[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.15,0:00:02.52,Default,,0000,0000,0000,,- Let's trace a program step by step.
Dialogue: 0,0:00:02.52,0:00:04.05,Default,,0000,0000,0000,,This is a common pattern we'll use
Dialogue: 0,0:00:04.05,0:00:06.75,Default,,0000,0000,0000,,to understand what the computer\Nis doing under the hood
Dialogue: 0,0:00:06.75,0:00:08.49,Default,,0000,0000,0000,,when we press the Run button.
Dialogue: 0,0:00:08.49,0:00:10.71,Default,,0000,0000,0000,,Tracing program execution like this
Dialogue: 0,0:00:10.71,0:00:12.60,Default,,0000,0000,0000,,helps us better read and write programs
Dialogue: 0,0:00:12.60,0:00:14.40,Default,,0000,0000,0000,,because we can start to predict
Dialogue: 0,0:00:14.40,0:00:16.59,Default,,0000,0000,0000,,what the computer's going\Nto do with each instruction
Dialogue: 0,0:00:16.59,0:00:17.64,Default,,0000,0000,0000,,without having to go through
Dialogue: 0,0:00:17.64,0:00:19.77,Default,,0000,0000,0000,,a long process of trial and error.
Dialogue: 0,0:00:19.77,0:00:21.66,Default,,0000,0000,0000,,For now, we'll write this all out together
Dialogue: 0,0:00:21.66,0:00:23.34,Default,,0000,0000,0000,,so we can make sense of\Nwhat's happening here,
Dialogue: 0,0:00:23.34,0:00:26.01,Default,,0000,0000,0000,,but eventually, we'll get\Nfamiliar enough with Python
Dialogue: 0,0:00:26.01,0:00:27.82,Default,,0000,0000,0000,,where we can trace small\Nblocks of code like this
Dialogue: 0,0:00:27.82,0:00:29.01,Default,,0000,0000,0000,,in our heads.
Dialogue: 0,0:00:30.36,0:00:31.41,Default,,0000,0000,0000,,When you press Run,
Dialogue: 0,0:00:31.41,0:00:33.69,Default,,0000,0000,0000,,the computer looks at\Nyour program line by line.
Dialogue: 0,0:00:33.69,0:00:35.58,Default,,0000,0000,0000,,It isn't smart enough to zoom out
Dialogue: 0,0:00:35.58,0:00:38.13,Default,,0000,0000,0000,,and look at the big picture\Nand try and understand
Dialogue: 0,0:00:38.13,0:00:40.59,Default,,0000,0000,0000,,what the program is\Ntrying to do as a whole.
Dialogue: 0,0:00:40.59,0:00:43.35,Default,,0000,0000,0000,,When I say line by line, I literally mean
Dialogue: 0,0:00:43.35,0:00:46.53,Default,,0000,0000,0000,,that the computer is going to\Nload each line of your program
Dialogue: 0,0:00:46.53,0:00:48.96,Default,,0000,0000,0000,,into its working memory one by one.
Dialogue: 0,0:00:48.96,0:00:50.55,Default,,0000,0000,0000,,And then with one instruction
Dialogue: 0,0:00:50.55,0:00:51.84,Default,,0000,0000,0000,,loaded into its working memory,
Dialogue: 0,0:00:51.84,0:00:54.66,Default,,0000,0000,0000,,it's going to interpret that\Ninstruction in isolation,
Dialogue: 0,0:00:54.66,0:00:57.00,Default,,0000,0000,0000,,in total isolation.
Dialogue: 0,0:00:57.00,0:00:59.43,Default,,0000,0000,0000,,Now, remember that the\Ncomputer is not a mind reader.
Dialogue: 0,0:00:59.43,0:01:01.71,Default,,0000,0000,0000,,So it doesn't understand any nuance
Dialogue: 0,0:01:01.71,0:01:03.87,Default,,0000,0000,0000,,or intention behind your instruction.
Dialogue: 0,0:01:03.87,0:01:06.66,Default,,0000,0000,0000,,It's only going to do literally\Nwhat that instruction says
Dialogue: 0,0:01:06.66,0:01:09.35,Default,,0000,0000,0000,,according to whatever rules of Python.
Dialogue: 0,0:01:09.35,0:01:12.18,Default,,0000,0000,0000,,Let's see that in action\Nby tracing this program.
