For Loops and While Loops | Python for Beginners Lesson 6 | Code with Kylie

00:17:30
https://www.youtube.com/watch?v=zXaQ-ift74A

摘要

TLDRIn this lesson, we explore loops in programming, focusing on for loops and while loops. For loops allow iteration over sequences, such as strings and lists, executing a block of code for each item. An example demonstrates printing each character of a string and values from a list. While loops, on the other hand, execute a block of code as long as a specified condition is true, with an example of counting down from a number. The lesson emphasizes the importance of avoiding infinite loops by ensuring termination conditions are met. The video concludes with an invitation to join a Discord server for further interaction and support.

心得

  • 🔄 Loops allow repeated execution of code blocks.
  • 📜 For loops iterate over sequences like strings and lists.
  • 🔍 While loops run until a condition is false.
  • ⚠️ Infinite loops can occur without termination conditions.
  • ✅ Always ensure a loop can eventually end.
  • 📝 Practical applications of loops will be discussed next.
  • 💬 Join the Discord server for student interaction.
  • 📚 Check out linked notes for additional resources.
  • 👨‍💻 Examples help clarify loop usage in programming.

时间轴

  • 00:00:00 - 00:05:00

    The video introduces loops, specifically focusing on for loops and while loops. It explains that loops allow for the repeated execution of a block of statements, using the example of iterating through a string. The presenter demonstrates a for loop that iterates through the characters of the string 'cat', printing each character and a final 'done' statement after the loop completes. The importance of indentation in Python to define the scope of the loop is emphasized.

  • 00:05:00 - 00:10:00

    The presenter provides another example using a list of numbers (0, 1, 2) to demonstrate the versatility of for loops. The loop prints each value in the list and its double, followed by a 'done' statement. The output is confirmed by running the code, showing that the loop correctly iterates through the list and executes the intended print statements.

  • 00:10:00 - 00:17:30

    The video transitions to while loops, explaining that they continue executing as long as a specified condition is true. An example is given where a variable 'n' is decremented until it reaches zero, at which point a 'go' statement is printed. The presenter warns about the potential for infinite loops if the condition never becomes false, illustrating this with a 'while true' example. The lesson concludes with a reminder to ensure termination conditions are present in while loops to avoid infinite execution.

思维导图

视频问答

  • What are the two types of loops discussed?

    The two types of loops discussed are for loops and while loops.

  • What does a for loop do?

    A for loop allows you to iterate over a sequence, executing a block of code for each item.

  • What is a while loop?

    A while loop continues to execute a block of code as long as a specified condition is true.

  • What is an infinite loop?

    An infinite loop occurs when a loop continues to execute without a termination condition, causing the program to run indefinitely.

  • How can you avoid infinite loops?

    To avoid infinite loops, ensure that there is a termination condition that will eventually be met.

  • Can you iterate through lists with for loops?

    Yes, you can iterate through lists, strings, tuples, and other sequences using for loops.

  • What happens when the condition in a while loop becomes false?

    When the condition in a while loop becomes false, the loop terminates and the program continues with the next statement after the loop.

  • What is the output of the example with n equal to 3 in a while loop?

    The output is '3 2 1 go'.

  • What should you check before using a while loop?

    You should check that there is a termination condition to prevent infinite loops.

  • What practical applications of loops will be discussed next?

    The next class will discuss examples of each loop type and practical applications.

