Intro to tracing program execution | Intro to computer science - Python | Khan Academy

- Let's trace a program step by step.

This is a common pattern we'll use

to understand what the computer
is doing under the hood

when we press the Run button.

Tracing program execution like this

helps us better read and write programs

because we can start to predict

what the computer's going
to do with each instruction

without having to go through

a long process of trial and error.

For now, we'll write this all out together

so we can make sense of
what's happening here,

but eventually, we'll get
familiar enough with Python

where we can trace small
blocks of code like this

in our heads.

When you press Run,

the computer looks at
your program line by line.

It isn't smart enough to zoom out

and look at the big picture
and try and understand

what the program is
trying to do as a whole.

When I say line by line, I literally mean

that the computer is going to
load each line of your program

into its working memory one by one.

And then with one instruction

loaded into its working memory,

it's going to interpret that
instruction in isolation,

in total isolation.

Now, remember that the
computer is not a mind reader.

So it doesn't understand any nuance

or intention behind your instruction.

It's only going to do literally
what that instruction says

according to whatever rules of Python.

Let's see that in action
by tracing this program.

The computer takes the
first line of the program

and loads that into working memory.

Then the first thing it's gonna do

is look for any expressions to evaluate.

Remember that evaluating an expression

just simplifies it down to a single value.

Here we have the expression,

the integer four plus the float, 20.55.

And that evaluates to the float 24.55.

Now, all our expressions
have been simplified,

so the computer's gonna
peek outside the parentheses

and ask, "Well, what did you
want me to do with this value?"

The instruction print
tells it to take the value

inside the parentheses and go
display that in the console.

So it goes over here and it prints 24.55.

This instruction is complete,

so the computer's ready
to move to the next step,

but first it wants to
optimize its brain space.

It doesn't really need to
remember this instruction anymore,

it doesn't need this information.

So it just clears out its
working memory and forgets,

and that makes room for
the next instruction.

Now, the computer loads the second line

into its working memory.

And again, it looks for any
expressions to evaluate.

It sees the expression three plus two,

and it simplifies that
down to the integer five.

Notice that there's no
print instruction here.

We didn't actually ask the computer

to do anything with that value.

So the computer's thinking,

"Well, hey, I just did all this work.

I figured out the answer's five,

but I guess you don't want me to tell you.

So it shrugs whatever, and it
clears its working memory out,

forgets that five, and just moves on.

Third line, the computer loads,

print the string learn
plus the string space.

Careful, this is not the empty string.

There's one little space character

in between these quotation marks.

Plus the string more.

But there are two operators
in this expression.

There's two plus signs.

So the computer's actually going
to evaluate this expression

in two steps, reading left to right.

First, it evaluates the
expression, learn plus space.

Now, when we add strings,

remember that we are concatenating.

We are smushing together.

So we get the string learn space.

Then we add the string more.

We concatenate and we
get learn space more.

We're down to a single value,

so the computer peaks
outside the parentheses,

sees that we wanted to print that value,

and it prints learn space
more in the console.

Finally, it clears its working memory

and it moves to the next line.

What do you think the last
two lines of this program do?

Take a second and try
and trace it yourself.

Okay, this instruction has the expression,

the string 81 plus the string 19.42.

Now, these may look like
integers and floats,

but because there are
quotation marks around them,

the computer's going to
treat them like strings.

So when we evaluate this expression,

we're concatenating strings

and we get the string 8119.42.

Nothing left to simplify here.

So the computer pops
outside the parentheses,

sees the print, and then
prints 8119.42 to the console.

Then it clears out working memory

and moves to the last line.

The computer loads the
last line of the program

into working memory.

Notice that this whole thing
inside the parentheses here

is surrounded by quotation marks.

That means this is already a single value.

It's the string, the two character,

space character, plus
character, space character,

two character.

It's not the expression
the integer two plus two.

Because we already have a single value,

there's nothing to simplify here.

So the computer pops out the parentheses,

sees the instruction print,

prints two space plus
space two to the console,

and clears working memory.

Then it jumps to the
next line of the program,

and oh, there is no next line.

We are at the end, we did it.

So the computer terminates
the program execution,

it exits, and we have a final
result here in the console.

Wanna check my work?

Copy this program into a code editor

and run it for yourself.

Is the result the same?