List

Aladdin

Sep 15, 2019 15:18

I need to print alterbate elements of a list

List is:

numbers=[67, 89,23,09,23,18,10,13]

I tried

for I in range (0, 8) :

@JohnRennie imgur.com/a/7nq7olQ

I am getting syntax error.... What's my error

Wait I forgot semicolon after for..

But even after using it, I am getting error

John Rennie

if i % 2== 0 not for i % 2 == 0

Aladdin

Why if not for

John Rennie

for is a looping command i.e. it executes a block of code repeatedly.

So for i in range(0,9) executes the block code that follows it ten times, each time with i set to 0, 1, 2 etc up to i=9 the last time.

then for each repetition you want to check the value of i and print the list member if i is even.

Aladdin

Yes

John Rennie

In any case a better way to do it would be for i in range(0,9,2)

The final ,2 means increase i by 2 for each loop

Aladdin

Sep 15, 2019 15:27

Okay

Tbh I am still getting syntax error

imgur.com/a/b5Xl9bW

What's wrong now

John Rennie

numbers = numbers[i]

That is doing something rather odd.

You're trying to set the whle list numbers to one of its elements.

Why not simply:

Aladdin

Didn't understand

John Rennie

= is an assignment operator

Aladdin

Yep

John Rennie

So numbers = numbers[i] sets the variable numbers to the value of numbers[i]

>>> numbers = [1,2,3]
>>> print(numbers)
[1, 2, 3]
>>> numbers = numbers[1]
>>> print(numbers)
2

Initially numbers is a list, but by setting numbers = numbers[1] it is changed to an int.

You just want to print alternate elements don't you?

Aladdin

Sep 15, 2019 15:35

Yes

John Rennie

numbers = [67,89,23,9,23,18,10,10]
for i in range(0, len(numbers))
    if i % 2 == 0:
        print(numbers[i])

Try that.

Aladdin

I tried differently and got it

John Rennie

oops, I missed a colon on the for line ...

Aladdin

Urs is shorter

Let me try

John Rennie

numbers = [67,89,23,9,23,18,10,10]
for i in range(0, len(numbers), 2):
    print(numbers[i])

is shorter still.

Aladdin

Sep 15, 2019 15:39

@JohnRennie imgur.com/a/qYC1eX7

I tried this

John Rennie

That's a perfectly reasonable way to do it if you might need a list of the alternate numbers

Aladdin

But I think len(numbers) should have been more general

Instead of 8 . I made the list can be varying

John Rennie

It's always best to avoid hard coding numbers into your code if you can.

Aladdin

Where would numbers [I]be printed

How do u know it will print in list

John Rennie

@Aladdin I'm not sure what you are asking

Aladdin

Sep 15, 2019 15:45

Shouldn't the output be like:

6

23

John Rennie

You mean one number per line?

Aladdin

No ur program will print one person line right

But I want a list

John Rennie

D:\rhs\Python>python numbers.py
67
23
23
10

Yes, mine prints one number per line.

If you want all the numbers on one line you can use formatted printing or you can just do what you do and build up a new list.

Aladdin

Is there a way to convert to list

John Rennie

@Aladdin well that's what your code does. It builds up a new list from every alternate nuber.

Aladdin

Sep 15, 2019 15:49

So to make another list, we need an empty list

Without it, there's no way

John Rennie

That would be a good way to do it because it's nice clear code.

Aladdin

Ah OK

John Rennie

You could modify the list in place i.e. something like

for i in range(0,len(numbers)/2):
    numbers[i] = numbers[2*i]

Aladdin

Nice

I think this will work

John Rennie

I wouldn't do it that way because it looks confusing.

Clear and obvious code is always best.

Aladdin

Sep 15, 2019 15:53

Say I have to write a list to replace numbers with their square

Then I would need empty list for that?