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  • 00:00:00
    today we're talking about loops loops
  • 00:00:03
    looped previously we saw that we could
  • 00:00:04
    iterate through sequences using Loops
  • 00:00:07
    well iteration allows us to actually run
  • 00:00:11
    a block of statements repeatedly kind of
  • 00:00:14
    just like Doctor Strange when he come to
  • 00:00:17
    bargain
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    now there are two kinds of Loops there's
  • 00:00:30
    the for Loop and there's the while loop
  • 00:00:33
    in this lesson we'll be discussing both
  • 00:00:35
    of them the first type of loop that
  • 00:00:37
    we're going to discuss is known as a for
  • 00:00:38
    Loop so a for Loop allows to iterate
  • 00:00:41
    over a sequence so this might be looping
  • 00:00:43
    through each character of a string for
  • 00:00:46
    example I might have some code that says
  • 00:00:48
    for Char in cat
  • 00:00:54
    print
  • 00:00:56
    char
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    and then at the very end I'm going to
  • 00:01:00
    print you know something that signifies
  • 00:01:02
    I'm done with this Loop so I might say
  • 00:01:05
    Okay print
  • 00:01:08
    done
  • 00:01:11
    all right so now what's this going to
  • 00:01:13
    look like in our virtual computer world
  • 00:01:15
    over here so this is our virtual
  • 00:01:17
    computer World okay so we have some
  • 00:01:20
    string here cat okay so I'm going to
  • 00:01:24
    represent that string as I did before
  • 00:01:26
    with this little sequence of boxes and
  • 00:01:29
    so that stands for letter C then a then
  • 00:01:33
    t
  • 00:01:34
    okay so now what this is doing so this
  • 00:01:38
    is what I'm iterating over what this
  • 00:01:41
    Loop is doing it's saying so I'm going
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    to come up with this variable char
  • 00:01:45
    and let me use green to denote that so
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    in my virtual computer world I now have
  • 00:01:50
    a variable named char
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    and I'm going to go through each of the
  • 00:01:55
    boxes in cat
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    what's the first box it's the C so at
  • 00:02:00
    first Char is equal to the C and let me
  • 00:02:02
    just erase this red to make it very
  • 00:02:04
    clear and once I've you know set Char to
  • 00:02:08
    be that first box in cat I go ahead and
  • 00:02:11
    execute whatever is underneath that
  • 00:02:14
    indented statement so that's anything
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    down here and right now we only have one
  • 00:02:20
    statement which is print Char so
  • 00:02:24
    what we're doing here is right now what
  • 00:02:27
    is Char well Char is C right so let me
  • 00:02:32
    draw out you know what my output might
  • 00:02:35
    be from here
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    and currently we want to print Char so
  • 00:02:44
    okay we come here Char is equal to C and
  • 00:02:48
    so we print C
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    now what happens
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    okay well
  • 00:02:54
    we've done everything under this Loop
  • 00:02:57
    right here we've done every we've done
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    this whole block of statements that's
  • 00:03:01
    indented
  • 00:03:02
    so what we actually are going to do is
  • 00:03:05
    go over to the next box
  • 00:03:08
    so I'm going to take this Char variable
  • 00:03:10
    and just slide it over by one box
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    and I'm going to do the same thing again
  • 00:03:15
    so now I'm going to come down here and
  • 00:03:16
    execute everything
  • 00:03:18
    that's indented in this for Loop
  • 00:03:21
    which is print that whatever Char is and
  • 00:03:25
    what is Char while we look over here at
  • 00:03:28
    our virtual computer world and Char is
  • 00:03:30
    pointing to a
  • 00:03:32
    so I come down here into my output and I
  • 00:03:35
    now write a because Char is a
  • 00:03:39
    okay and then we slide this over one
  • 00:03:42
    more time because we can and so now Char
  • 00:03:45
    is T and we're going to go ahead and
  • 00:03:48
    execute everything here so that we're
  • 00:03:50
    printing Char which is T so we print a t
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    and then we try and we want to slide
  • 00:03:57
    this chart over one more
  • 00:04:00
    and then we realize okay there's nothing