Dialogue: 0,0:01:12.18,0:01:14.13,Default,,0000,0000,0000,,The computer takes the\Nfirst line of the program
Dialogue: 0,0:01:14.13,0:01:16.14,Default,,0000,0000,0000,,and loads that into working memory.
Dialogue: 0,0:01:16.14,0:01:17.58,Default,,0000,0000,0000,,Then the first thing it's gonna do
Dialogue: 0,0:01:17.58,0:01:20.49,Default,,0000,0000,0000,,is look for any expressions to evaluate.
Dialogue: 0,0:01:20.49,0:01:22.16,Default,,0000,0000,0000,,Remember that evaluating an expression
Dialogue: 0,0:01:22.16,0:01:25.35,Default,,0000,0000,0000,,just simplifies it down to a single value.
Dialogue: 0,0:01:25.35,0:01:26.88,Default,,0000,0000,0000,,Here we have the expression,
Dialogue: 0,0:01:26.88,0:01:30.27,Default,,0000,0000,0000,,the integer four plus the float, 20.55.
Dialogue: 0,0:01:30.27,0:01:34.44,Default,,0000,0000,0000,,And that evaluates to the float 24.55.
Dialogue: 0,0:01:34.44,0:01:36.54,Default,,0000,0000,0000,,Now, all our expressions\Nhave been simplified,
Dialogue: 0,0:01:36.54,0:01:38.79,Default,,0000,0000,0000,,so the computer's gonna\Npeek outside the parentheses
Dialogue: 0,0:01:38.79,0:01:41.79,Default,,0000,0000,0000,,and ask, "Well, what did you\Nwant me to do with this value?"
Dialogue: 0,0:01:41.79,0:01:44.34,Default,,0000,0000,0000,,The instruction print\Ntells it to take the value
Dialogue: 0,0:01:44.34,0:01:47.61,Default,,0000,0000,0000,,inside the parentheses and go\Ndisplay that in the console.
Dialogue: 0,0:01:47.61,0:01:51.84,Default,,0000,0000,0000,,So it goes over here and it prints 24.55.
Dialogue: 0,0:01:51.84,0:01:53.28,Default,,0000,0000,0000,,This instruction is complete,
Dialogue: 0,0:01:53.28,0:01:55.29,Default,,0000,0000,0000,,so the computer's ready\Nto move to the next step,
Dialogue: 0,0:01:55.29,0:01:58.08,Default,,0000,0000,0000,,but first it wants to\Noptimize its brain space.
Dialogue: 0,0:01:58.08,0:02:01.14,Default,,0000,0000,0000,,It doesn't really need to\Nremember this instruction anymore,
Dialogue: 0,0:02:01.14,0:02:02.55,Default,,0000,0000,0000,,it doesn't need this information.
Dialogue: 0,0:02:02.55,0:02:06.15,Default,,0000,0000,0000,,So it just clears out its\Nworking memory and forgets,
Dialogue: 0,0:02:06.15,0:02:08.94,Default,,0000,0000,0000,,and that makes room for\Nthe next instruction.
Dialogue: 0,0:02:08.94,0:02:10.77,Default,,0000,0000,0000,,Now, the computer loads the second line
Dialogue: 0,0:02:10.77,0:02:12.21,Default,,0000,0000,0000,,into its working memory.
Dialogue: 0,0:02:12.21,0:02:15.39,Default,,0000,0000,0000,,And again, it looks for any\Nexpressions to evaluate.
Dialogue: 0,0:02:15.39,0:02:17.52,Default,,0000,0000,0000,,It sees the expression three plus two,
Dialogue: 0,0:02:17.52,0:02:21.30,Default,,0000,0000,0000,,and it simplifies that\Ndown to the integer five.
Dialogue: 0,0:02:21.30,0:02:24.15,Default,,0000,0000,0000,,Notice that there's no\Nprint instruction here.
Dialogue: 0,0:02:24.15,0:02:26.40,Default,,0000,0000,0000,,We didn't actually ask the computer
Dialogue: 0,0:02:26.40,0:02:28.20,Default,,0000,0000,0000,,to do anything with that value.
Dialogue: 0,0:02:28.20,0:02:29.41,Default,,0000,0000,0000,,So the computer's thinking,
Dialogue: 0,0:02:29.41,0:02:31.38,Default,,0000,0000,0000,,"Well, hey, I just did all this work.
