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lab27
Loops!
via `range`

In the previous lab, we had a bunch of nearly-repeated code, to make a stripe 5 pixels long ¹:

px = getPixelAt(pict0, 20, 10+0)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 10+1)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 10+2)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 10+3)
setRed(px,255)
setBlue(px,255)
px = getPixelAt(pict0, 20, 10+4)
setRed(px,255)
setBlue(px,255)

If we wanted to make this stripe be 8 pixels long instead, we could copy/paste and twiddle the numbers three more times. It wouldn't be that bad, but it wouldn't be anything fun either. And if we wanted to make the strip be 100 pixels long, the copy/paste approach suddenly seems very annoying!

We just want to count the offset-from-10 being 0,1,2,3,4. Can't we get the computer to do this counting for us? Yes! That's exactly what computers are good at (and humans are error-prone at). We can re-write the above 15 lines of code into just 4, using a for loop:

for offset in range(5):
  px = getPixelAt(pict0, 20, 10+offset)
  setRed(px,255)
  setBlue(px,255)

This says:

Hey computer:
Do the body of the loop — the three indented lines — with offset being 0.
Then, do the body again with the offset being 1.
Then, do the body again with the offset being 2.
Then, do the body again with the offset being 3.
Then, do the body again with the offset being 4.

Or more concisely:

Hey computer:
For every value in the range [0,5),
set offset to that value
, and do the loop-body.

What if we change range(5) to be range(100)? Then offset ranges from 0 up to (but not including) 100, and for each offset we do the loop-body. Yippee!

Notes:

“offset” is just a variable; we can choose whatever name we want, as usual.
The indentation is important! It's how python knows what is the loop-body, from any other instructions that follow (but are not to be repeated inside the loop).
In particular: there is also indentation to indicate what statements are inside a function-definition (as opposed to lines that come after that function definitions). So you must have nested indentation, if you have a for-loop inside a function:
def someFunction(n): print( "hi" ) for i in range(10): print "Yo!" print "i is " + str(i) print( "bye" ) return
For fun: in the interaction window, evalute range(5). What does it return? This is a list-of-integers — a new type! range : int → list<int>.

If you have not completed the previous lab, you can check that off up until the end of today (last chance). Then, you can check off the following tasks today, or next time. (We'll also be talking more about loops next time.)

TODO: For your previous lab, do a Save As… to create a new file called lab28.py (so you keep around both the previous lab, and this one).
TODO: Update stripeAtX10, so that it makes a stripe 100 tall.
Your code will use a for loop, and will be much shorter than before and more powerful!.
TODO: Update squareAtX10, so that it makes a square 30 wide, and 100 tall. (You can re-name it rectAtX10 if you like.)
Your code will use a for loop, and will be shorter than before and more powerful!.

¹ The code given here is slightly different than the previous lab — I have replaced 12 with 10+2, and so on. Hey, that's kinda like what you had to do for stripeAtX20, once 10 was replaced with any x! ↩

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©2014, Ian Barland, Radford University
Last modified 2014.Mar.20 (Thu) Please mail any suggestions
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lab27 Loops!via `range`

lab27
Loops!
via `range`