  • 00:04:01
    there at this point our computer says
  • 00:04:04
    okay we we can't go over one more
  • 00:04:07
    hmm well that means that now we're done
  • 00:04:11
    with this for Loop because we've
  • 00:04:13
    iterated through everything that we can
  • 00:04:14
    and there's nothing left to go through
  • 00:04:16
    anymore which means that we're left with
  • 00:04:19
    this final statement that's outside of
  • 00:04:21
    the loop so
  • 00:04:23
    one thing to note is that how do we know
  • 00:04:26
    what's in the loop what's outside the
  • 00:04:27
    loop we just look at this white space
  • 00:04:30
    here
  • 00:04:31
    so anything that's indented that's after
  • 00:04:33
    these dots that's inside of that for
  • 00:04:36
    Loop so whenever we go through one by
  • 00:04:38
    one we execute everything there if it's
  • 00:04:41
    outside if it's like the same uh
  • 00:04:44
    indentation as that original four then
  • 00:04:47
    that's outside the for Loop and we
  • 00:04:48
    execute that after we're done with the
  • 00:04:51
    loop so here because this print done is
  • 00:04:55
    the same indentation as four
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    then after we're done with this loop at
  • 00:04:59
    the very end we print done
  • 00:05:02
    okay so our output from this code should
  • 00:05:05
    be c a t done now let's go over to the
  • 00:05:08
    computer and try that out okay so here
  • 00:05:11
    I've typed for Char in cat print Char
  • 00:05:14
    and then at the very end we're going to
  • 00:05:16
    print done so let's run this
  • 00:05:19
    and we get cat done just like how I just
  • 00:05:24
    explained it so let's go through a
  • 00:05:26
    different example we've looked at
  • 00:05:28
    strings but actually we can iterate
  • 00:05:30
    through any sequence using a for Loop so
  • 00:05:32
    we can iterate through lists tuples and
  • 00:05:35
    even dictionary keys or you know we
  • 00:05:37
    could also do the values and items but
  • 00:05:39
    that requires an extra step we can also
  • 00:05:41
    do iteration through sets so let's look
  • 00:05:45
    at a list
  • 00:05:46
    l is equal to some list 0 1 2.
  • 00:05:52
    okay and then I'm going to say for vowel
  • 00:05:57
    in l
  • 00:05:59
    let me put a little tail on that L to
  • 00:06:02
    make it very clear that it's an L
  • 00:06:05
    um let's print that value
  • 00:06:09
    and then let's also print
  • 00:06:11
    that value times 2.
  • 00:06:16
    okay and then at the very end we're
  • 00:06:18
    going to print done again
  • 00:06:22
    okay so I'm going to put my virtual
  • 00:06:24
    computer world
  • 00:06:26
    up here
  • 00:06:27
    and then I'm going to write my output
  • 00:06:29
    down here
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    okay sorry these are not great boxes but
  • 00:06:37
    that's okay
  • 00:06:39
    so first I have some list L and I'm
  • 00:06:42
    going to set that to this like
  • 00:06:46
    representation of the list that I came
  • 00:06:47
    up with in the last class
  • 00:06:50
    um so here I have value 0 1 and 2 in my
  • 00:06:54
    list right
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    so I'm coming up with a variable vowel
  • 00:06:59
    and I'm pointing it towards the first
  • 00:07:01
    item when I go into this
  • 00:07:04
    for Loop so Val okay that's equal to
  • 00:07:08
    zero right now and I'm going to dive
  • 00:07:11
    into the indented part of that for Loop
  • 00:07:13
    so I'm going to print Val which is zero
  • 00:07:16
    okay then I'm going to print Val times
  • 00:07:18
    two okay well what is Val times two
  • 00:07:20
    vowel zero so Val times two zero times
  • 00:07:24
    two is zero so I'm going to Output 0
  • 00:07:26
    again
  • 00:07:28
    now I'm done with this block
  • 00:07:30
    so
  • 00:07:31
    I go and I scoot Val over to the next
  • 00:07:35
    item in my sequence
  • 00:07:38
    and now vowel is equal to one we go into
  • 00:07:42
    this indented part of the code again and
  • 00:07:44
    we're going to print Val which is one
  • 00:07:46
    and then we're going to print vowel
  • 00:07:48
    times two so one times two
  • 00:07:50
    that's two
  • 00:07:53
    okay so then we're going to scoot this
  • 00:07:56
    over one more time
  • 00:07:58
    till we get to 2.
  • 00:08:00
    okay and now we're going to print Val
  • 00:08:01
    again so that's two
  • 00:08:03
    and then we're going to print Val times
  • 00:08:06
    two so two times two that's four
  • 00:08:10
    and
  • 00:08:11
    now we're going to try to scoot this
  • 00:08:13
    over one more time but there's nothing
