Lecture 4
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Log into Linux
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Textbook is in the bookstore
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Reminder: Homework 2 program is due by
Friday 4:30pm. The submission system is set
up to accept submissions.
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Reminder: Sign up for Career Forum by Friday.
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Questions?
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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Outline
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Formatting tables
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Course goals
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Recursion
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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Formatting Tables
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To format data into tables, want to output data
into a fixed-width field.
In C++, done using I/O manipulators defined in
<iomanip>. Manipulators are "output" to a
stream using insertion operator (<<).
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setw(n) – output the next item in a field of size n
left, right – change justification within fields (default
is right). This is in force until the next justification
manipulator.
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CS 215 Fundamentals of Programming II - Lecture 4
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Formatting Tables
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Example:
cout << left;
cout << setw(4) << "n" << " " << setw(6) << "2^n" << endl;
cout << setw(4) << "­­­­" << " " << setw(6) << "­­­­­­" << endl;
cout << right;
for (int i = 0; i < 16; i=i+4)
cout << setw(4) << i << " " << setw(6) << pow(2,i) << endl;
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Output:
n 2^n
­­­­ ­­­­­­
0 1
4 16
8 256
12 4096
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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Course Goals
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Continue studying software engineering
techniques with emphasis on implementing
abstract data types using classes.
Mastery of programming skills: files and other
streams, recursion, classes, dynamic allocation,
exception handling, templates.
Introduction to algorithm analysis.
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Example: How to compute the sum from 1 to n?
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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Software Life Cycle
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Specification of the problem/task
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Design of a solution
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Implementation (coding) of the solution
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Testing and debugging
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Maintenance and evolution of the system
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Obsolescence
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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What is Recursion?
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Recursion is a programming technique in
which a problem is broken down into smaller
versions of itself until the solution is obvious.
General strategy for thinking about recursive
solutions:
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Identify one or more base (anchor) cases, also
called stopping conditions: Cases that are solved
directly.
Identify one or more recursive (inductive) steps:
Solve the problem by dividing it into smaller version
of itself. (Why must must new version be smaller?)
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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Recursion
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Example: Write a program that displays a nonnegative integer to the screen so that its digits
are stacked vertically. E.g. 1234 should display
as:
1
2
3
4
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What is the simplest version of this problem?
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How do we identify it?
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CS 215 Fundamentals of Programming II - Lecture 4
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Recursion
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For a number 10 or greater, how can we break
up this problem so that we get a smaller version
of the original problem?
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Consider different possible inputs
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How can we get a single digit of a number?
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Look for ways that are easy to compute
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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In-class Exercise, Part 1
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Copy file from csserver:
/home/hwang/cs215/lecture04/inclass4.cpp
Write the code for WriteVertical, the
recursive function that displays the digits of a
non-negative number vertically on the screen.
This function receives the positive number it is
to display. (There are no other parameters, nor
does it return a result.)
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CS 215 Fundamentals of Programming II - Lecture 4
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How Does Recursion Work?
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Trace functions as follows
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For every function call, draw an arrow from the call
to a new function box.
Inside the box, write down the parameter names
and their correspondence to the actual arguments.
Also write down any local variable names.
Trace the function code starting with the first line.
Substitute parameter values.
When the function returns, draw an arrow back
(with the result, if any) to the call and continue
tracing the code after the call.
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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How Does Recursion Work?
WriteVertical(1234)
Fourth output
number: 1234
number < 10 X
WriteVertical(1234/10)
cout << 1234%10 << endl;
number: 123
123 < 10 X
WriteVertical(123/10)
cout << 123%10 << endl;
Second output
number: 12
number < 10 X
WriteVertical(12/10)
cout << 12%10 << endl
number: 1
number < 10 
cout << 1 << endl
Third output
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
First output
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Iterative Version
// From Savitch, _Problem Solving with C++_
void WriteVertical(int n) {
int tensInN = 1;
int leftEndPiece = n;
// tensInN is a power of ten that has the same number // of digits as n.
while (leftEndPiece > 9) {
leftEndPiece = leftEndPiece/10;
tensInN = tensInN * 10;
}
// Successively print out digits from left to right
for (int powerOf10 = tensInN; powerOf10 > 0; powerOf10 = powerOf10 / 10) {
cout << (n / powerOf10) << endl;
n = n % powerOf10;
}
}
Wednesday, January 19
CS 215 Fundamentals of Programming II - Lecture 4
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In-class Exercise, Part 2
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Extend WriteVertical so that it handles
negative numbers by displaying the negative
sign above the digits.
Example: WriteVertical(­361) would
display
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3
6
1
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CS 215 Fundamentals of Programming II - Lecture 4
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