### Session 10: Manipulating Images with Loops (cont.)

#### Homework 2: Solution for Question 1

Don't think of a solution to the entire problem, think of the simplest step towards a solution. Implement that step, then consider the next simplest step - continue until problem is solved. Following is a very detailed example of one possible solution to question one.

You need to generate a square.

You generate that square by printing characters.

You print characters one line at a time (i.e., we don't print in columns)

Let's try doing this for a square of x's with side=5 (we'll worry about passing parameters later).

Look at the sample result in the task:

xxxxx x x x x x x xxxxx

Let's consider this one line at a time:

line 1: print 5 "x" line 2: print "x", then 3 spaces, then "x" line 3: print "x", then 3 spaces, then "x" line 4: print "x", then 3 spaces, then "x" line 5: print 5 "x"

Recall the material of chapter 3: we build up strings using concatenation. However, if we need to repeat a character a number of times, we can use multiplication (which is repeated concatenation!) So, for line 1 and 5 we can use:

print "x" * 5

What about lines 2-4? How can we express 3 in terms of 5 (which is the size)? How about 5-2? That gives us:

print "x" + " " * (5 - 2) + "x"

Thus, we have the following code:

print "x" * 5 print "x" + " " * (5 - 2) + "x" print "x" + " " * (5 - 2) + "x" print "x" + " " * (5 - 2) + "x" print "x" * 5

Consider the three inner print statements - that's repetition - and for that we should use a loop:

for i in range(0, 3): print "x" + " " * (5 - 2) + "x"

Let's put it together, substituting 5-2 for 3, and adding the outer print statements:

print "x" * 5 for i in range(0, (5 - 2)): print "x" + " " * (5 - 2) + "x" print "x" * 5

Last step: see those 5's and x's? Those are our parameters! On to the final solution:

def textsquare(char, size): print char * size for i in range(0, (size - 2)): print char + " " * (size - 2) + char print char * size

You can improve this slightly - eliminating minor duplication, but that is left as an exercise for the reader.

Main idea: don't try to solve the whole thing at once; break down your problem into smaller problems - and solve them one step at a time, building up towards the solution.

#### Back to Image Manipulation

[to be posted later this week..]