  • 00:08:14
    over here so
  • 00:08:16
    we're you know basically this is telling
  • 00:08:19
    us all right we are done with these
  • 00:08:22
    lines we're done with this for Loop and
  • 00:08:25
    then we can move on to the very last
  • 00:08:26
    statement which is print done
  • 00:08:30
    so then we print done at the very end
  • 00:08:33
    so let's go and try that out so let's
  • 00:08:35
    set L equal to 0 1 and 2. and then four
  • 00:08:40
    each vowel in L I'm going to print
  • 00:08:45
    Bell and I'm going to print Val times
  • 00:08:49
    two now at the very end I'm going to
  • 00:08:52
    print done
  • 00:08:54
    okay so if I run this
  • 00:08:57
    then I get zero zero one two two four
  • 00:09:01
    done which is exactly what we just wrote
  • 00:09:05
    down
  • 00:09:06
    so now let's go over while loops
  • 00:09:09
    so sometimes what we want to do instead
  • 00:09:12
    of iterating through a sequence is we
  • 00:09:14
    want to iterate through a block of code
  • 00:09:15
    until a certain conditional statement is
  • 00:09:19
    met so until something becomes false and
  • 00:09:23
    so in the doctor strange case it's until
  • 00:09:26
    dermamu could not tolerate with Doctor
  • 00:09:29
    Strange anymore and decides to concede
  • 00:09:32
    then they finally break that Loop never
  • 00:09:37
    come back
  • 00:09:38
    do it
  • 00:09:39
    and I'll break the loop
  • 00:09:42
    [Music]
  • 00:09:44
    so this is where something known as a
  • 00:09:46
    while loop come
  • 00:09:48
    so in a while loop we continue looping
  • 00:09:51
    while some condition is true and then
  • 00:09:53
    once that condition becomes false then
  • 00:09:56
    we break from that Loop all right so
  • 00:09:58
    let's do an example with some real code
  • 00:10:00
    so let's say that n is equal to three
  • 00:10:03
    okay so in my Loop I'm going to say
  • 00:10:05
    while n is greater than zero I'm going
  • 00:10:09
    to print
  • 00:10:11
    n and then I'm going to do a subtraction
  • 00:10:14
    so let's do n equals n minus 1. okay and
  • 00:10:19
    then at the very end
  • 00:10:21
    I'm going to print
  • 00:10:23
    go
  • 00:10:25
    all right so what happens here let's
  • 00:10:27
    draw out our
  • 00:10:29
    virtual computer world and then let's
  • 00:10:31
    draw out our output
  • 00:10:34
    basically okay let's start here n is
  • 00:10:38
    equal to 3. okay so we have some
  • 00:10:40
    variable n and we set that equal to 3.
  • 00:10:44
    and now our condition is while n is
  • 00:10:48
    greater than zero so right now what this
  • 00:10:50
    is doing is it's checking okay what is n
  • 00:10:53
    n is 3 is 3 greater than zero okay well
  • 00:10:58
    this is true
  • 00:10:59
    so we're going to go through our
  • 00:11:02
    statements here so we're going to print
  • 00:11:03
    n which is three
  • 00:11:05
    and then what we're going to do is
  • 00:11:07
    subtract 1 from n and set that back to n
  • 00:11:11
    so
  • 00:11:12
    over here n equals three okay so what's
  • 00:11:15
    three minus one it's two and we're going
  • 00:11:17
    to set that back to n so instead of
  • 00:11:20
    three now n is 2.
  • 00:11:22
    so
  • 00:11:23
    okay we come back up here and we check
  • 00:11:26
    our condition again so now n is equal to
  • 00:11:30
    2. so we're checking 2 greater than zero
  • 00:11:32
    well that's true again so we go into
  • 00:11:35
    here and we print n which is 2 and then
  • 00:11:39
    again we do okay n equals n minus one so
  • 00:11:43
    n minus 1 is 1 because 2 minus 1 is 1.
  • 00:11:47
    and we're going to set that as n so let
  • 00:11:50
    me just clean that up
  • 00:11:52
    so now n is one right
  • 00:11:54
    okay now we're done with you know that
  • 00:11:57
    block of statements again we go back to
  • 00:11:59
    the while and we say okay is one greater
  • 00:12:02
    than zero
  • 00:12:04
    okay well that's true again so we go
  • 00:12:06
    into our statements again so we're
  • 00:12:08
    printing one and then again well n
  • 00:12:12
    equals n minus one so we're setting n to
  • 00:12:15
    one minus one which is zero
  • 00:12:19
    so when we come back up to this while
  • 00:12:21
    loop the statement we're checking okay
  • 00:12:24
    is zero greater than zero
  • 00:12:28
    that's not true this time so
  • 00:12:31
    we don't do any of this stuff down here
  • 00:12:33
    and in fact we are done with this Loop
  • 00:12:35
    once that becomes false we're done with
  • 00:12:37
    the loop so the last thing that we have
  • 00:12:40
    left is to print go
  • 00:12:42
    so down here I print go so our output
