CS 31 Discussion, Week 3

CS 31 Discussion, Week 3

Faisal Alquaddoomi, [email protected]

Office Hours: BH 2432,MW 4:30-6:30pm,

F 12:30-1:30pm (today)

mailto:[email protected]

Agenda

• Understanding program flow– the “Waterfall” metaphor– “If” statements review

• Loops: While, Do-While, For– How to transform one into another

• Project 2/Homework Questions• Time permitting: basic string operations• Even more time permitting: characters

Waterfall Metaphor

• Imagine that your program is like a waterfall– water comes in through the top and can only take

one path through your program– If statements are like “gates” that allow the water

to only take one path and exclude other possible paths• This only applies to alternate paths at the same “level”

as the if statement

• We’ll demonstrate this with a few examples

A Familiar Example#include <iostream>

using namespace std;

int main() {cout << "Hello, world!" << endl;

if (true) {cout << "Path A was taken" << endl;

}

return 0;}

Extension: If-Else#include <iostream>



if (true) {cout << "Path A was taken" << endl;

}else {

cout << “Path B was taken” << endl;}

return 0;}

Multiple, Separate If-Else Statements

int x = 32;

if (x > 5) {cout << "Path A was taken" << endl;x = 16;

}else {


cout << “In between” << endl;

if (x > 30) {cout << “The blue path" << endl;

}else {

cout << “The red path” << endl;}

Nested If Statementsint x = 32;


if (x < 50) {cout << “Inner true path" << endl;

}else {

cout << “Inner false path” << endl;}

}else if (x < 1000) {


cout << “At the end” << endl;

Inside a Nested Ifint x = 32;


if (x < 50) {cout << “Inner true path" << endl;

}else {

cout << “Inner false path” << endl;}

}else if (x < 1000) {


cout << “At the end” << endl;

Loops

• Think of these as “whirlpools” • Come in three flavors: While, Do-While, and

For• They’re actually all equivalent to each other,

as we’ll see later• Which one you decide to use is largely a

matter of convenience

While Loops

• The most basic type of loop• Much like an “If”, has a part that can be true

or false (called the predicate)– If the predicate is true, the loop begins– if it’s false, the program skips over the loop

• Unlike an If, when the program reaches the bottom of the loop, it jumps back to the top– If the predicate is still true, the loop continues– Otherwise, it skips to the bottom and continues

While Loop Flow, Part 1int x = 1;

while (x < 10) {cout << “x is: “ << x << endl;

if (x > 8) {cout << “Almost there…”;

}

x = x + 1;}

cout << “Done! x is ” << x << endl;

While Loop Flow, Part 2int x = 1;

while (x < 10) {cout << “x is: “ << x << endl;


}

x = x + 1;}


Do-While Loops

• Exactly the same as a While loop, with one major difference: the predicate is checked when the loop ends, not when it starts

• Once the program reaches the bottom of the loop, it checks the predicate– If it’s true, the loop jumps back to the top, just like a while,

and starts running again– If it’s false, the loop ends and the program starts up below

the do-while loop• Because the predicate is checked at the end, do-while

loops always run at least once

Do-While Loop Flow, Part 1int x = 1;

do {cout << “x is: “ << x << endl;


}

x = x + 1;} while (x < 10); // <- note the semicolon


Do-While Loop Flow, Part 2int x = 1;

do {cout << “x is: “ << x << endl;


}

x = x + 1;} while (x < 10); // <- note the semicolon


While Loops Revisited

int x = 1;while (x < 10) {

cout << “x is: “ << x << endl;

x = x + 1;}

• To review, a while loop can have up to three parts:– The predicate (required)– A control variable (optional)– A change to the control variable (also optional)

For Loops, Part 1

• A For loop is just shorthand for a while loop with a control variable that gets changed

int x = 1;while (x < 10) {

cout << “x is: “ << x << endl;

x = x + 1;}

For Loops, Part 2

• A For loop is just shorthand for a while loop with a control variable that gets changed

• All the parts are still there, just written in a shorter form// int x = 1for (int x = 1; x < 10; x = x + 1) {

cout << “x is: “ << x << endl;// x = x + 1

}

Questions about the HW or Project 2?

String Basics

• A string is a type of variable that holds a series of letters (e.g. a paragraph, a sentence, a word...)– Even just a single letter, but there’s a special type for

just one letter, too• Seen in the project as clientName, where the

client’s name is stored• Declaring and assigning to a string:

string myName = “Faisal”;

Basic String Operations

• Printing:cout << “My name: “ << myName << endl;

• Reading:getline(cin, myName);

• Getting the Number of Letters:string myFullName = “Faisal Alquaddoomi”;if (myFullName.size() >= 10)

cout << “Your name is crazy long!”;

– Note that this includes spaces, punctuation, etc.

