Hello,

I didn't understand the functional approach with the loop'for'. Could you explain me that?

U = >>> [[x * y for x in range (3)] for y in range (3)]

U >>>

[[0, 0, 0], [0, 1, 2], [0, 2, 4]]

I tried to understand with print but it does not tell me much /> :

>>> print([i*j for j in range(3)])

[0, 2, 4]

>>> print([i*j for i in range(3)])

[0, 2, 4]

# function range() and loop 'for'

Page 1 of 1## 5 Replies - 894 Views - Last Post: 21 January 2014 - 01:24 PM

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**Replies To:** function range() and loop 'for'

### #2

## Re: function range() and loop 'for'

Posted 21 January 2014 - 12:34 PM

I'm not sure exactly what you're after here, but I can unpack this line a little.

What you have here is called a list comprehension - that's a good phrase to search up and read up on.

In fact, it's a nested list comprehension, so let's take the inner one to start with:

This evaluates to a list whose elements are x*y for some xs and ys. What are the xs? They are the elements of the range(3), which is to say, the list [0,1,2]. That is, they are a sequence of three elements, and x takes each of those values in turn. So what is x? x is not specified in the comprehension, so we have to get it from outside.

Let's look at the outer loop:

[?????? for y in range (3)]

This generates a list of ?????? for each value in the range (3), and for each of those somethings in the list, y is available as a value from that range. What is that mysterious thing? Well, it's just that inner comprehension - so now we know where y comes from.

So we can read this as "make me a list with one element for each value y in range(3), and make each of those elements a list, itself containing one element x*y for each value x in the range (3)"

You can read this as a special sort of for loop if you like:

list1 = []

for y in range (3):

list2 = []

for x in range(3):

list2.append(x*y)

list1.append(list2)

In general, a list comprehension can be rewritten as a for loop in this fashion, but it doesn't work the other way: you can't rewrite all for loops as list comprehensions.

What you have here is called a list comprehension - that's a good phrase to search up and read up on.

In fact, it's a nested list comprehension, so let's take the inner one to start with:

[x * y for x in range (3)]

This evaluates to a list whose elements are x*y for some xs and ys. What are the xs? They are the elements of the range(3), which is to say, the list [0,1,2]. That is, they are a sequence of three elements, and x takes each of those values in turn. So what is x? x is not specified in the comprehension, so we have to get it from outside.

Let's look at the outer loop:

[?????? for y in range (3)]

This generates a list of ?????? for each value in the range (3), and for each of those somethings in the list, y is available as a value from that range. What is that mysterious thing? Well, it's just that inner comprehension - so now we know where y comes from.

[[x * y for x in range (3)] for y in range (3)]

So we can read this as "make me a list with one element for each value y in range(3), and make each of those elements a list, itself containing one element x*y for each value x in the range (3)"

You can read this as a special sort of for loop if you like:

list1 = []

for y in range (3):

list2 = []

for x in range(3):

list2.append(x*y)

list1.append(list2)

In general, a list comprehension can be rewritten as a for loop in this fashion, but it doesn't work the other way: you can't rewrite all for loops as list comprehensions.

### #3

## Re: function range() and loop 'for'

Posted 21 January 2014 - 12:59 PM

Thanks for your explanation

if i have to create the matrix Z = [[1,2,3],[4,5,6]] (2 lines and 3 columns), and to use that special loop but my codes don't work good :

if i have to create the matrix Z = [[1,2,3],[4,5,6]] (2 lines and 3 columns), and to use that special loop but my codes don't work good :

>> squares = [] >>> for x in range(6): squares.append(x+1) Z = [squares] >>> Z [[1, 2, 3, 4, 5, 6]] >>> squares [1, 2, 3, 4, 5, 6]

>>> [[x+1 for x in range(6)]] [[1, 2, 3, 4, 5, 6]] >>> [[x+1 for x in range(6)]] [[1, 2, 3, 4, 5, 6]] >>> [[x+y for x in range(1,4)] for y in range(0,2)] [[1, 2, 3], [2, 3, 4]]

### #4

## Re: function range() and loop 'for'

Posted 21 January 2014 - 01:03 PM

That's a nice little puzzle. I'm going to let you work it out. Here's a hint: [1,2,3] and [4,5,6] are the same sort of progression. What sort of operator do you need here?

### #5

## Re: function range() and loop 'for'

Posted 21 January 2014 - 01:18 PM

i think that's the operator 'addition' to get this matrix Z = [[1,2,3],[4,5,6]]

>>> [x+1 for x in [1,2,3]] [2, 3, 4] >>> [x for x in [1,2,3]] [1, 2, 3] >>> [[x for x in [1,2,3]] for y in [4,5,6]] [[1, 2, 3], [1, 2, 3], [1, 2, 3]] >>> [x for x in [4,5,6]] [4, 5, 6] >>> [[x+y for x in [1,2,3]] for y in [4,5,6]] [[5, 6, 7], [6, 7, 8], [7, 8, 9]] >>> [[x+y for x in [1,2,3]] for y in range(2)] [[1, 2, 3], [2, 3, 4]]

### #6

## Re: function range() and loop 'for'

Posted 21 January 2014 - 01:24 PM

That's correct. You're almost there now - you just need to figure out how to express "the thing to add to each member of that range" (what you might call the "offset". You can do that in a few ways. For example, you can do it with an explicit list of two values, or you can do it arithmetically, or you could use a range() with the step parameter.

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