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CS1112 Lecture 7 2009/2/
Lecture slides 1
Previous Lecture: Iteration using while Introduce nested loops
Today’s Lecture: Developing algorithms Nested loops
Announcements: Section in the computer lab this week Project 2 due 2/12 (Thurs) at 11pm Prelim 1 on 2/19 (Thurs) 7:30pm. If you have a conflict, tell us (email Bill Hogan) now—no later than 2/12.
February 10, 2009 Lecture 7 5
Example: Nested Stars
February 10, 2009 Lecture 7 8
Knowing how to draw
How difficult is it to draw
February 10, 2009 Lecture 7 9
Pattern for doing something n times
n= _____
for k= 1:n
% code to do
% that something
end
February 10, 2009 Lecture 7 10
x=0; y=0; % figure centered at (0,0) s= 2.1; % side length of square DrawRect(x-s/2,y-s/2,s,s,’k’) r= 1; k= 1; while r > 0.1 %r still big % draw a star if rem(k,2)==1 %odd number DrawStar(x,y,r,’m’) %magenta else DrawStar(x,y,r,’y’) %yellow end % reduce r r= r/1.2; k= k + 1; end
Example: Are they prime?
Given integers a and b, write a program that lists
all the prime numbers in the range [a, b].
Assume a>1, b>1 and a<b.
CS1112 Lecture 7 2009/2/
Lecture slides 2
February 10, 2009 Lecture 7 15
Example: Are they prime?
Subproblem: Is it prime?
Write a program fragment to determine whether
a given integer n is prime, n>1.
Reminder: rem(x,y) returns the remainder of x
divided by y.
Developing the
algorithm for the
times table
disp('Show the times table for specified range') lo= input('What is the lower bound? '); hi= input('What is the upper bound? ');
February 10, 2009 Lecture 7 26
Find the biggest rectangle
Draw 5 rectangles
that the user specifies
using mouse clicks
Color the biggest one
red
Here's the biggest rectangle you drew!
February 10, 2009 Lecture 7 29
Find the mode in a sequence
A mode is the number in a sequence that appears the most number of times Develop an algorithm for calculating the mode of a user- entered sequence that is Non-negative Entered one-by-one in non-decreasing order Terminated by a negative number E.g., sequence 87, 92, 92, 98, 98, 98, 100 has a mode… Write the algorithm and then the code on your own for practice!
February 10, 2009 Lecture 7 31
The savvy programmer…
Learns useful programming patterns and use them where appropriate Seeks inspiration by working through test data “by hand” Asks, “What am I doing?” at each step Sets up a variable for each piece of information maintained when working the problem by hand Decomposes the problem into manageable subtasks Refines the solution iteratively, solving simpler subproblems first Remembers to check the problem’s boundary conditions Validates the solution (program) by trying it on test data
