Engineering 1020

Engineering 1020

Introduction to Programming

Peter [email protected]

www.engr.mun.ca/~peter

Winter 2010

ENGI 1020: Update

• Midterm– Date: Feb 17th (unchanged)

– Time: 7pm to 8:15pm• This is the same as all other core courses

– Location: To Be Announced

ENGI 1020: Control Statements

• Up to now all our programming has followed the following flow:

– Start program at main– Execute each instruction once– For instructions that are function calls,

• go to the function declaration and start at first instruction

– When instruction is finished, go to next instruction


• How can we solve problems like– Given two numbers, return the largest one

– Find the square root of a number, but only when it's greater than zero

– Given a person's age, output whether they are a child, teenager, adult, or senior

– Given the distance to a wall, tell a robot to stop when it is less than 4m from a wall


• We need to enable the program to make “decisions”

• Or more formally

– Depending on some condition(s), execute alternative sections of code

• At some point in the code, we will choose to execute one block, instead of another


• How is this done you ask?

• The If statement

– “if the door is locked, I will get a key”– “if traffic is bad, I will walk to work”– “if it's later than 9pm, I will go home”


• You've all probably seen this:

CheckBank

Account

Have more than $10k?

Buy Car Get Job


• This is an if statement

• Depending on some condition, we will take a particular path

Have more than $10k?


• Let's see it in C++

• If (some condition) do something

• if (some condition){do somethingdo something… …do something}


• Example

if (x > 0) cout << “x is positive.” << endl;

if (x < 0){cout << “ x is negative.”cout << endl;}


• The if is a keyword• The ( condition ) is an expressions that is

evaluated as either true or false

– x > 0– y != 5– z == 2*y

• When the condition is true, the statement (or block of statements) are executed

• If not true, then the statements are ignored


• Lets look at the conditions– They are boolean expressions– They are evaluated to either true or false

– We can utilize multiple conditions using the • && → and operator• || → or operator

– If x is greater than 5 and y is less than 2, proceedIf (x > 5 && y < 2)

proceed();


What if we want to select one or the other statements, based on a single condition?

“IF there is any 7-up, I'll have that, else I'll have a Sprite”


The if-else statement Picks between two alternatives

if (x >0) Cout << “x is positive” << endl;

else Cout << “x is negative” << endl;


Or Since we know only one of the statements will get

executed

if (x >0) Cout << “x is positive”;

else Cout << “x is negative”;

cout << endl;


If the condition is true We execute under the if

If the condition is not true (false) We execute under the else


We can also nest our if statements What does that mean?

If time is later than 12pm and earlier than 1pm, eat lunch

if (time > 12){If (time < 13){

eatLunch();}

}


Statement blocks after the if can contain any valid code, even other if statements


One more variation Instead of doing this

if (x < 1)doThis();

elseif(x <2)

DoThat();else

if(x <3)DoSomething();


We can do this

if (x < 1)dothis();

else if(x < 2)doThat();

else if(x < 3)doSomthing();

elsedoNothing();


Grading Examples

Engineering 1020

Documents

Transcript of Engineering 1020