More String Operations

• Getting a Particular Letter by Position:string myLetters = “abcdefg”;for (int i = 0; i < myLetters.size(); i++)

cout << myLetters[i] << “...”;

• When using myLetters[i], what you get back is a type called a char– short for “character”, i.e. an individual letter

• There are a few elementary operations for chars, but first…

What’s a Char, Exactly?

• A char (again, short for “character”) is actually a small number• Computers “encode” letters as numbers, because at a

hardware level computers only understand numbers– An “encoding” is just a table where every letter (in English, 26 of

them) has a corresponding number– Technically, there’s an entry for each uppercase letter, each

lowercase letter, punctuation, spaces…anything that can go in a string needs to have an entry in the table

• The most widely accepted character encoding is ASCII, the American Standard Code for Information Interchange

• To convert to ASCII, we take our letter (say, ‘A’), and look up the value for it in the ASCII table…

The ASCII Table

Looking up ‘A’ in ASCII

• So, if we look up ‘A’, we find it has value 65• Similarly, lowercase ‘a’ has the value 97• There are a whole bunch of letters in the ASCII

table that are “special” – you don’t need to worry about that

• In code,char someChar = ‘a’;

is exactly the same as sayingchar someChar = 97;

Some Char Operations

• In all the following examples, ‘c’ is a char• isdigit(c): true if it’s a digit (e.g. 0 to 9), false otherwise• isalpha(c): true if it’s a letter, case-independent• islower(c): true if it’s lowercase (e.g. a to z)• isupper(c): true if it’s uppercase (e.g. A to Z)• tolower(c): returns a lowercase version of c if c is a letter,

returns it verbatim otherwise• toupper(c): returns an uppercase version of c if c is a letter,

returns it verbatim otherwise• You can make any of these yourself

– e.g. (c >= ‘a’ && c <= ‘z’) is the same thing as islower(c)

Questions?

Office Hours are immediately, downstairs in room 2432

(from 12:30-1:30pm, but I’m usually there slightly longer)

Functions by Example

#include <iostream>



return 0;}

• The code below should look very familiar by now• Let’s try to figure out what all the “magic”

means…

Functions by Example, Part 2

#include <iostream>



return 0;}

• All functions have three parts,– A name– A return type– A list of parameters (main has no parameters)

Functions by Example, Part 2

#include <iostream>



return 0;}

• The highlighted part below is the body of the function

• When the function is “called”, the body starts running from the top

Multiple Functions#include <iostream>


int sum(int x, int y) {int result = x + y;return result;

}

int main() {int firstNum, secondNum;cout << “Enter two numbers: “ << endl;cin >> firstNum >> secondNum;cout << “Sum of the two is: “ << endl;cout << sum(firstNum, secondNum);

return 0;}




}

int main() {int firstNum, secondNum;cout << “Enter two numbers: “ << endl;cin >> firstNum >> secondNum;cout << “Sum of the two is: “ << endl;int answer = sum(firstNum, secondNum);cout << “Answer: “ << answer << endl;return 0;

}

Call from main() to sum()…




}


}

…returns from sum() back to where we left off in main(), with a value

Functions Review

• main() is a special function that gets called when your program starts– Besides that it’s a normal function

• A function has a name, a return type, a parameter list, and a body

• Like variables, functions must be declared before they’re used– We’ll go over techniques on how to fix that for

program clarity later on

Functions Review, Part 2

• You call a function via myFunctionName(argument1, argument2)– myFunctionName is the name of your function– There must be one argument specified for each

parameter that the function takes• A function *must* return a value (with some

exceptions)• What’s returned must be the same type as the

function

The Exception: Void Functions#include <iostream>


void sum(int x, int y) {int result = x + y;

cout << “The sum is: ” << result;}


}

Is there anything wrong with this picture?

Void Functions, Part 2#include <iostream>


void sum(int x, int y) {int result = x + y;cout << “The sum is: “ << result;

}


}

Yes; a void function has no type, so it can’t be used in expressions

Void Function Review

• Void functions don’t pass back values, but they do have “side effects”– Printing things to the screen, for instance

• Since void functions have no type, they can’t be used in expressions (including assignments)

• They can, however, contain return statements– The return statements look like this:

return; // note the lack of a value

Documents

CS 31 Discussion, Week 3