  • 00:12:45
    becomes three two one go
  • 00:12:49
    now let's see that in action
  • 00:12:51
    okay so if I have n equals three and I
  • 00:12:56
    say while n is greater than zero
  • 00:13:00
    I'm going to print n
  • 00:13:03
    and then I'm going to set n equal to n
  • 00:13:06
    minus 1. so I'm basically just
  • 00:13:08
    subtracting 1 each time and then at the
  • 00:13:10
    very end I'm going to print go
  • 00:13:14
    okay if I run this
  • 00:13:16
    then I get three two one go
  • 00:13:20
    so you can actually kind of read this
  • 00:13:22
    while statement as if it were English
  • 00:13:25
    so in this specific context you can say
  • 00:13:27
    okay while n is greater than zero
  • 00:13:32
    then display this value of N and then
  • 00:13:35
    subtract 1 from n and when you get to
  • 00:13:38
    zero then we're going to print go
  • 00:13:42
    so we do have to be careful when we use
  • 00:13:44
    while loops
  • 00:13:45
    take a second here and think about why
  • 00:13:48
    do we have to be careful
  • 00:13:49
    again
  • 00:13:51
    and again
  • 00:13:54
    and again forever
  • 00:13:57
    well what would happen if I did
  • 00:13:59
    something like this
  • 00:14:01
    this while true
  • 00:14:04
    print
  • 00:14:06
    hello world
  • 00:14:12
    hmm
  • 00:14:14
    what would happen here
  • 00:14:16
    well
  • 00:14:19
    true is always going to be true
  • 00:14:23
    basically forever well not basically it
  • 00:14:26
    is going to be true forever right
  • 00:14:29
    so if that's true forever then when do
  • 00:14:32
    we ever stop printing hello world
  • 00:14:36
    we don't right and again forever in our
  • 00:14:40
    computer we'll go crazy like dormammu so
  • 00:14:43
    in this case it's actually pretty easy
  • 00:14:45
    to tell that this will become what's
  • 00:14:48
    known as an infinite Loop
  • 00:14:53
    so sometimes it's actually fairly
  • 00:14:55
    straightforward to tell if something's
  • 00:14:56
    going to be an infinite Loop but other
  • 00:14:59
    times it's not so for example if I have
  • 00:15:02
    n equals 27 and I say while you know n
  • 00:15:06
    does not equal one
  • 00:15:08
    I'm going to print n and within you know
  • 00:15:13
    the block of code underneath a loop you
  • 00:15:17
    can have other valid codes so let me say
  • 00:15:18
    like
  • 00:15:19
    let me put a conditional in here so if n
  • 00:15:23
    uh is even so this if the remainder
  • 00:15:27
    divided by 2 is 0
  • 00:15:29
    then I'm going to just divide n in half
  • 00:15:32
    and set the new n equal to that value
  • 00:15:35
    and otherwise so basically n is odd here
  • 00:15:40
    and here n is even
  • 00:15:45
    so otherwise we're going to do n equals
  • 00:15:47
    n times 3 plus 1.
  • 00:15:51
    okay
  • 00:15:52
    so is it obvious to you that in this
  • 00:15:54
    specific case we
  • 00:15:58
    you know this this never ends
  • 00:16:01
    not so obvious right
  • 00:16:04
    because n is sometimes increasing and
  • 00:16:06
    sometimes decreasing and
  • 00:16:09
    there are actually certain values of n
  • 00:16:11
    where the program will terminate so it
  • 00:16:15
    will sometimes become one but we can't
  • 00:16:18
    prove that this actually ends for all
  • 00:16:21
    positive values of n and there's
  • 00:16:23
    actually some famous example that I took
  • 00:16:25
    from one of mit's notes but basically
  • 00:16:28
    the warning here is that infinite Loops
  • 00:16:30
    are not always as simple as you know
  • 00:16:34
    while true so you have to be careful
  • 00:16:37
    when you use while Loops you need to
  • 00:16:38
    make sure that there's a termination
  • 00:16:40
    condition you need to make sure that
  • 00:16:43
    there will be an end if you start this
  • 00:16:44
    Loop otherwise you will drive your
  • 00:16:46
    computer crazy
  • 00:16:47
    in this lesson we learned about for
  • 00:16:49
    loops and while loops and how to use
  • 00:16:52
    each one now in the next class what
  • 00:16:54
    we're going to be talking about is
  • 00:16:56
    examples of each and why you might want
  • 00:17:00
    to use a loop or what is a practical
  • 00:17:02
    application of a loop so stay tuned
  • 00:17:06
    by the way make sure you guys check out
  • 00:17:07
    the Discord server where you can
  • 00:17:09
    interact with other students and ask
  • 00:17:11
    questions and also check out the notes
  • 00:17:13
    that are linked below
  • 00:17:15
    good luck
标签
  • loops
  • for loop
  • while loop
  • iteration
  • infinite loop
  • programming
  • sequences
  • termination condition
  • coding examples
  • Discord server