Dialogue: 0,0:02:31.38,0:02:32.91,Default,,0000,0000,0000,,I figured out the answer's five,
Dialogue: 0,0:02:32.91,0:02:35.13,Default,,0000,0000,0000,,but I guess you don't want me to tell you.
Dialogue: 0,0:02:35.13,0:02:39.60,Default,,0000,0000,0000,,So it shrugs whatever, and it\Nclears its working memory out,
Dialogue: 0,0:02:39.60,0:02:42.72,Default,,0000,0000,0000,,forgets that five, and just moves on.
Dialogue: 0,0:02:42.72,0:02:44.33,Default,,0000,0000,0000,,Third line, the computer loads,
Dialogue: 0,0:02:44.33,0:02:47.97,Default,,0000,0000,0000,,print the string learn\Nplus the string space.
Dialogue: 0,0:02:47.97,0:02:49.95,Default,,0000,0000,0000,,Careful, this is not the empty string.
Dialogue: 0,0:02:49.95,0:02:51.33,Default,,0000,0000,0000,,There's one little space character
Dialogue: 0,0:02:51.33,0:02:53.40,Default,,0000,0000,0000,,in between these quotation marks.
Dialogue: 0,0:02:53.40,0:02:54.84,Default,,0000,0000,0000,,Plus the string more.
Dialogue: 0,0:02:55.71,0:02:58.04,Default,,0000,0000,0000,,But there are two operators\Nin this expression.
Dialogue: 0,0:02:58.04,0:02:59.88,Default,,0000,0000,0000,,There's two plus signs.
Dialogue: 0,0:02:59.88,0:03:02.64,Default,,0000,0000,0000,,So the computer's actually going\Nto evaluate this expression
Dialogue: 0,0:03:02.64,0:03:04.92,Default,,0000,0000,0000,,in two steps, reading left to right.
Dialogue: 0,0:03:04.92,0:03:08.79,Default,,0000,0000,0000,,First, it evaluates the\Nexpression, learn plus space.
Dialogue: 0,0:03:08.79,0:03:10.41,Default,,0000,0000,0000,,Now, when we add strings,
Dialogue: 0,0:03:10.41,0:03:12.09,Default,,0000,0000,0000,,remember that we are concatenating.
Dialogue: 0,0:03:12.09,0:03:13.44,Default,,0000,0000,0000,,We are smushing together.
Dialogue: 0,0:03:13.44,0:03:16.20,Default,,0000,0000,0000,,So we get the string learn space.
Dialogue: 0,0:03:16.20,0:03:18.18,Default,,0000,0000,0000,,Then we add the string more.
Dialogue: 0,0:03:18.18,0:03:21.15,Default,,0000,0000,0000,,We concatenate and we\Nget learn space more.
Dialogue: 0,0:03:21.99,0:03:23.34,Default,,0000,0000,0000,,We're down to a single value,
Dialogue: 0,0:03:23.34,0:03:25.98,Default,,0000,0000,0000,,so the computer peaks\Noutside the parentheses,
Dialogue: 0,0:03:25.98,0:03:27.87,Default,,0000,0000,0000,,sees that we wanted to print that value,
Dialogue: 0,0:03:27.87,0:03:31.59,Default,,0000,0000,0000,,and it prints learn space\Nmore in the console.
Dialogue: 0,0:03:31.59,0:03:33.39,Default,,0000,0000,0000,,Finally, it clears its working memory
Dialogue: 0,0:03:33.39,0:03:35.67,Default,,0000,0000,0000,,and it moves to the next line.
Dialogue: 0,0:03:35.67,0:03:38.85,Default,,0000,0000,0000,,What do you think the last\Ntwo lines of this program do?
Dialogue: 0,0:03:38.85,0:03:41.05,Default,,0000,0000,0000,,Take a second and try\Nand trace it yourself.
Dialogue: 0,0:03:42.92,0:03:45.66,Default,,0000,0000,0000,,Okay, this instruction has the expression,
Dialogue: 0,0:03:45.66,0:03:49.83,Default,,0000,0000,0000,,the string 81 plus the string 19.42.
Dialogue: 0,0:03:49.83,0:03:52.47,Default,,0000,0000,0000,,Now, these may look like\Nintegers and floats,
Dialogue: 0,0:03:52.47,0:03:54.99,Default,,0000,0000,0000,,but because there are\Nquotation marks around them,
Dialogue: 0,0:03:54.99,0:03:57.33,Default,,0000,0000,0000,,the computer's going to\Ntreat them like strings.