Can we do modifications inside the list itself to get this condition

John Rennie

You could do, but I would create a new list because it's clearer

numbers = [1,2,3]
squares = []
for i in range(0,len(numbers)):
  squares.append(numbers[i]**2)

Aladdin

Yep

But the question says replace

John Rennie

OK if the question tells you to replace the number by their squares use:

numbers[i] = numbers[i]**2

Aladdin

Sep 15, 2019 16:13

@JohnRennie hi

John Rennie

@Aladdin hi

Aladdin

imgur.com/a/n7sLmiW

I need my list to contain numbers which have squares greater than 5 and less than 100

But my code is not working

John Rennie

Because the line list[i] = list[i] doesn't do anything. It leaves list[i] unchanged.

Are you allowed to create a new list, or does the question tell you to update the list in place?

Aladdin

I thought it would only be executed if condition is true

No new list

Wait I misinterpret the question

The question is

Create a list with numbers 0 snd 9 in which square of numbers are between 5 and 100

But I still don't understand what's wrong in my program

John Rennie

numbers = []
for i in range (0,10):
    if i**2 >= 5 and i**2 <= 100:
        numbers.append(i)

Aladdin

Sep 15, 2019 16:21

@JohnRennie can you explain what's wrong with bmy program

I didn't understand that line

Because the line list[i] = list[i] doesn't do anything. It leaves list[i] unchanged.

John Rennie

the line list[i]=list[i]sets the ith element of the list equal to the ith element of the list. It's like writing x=x.

Aladdin

Ah oops

Do u know the correction

John Rennie

@Aladdin Build up a new list using append

Aladdin

Guess that's the only way

@JohnRennie how did u learn coding

On ur own

John Rennie

Yes, but computers were far simpler in 1978 when my school got it's first computer :-)

Aladdin

Sep 15, 2019 16:31

1970's era

John Rennie

Yes :-)

Aladdin

How were teachers back in those days

We're they strict or kinda loose

John Rennie

My school was quite strict.

Though the teachers would encourage you if you were interested to learn.

I need to go now. I'll be back tomorrow.

Aladdin

Sure. Good night

Aladdin

Sep 16, 2019 15:01

@JohnRennie hi

John Rennie

Sep 16, 2019 15:20

@Aladdin hi

Aladdin

How to reverse list elements

John Rennie

@Aladdin In place? Or into a new list?

Aladdin

In place

John Rennie

Suppose you have a list [a,b,c,d,e]

What you do is start at the end and work towards the centre swapping elements as you go.

Aladdin

I did

John Rennie

Sep 16, 2019 15:27

So we use a for loop for i in range (0,len(mylist)/2)

That goes from the left end to the middle.

I would set a new variable j = len(mylist) - i - 1

Aladdin

Lst=[1, 2,3,4]

John Rennie

Then you'll swap the elements [i] and [j].

Aladdin

for I in range (lena(lst)):

Lst[I]==Lst[-i-1]

print (lst)

@JohnRennie but this prints the same list...

John Rennie

Well let's go through it by hand. len(lst) is 4 so your four loop will execute 4 times with i set to 0, 1, 2 and 3. OK so far?

Aladdin

Ok

John Rennie

Sep 16, 2019 15:33

the second line is lst[i] = lst[-i-1]. Is that correct? On the right hand side you have a negative number as the index i.e. -i-1.

Did you mean lst[i] = lst[len(lst)-i-1] ?

Aladdin

Yes

John Rennie

Also, did you really mean == ?

That's the comparison operator not the assignment operator.

Aladdin

I meant =

John Rennie

So let's just check this. Your code is:

lst=[1,2,3,4]
for i in range(0,len(lst)):
    lst[i] = lst[len(lst)-i-1]
print(lst)

Is that definitely correct?

Aladdin

Lst[I]=Lst [-i-1]

My code had this

John Rennie

Sep 16, 2019 15:44

That can't be correct because -i-1 is a negative number. You can't have a negative number as a list index.

We need to agree on what code you are running, then you need to try running the code we've agreed on to see what happens.

Ah, my bad, Python does allow negative indexes.

Ignore what I said

A negative index means you count from the right end of the list.

@Aladdin OK. So the code is:

lst=[1,2,3,4]
for i in range(0,len(lst)):
    lst[i] = lst[-i-1]
print(lst)

Yes?

Aladdin

Yed

But the problem is it's not printing backwards

I thought it could...

