How to assign values in a for loop from a list

Question

I have three vertices of a rectangle and need to find the fourth vertex, and I need to find the missing vertex for N numbers of rectangles.

Sadly, I could not figure out how I can assign the vertices after the first rectangle :/.

Here is the example text file for input:

2 # '2' is the number of rectangles.
5 5 # (x1, y1)
5 7 # (x2, y2)
7 5 # (x3, y3)
30 20 # (x1, y1)
10 10 # (x2, y2)
10 20 # (x3, y3)
 # (there could be more '**vertices**' and more than '**2**' cases)

Here was my approach:

import sys
def calculate(num):
 x1 = lines[n].split()[0]
 y1 = lines[n].split()[1]
 x2 = lines[n+1].split()[0]
 y2 = lines[n+1].split()[1]
 x3 = lines[n+2].split()[0]
 y3 = lines[n+2].split()[1]
 print x1, y1
 print x2, y2
 print x3, y3
 #Planning to write codes for calculation & results below inside this function.
readlines = sys.stdin.readlines() # reads
num = int(lines[0]) # assigns the number of cases
for i in range(0, num):
 item += 1
 calculate(item) # Calls the above function

When I run this code, I get the following:

5 5
5 7
7 5 
5 7
7 5
30 20 

What I wanted to get was:

5 5
5 7
7 5
30 20
10 10
10 20

• Frerich Raabe: Using the grouper recipe shown on the itertools documentation page, you could easily (and efficiently) consume your file in chunks of three.
• DavidC: It's my first time signing up and posting my question here. I do really appreciate your help!
• Ali: What is 'n? where did you declare it? it seems that the code you posted above is not complete
• DavidC: My bad. Yes, you are right. It should be 'n' instead of 'num'! :)

Answer 1

You want

item += 3

inside your loop.

---

Looking at it again, that's not enough to make it work. You want to pass the line numbers

1, 4, 7, 10 .....

to your calculate function. You can do this with the 3-argument version of range

for iLine in range( 1, 3*num-1, 3):
 calculate( iLine)

The third argument tells it to skip by 3 each time, and you need to start at line #1, not #0, as line #0 contains your count.

You also need to get the upper limit right. The final value you want to pass into calculate is actually 3*num-2, but remember that the range function does not include the upper limit, so we can use (highest desired value + 1) there, which is where the 3*num-1 comes from.

• DavidC: Thank you Jonathan, I tried that, but it gave me an out of range error. Here is what I got 7 5, 30 20, 10 10
• JonathanZ: Yes, I was confused by the loop variable being i and passing the variable item into your calculate function. If you loop over the same variable that you pass to calculate, like in my iLine example, I think it's clearer.
• DavidC: Thank you again Jonathan. I tried as you suggested, and it worked perfect! I really appreciate for your help!

Answer 2

It seems that the code above is not your complete code, but I think you should correct it in your real code as follows: instead of item +=1 you should write item = 1+ i*3.

• DavidC: Thank you for your help!
• Ali: @DavidC you're welcome. In stackoverflow when someone answers your question and you find it helpful please like the answer. Thanks
• DavidC: Oh okay, I would be happy to like your answer for your help. How do I click like? I clicked on the up arrow now! :) Thank you.
• Ali: You can like by clicking the up arrow in in the left hand side of my answer
• DavidC: I already did that, and when I click on it, it says that I need to have more than 15 reputations in order to show it publicly. I am not sure if that counted or not. Anyways, thank you for your help!

Answer 3

@Avilar - This is what happens in your code when num > 2 Your code suggestion is like this:

item = 1
for i in range(0, num):
 item += i*3

As we go through the loop

i = 0
item += 0 --> item = 1

then

i = 1
item += 3 --> item = 4

then

i = 2
item += 2*3 --> item = 10

then

i = 3
item += 3*3 --> item = 19

then

i = 4
item += 3*4 --> item = 31

You will generate the numbers

1, 4, 10, 19, 31, 46, 64

when we want

1, 4, 7, 10, 13, 16, 19