.off() green.on() else: green.off() red.on() Python Joel Grodstein 19

What does this do? current_color = None def muscle_report (musc1_on, changed): green = pyb.LED(2) red = pyb.LED(1) if musc1_on and changed: if (current_color == red): current_color = green green.on() red.off() else current_color = red red.on() green.off() Python Joel Grodstein 20

Follow-up activities Try the examples from this lecture yourself Vary them, or even mis-type some to see what happens More exercises. Write a program that Prints all of the even numbers in an array Print all the elements of array1 that are not in array2 Makes a new array with pos , neg or zero based on a first array of numbers Determines the indices of the largest and smallest numbers from an array Checks whether a specified value is contained in an array Tests if a number N is within 100 of 1000 or 2000 Prints all divisors of a given number Creates a 5-element array of random integers between 1 and 10, and then sums up only those elements that are 3 or higher Python Joel Grodstein 21

Another group exercise Take an array p. Multiply every element by a random number r, and then bound them all to be 0 and 1 You may want to Google numpy clip This will be useful for HW3 Python Joel Grodstein 22

Swapping Let s say we have two variables x and y, and we want to swap their values. Consider this code: x=y y=x y is 7, so now x is too x is 7, so copy that to y. But y already was 7, so that does nothing 7 5 x 7 y Once we set x to 7, there was no longer any variable anywhere with a value of 5. So there is no way to assign y=5 23 Python Joel Grodstein

Swapping take 2 As always, there s a trick to doing it. Try this code instead tmp=y y=x x=tmp y is 7, so now tmp is too x is 5, so copy that to y tmp=7, so copy 7 to x 7 5 x 7 5 y It works U 7 tmp Python Joel Grodstein 24