Dialogue: 0,0:03:57.33,0:03:59.19,Default,,0000,0000,0000,,So when we evaluate this expression,
Dialogue: 0,0:03:59.19,0:04:00.84,Default,,0000,0000,0000,,we're concatenating strings
Dialogue: 0,0:04:00.84,0:04:05.70,Default,,0000,0000,0000,,and we get the string 8119.42.
Dialogue: 0,0:04:05.70,0:04:06.99,Default,,0000,0000,0000,,Nothing left to simplify here.
Dialogue: 0,0:04:06.99,0:04:09.36,Default,,0000,0000,0000,,So the computer pops\Noutside the parentheses,
Dialogue: 0,0:04:09.36,0:04:14.07,Default,,0000,0000,0000,,sees the print, and then\Nprints 8119.42 to the console.
Dialogue: 0,0:04:14.07,0:04:15.45,Default,,0000,0000,0000,,Then it clears out working memory
Dialogue: 0,0:04:15.45,0:04:17.13,Default,,0000,0000,0000,,and moves to the last line.
Dialogue: 0,0:04:17.13,0:04:19.35,Default,,0000,0000,0000,,The computer loads the\Nlast line of the program
Dialogue: 0,0:04:19.35,0:04:20.79,Default,,0000,0000,0000,,into working memory.
Dialogue: 0,0:04:20.79,0:04:23.37,Default,,0000,0000,0000,,Notice that this whole thing\Ninside the parentheses here
Dialogue: 0,0:04:23.37,0:04:25.47,Default,,0000,0000,0000,,is surrounded by quotation marks.
Dialogue: 0,0:04:25.47,0:04:28.17,Default,,0000,0000,0000,,That means this is already a single value.
Dialogue: 0,0:04:28.17,0:04:30.45,Default,,0000,0000,0000,,It's the string, the two character,
Dialogue: 0,0:04:30.45,0:04:32.67,Default,,0000,0000,0000,,space character, plus\Ncharacter, space character,
Dialogue: 0,0:04:32.67,0:04:33.81,Default,,0000,0000,0000,,two character.
Dialogue: 0,0:04:33.81,0:04:37.20,Default,,0000,0000,0000,,It's not the expression\Nthe integer two plus two.
Dialogue: 0,0:04:37.20,0:04:39.39,Default,,0000,0000,0000,,Because we already have a single value,
Dialogue: 0,0:04:39.39,0:04:40.65,Default,,0000,0000,0000,,there's nothing to simplify here.
Dialogue: 0,0:04:40.65,0:04:43.14,Default,,0000,0000,0000,,So the computer pops out the parentheses,
Dialogue: 0,0:04:43.14,0:04:44.55,Default,,0000,0000,0000,,sees the instruction print,
Dialogue: 0,0:04:44.55,0:04:47.13,Default,,0000,0000,0000,,prints two space plus\Nspace two to the console,
Dialogue: 0,0:04:47.13,0:04:49.08,Default,,0000,0000,0000,,and clears working memory.
Dialogue: 0,0:04:49.08,0:04:50.76,Default,,0000,0000,0000,,Then it jumps to the\Nnext line of the program,
Dialogue: 0,0:04:50.76,0:04:52.41,Default,,0000,0000,0000,,and oh, there is no next line.
Dialogue: 0,0:04:52.41,0:04:54.54,Default,,0000,0000,0000,,We are at the end, we did it.
Dialogue: 0,0:04:54.54,0:04:56.97,Default,,0000,0000,0000,,So the computer terminates\Nthe program execution,
Dialogue: 0,0:04:56.97,0:05:00.93,Default,,0000,0000,0000,,it exits, and we have a final\Nresult here in the console.
Dialogue: 0,0:05:00.93,0:05:02.38,Default,,0000,0000,0000,,Wanna check my work?
Dialogue: 0,0:05:02.38,0:05:04.20,Default,,0000,0000,0000,,Copy this program into a code editor
Dialogue: 0,0:05:04.20,0:05:05.55,Default,,0000,0000,0000,,and run it for yourself.
Dialogue: 0,0:05:05.55,0:05:06.87,Default,,0000,0000,0000,,Is the result the same?