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Aprenda os Fundamentos da Programação

2435 classificações

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University of Toronto

Aprenda os Fundamentos da Programação

2435 classificações

Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.

Na lição

For Loops and Fancy String Manipulation

This module introduces one way to repeat code (using a for loop), how to manipulate strings, and how to use a debugger to watch a program execute step by step.

IDLE's Debugger4:00

Conheça os instrutores

Jennifer Campbell
Associate Professor, Teaching Stream
Department of Computer Science
Paul Gries
Associate Professor, Teaching Stream
Department of Computer Science

The Python Visualizer has limitations, it does not allow import statements and it

stops tracing after 500 steps. Idle comes with a debugger, which is a

tool much like the visualizer, but without the pretty pictures.

Here we see functions convert to minutes and convert to seconds, which we explored

in last week's lectures. Turn on this in IDLE's debugger.

We're going to make sure that the Python shell window is on top, and then select

debug-, debugger. This opens a window called debug control.

I am now going to, to resize this so that we can see everything as we execute.

We're going to check the check box for source.

Then, we're going to come back here to our module that we want to debug and select

run, run module. This middle area displays the call stack,

or at least the functions that have been called.

Down here are the variables inside the current call.

Notice that the variable for built ins is there, as our three other variables that

we haven't discussed yet in this course. Step is like forward in the visualizer.

We'll define our two functions. Notice we have variables down here.

All for them, referring to functions. This right here, that's a memory address.

You'll notice it does letters in it. That's because it's base sixteen, which

uses in, in a lot of programming the numbers zero through nine, plus the

letters A, B, C, D, E and F, for a total of sixteen characters.

Over here we see there's a grey highlight on the current line.

And line twenty happens to be that line. So we're told here which line is being

executed. When we step we're gonna do the function

called convert to seconds. We only have parameters num hours

available so far seconds convert to seconds.

If we want to see the, the variables in the module, we can just click there and

that switches the view for the local speed, the local variables.

We're gonna back to convert to seconds and step into the call to convert to minutes.

We now have three items on the call stack, involved in that program.

Let's step one more line to define our variable minutes.

Minutes is now 120. Just to check, we're gonna go back to the

frame for convert to seconds, and notice that it only has one variable num hours.

The reason is, we haven't finished this call to convert to minutes, and so this

variable minutes has not yet been created. I'm going to switch the view back to, the

top frame. And, when I step, it's actually just going

to execute the return statement all at once including popping the frame from the

stack. We don't get to see the return value

before the return happens. So we're back in convert to seconds, we're

about to evaluate 120 times 60 and assign the result to seconds, there we go 7,200

and we're going to return 7,200 back to. This call to convert to seconds, and.

It's going to create a variable seconds, in the.

Stack frame for the main module. Switching our view back and clicking step,

causes our. Program to end because, this was the last

statement in the program. Let's switch back to there and notice that

we do have a variable seconds and created. So it's a little confusing at the end of a

program because if on a return statement, and that return statement is inside a

function that is on the last instruction that gets executed, the program will end

and this stackframe won't be removed because the debugger doesn't update at the

end of a program once the program is done.

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