John Rennie

OK. Let's start with lst=[1,2,3,4]. On our first pass through the loop i=0 so the command in the loop does lst[0] = lst[-1]. Yes?

Aladdin

Yes

John Rennie

That changes lst to [4,2,3,4]. OK so far?

Aladdin

Yed

Ah I can see my mistake

This won't work

John Rennie

Sep 16, 2019 15:53

Yes :-)

You need an intermediate variable to do the swap.

Aladdin

So we use a for loop for i in range (0,len(mylist)/2)
That goes from the left end to the middle.
I would set a new variable j = len(mylist) - i - 1

Why did u take my list/2

Can u explain ur code

John Rennie

Because you are swapping the left and right halves of the list, so you only need to work your way through half the list.

Let's take your list [1,2,3,4] and see how this works. The first swap is with i=0 so j = 3. That swaps elements 0 and 3 so the list is now: [4,2,3,1]. OK so far?

Aladdin

What if length of list is odd

Ok

John Rennie

If the length is odd we don't process the middle element. But the middle element won't move anyway because it's in the middle.

So len(lst)//2 always works.

(I forgot we need // to do integer division)

Aladdin

Yep

John Rennie

Sep 16, 2019 16:01

My full code is:

lst = [1,2,3,4]

for i in range(0,len(lst)//2):
    j = len(lst) - i - 1
    x = lst[i]
    lst[i] = lst[j]
    lst[j] = x

print(lst)

Do you want to try running that on your PC?

Aladdin

Yes . On it

Yep it's working

John Rennie

:-)

Aladdin

Tbh I think these codes are more maths than coding

John Rennie

They are teaching you how to analyse a problem and construct an algorithm to solve it.

Aladdin

Sep 16, 2019 16:26

@JohnRennie hi

John Rennie

@Aladdin hi

Aladdin

imgur.com/a/AnJcWT6

I need to add elements of l1 and l2 into p

Such that no elements repeat

Replace c with p

@JohnRennie hello?

John Rennie

@Aladdin hi, sorry I was busy for a moment.

Aladdin

Oh okay. Ping me when u are back

John Rennie

I'm free now!

The way I would do this is to first put all elements of l1 into c.

Then I would step through l2, and I would append the elements of l2 to c only if that element isn't in l1.

Aladdin

Sep 16, 2019 16:34

Ah right

No need to make it complicated like mine

John Rennie

So that's my algorithm, and it now just remains to turn it into code.

Aladdin

Mine was

To first put those elements which are not Same in both lists

And then put those which are equal

Guess my algorithm is right?

John Rennie

I'm not sure how your algorithm works. Your description of it is a bit unclear to me.

Aladdin

First take a list then put those elements which are not in other list

Take the other list and put those elements which are not in other list

Finally take the elements which are common

@JohnRennie is this ok

John Rennie

Ah, I see. Yes that will work though it seems a bit inefficient because you are having to do lots of checking of whether elements are in lists or not.

Aladdin

Sep 16, 2019 16:42

Figured so. But just for practice I am gonna apply both mine and urs

John Rennie

OK, yes that's good practice.

Aladdin

@JohnRennie little help

John Rennie

@Aladdin yes?

Aladdin

For my code, how do I take the value of index only once for a list if the element it represents is not common

I can't use if statement.... It's taking i multiple times

imgur.com/a/7VikH1M

John Rennie

One of the reasons people like Python is that it makes lots of things easy. In particular to check if a value is in a list you can just do if x in mylist:

So suppose you are getting all the elements of l1 that are not in l2. The way to do this is:

for i in range(len(l1)):
    if not l1[i] in l2:
        c.append(l1[i])

Do you see how that works?

Aladdin

Sep 16, 2019 16:56

Hoe does if not work

Is it similar to if?

John Rennie

Yes. The operator not just converts true to false and false to true.

So:

if 1 == 1:
    print("Equal!")
else:
    print("Not equal!")

That prints "Equal!" which I guess is obvious.

Aladdin

OK

John Rennie

If you stick a not in:

if not 1 == 1:
    print("Equal!")
else:
    print("Not equal!")

Aladdin

Not equal

John Rennie

That will then print "Not equal!.

@Aladdin yes.

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