Engineering 1020 Introduction to Programming Peter King Winter 2010

ENGI 1020: Update Midterm –Date: Feb 17 th (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

ENGI 1020: Control Statements 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

ENGI 1020: Control Statements 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

ENGI 1020: Control Statements 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”

ENGI 1020: Control Statements You've all probably seen this: Check Bank Account Have more than $10k? Buy CarGet Job

ENGI 1020: Control Statements This is an if statement Depending on some condition, we will take a particular path Have more than $10k?

ENGI 1020: Control Statements Let's see it in C++ If (some condition) do something if (some condition){ do something … do something }

ENGI 1020: Control Statements Example if (x > 0) cout << “x is positive.” << endl; if (x < 0){ cout << “ x is negative.” cout << endl; }

ENGI 1020: Control Statements 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

ENGI 1020: Control Statements 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, proceed If (x > 5 && y < 2) proceed();

ENGI 1020: Control Statements 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”

ENGI 1020: Control Statements The if-else statement  Picks between two alternatives if (x >0)‏ Cout << “x is positive” << endl; else Cout << “x is negative” << endl;

ENGI 1020: Control Statements 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;

ENGI 1020: Control Statements If the condition is true  We execute under the if If the condition is not true (false)‏  We execute under the else

ENGI 1020: Control Statements 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(); }

ENGI 1020: Control Statements Statement blocks after the if can contain any valid code, even other if statements

ENGI 1020: Control Statements One more variation  Instead of doing this if (x < 1)‏ doThis()‏; else if(x <2)‏ DoThat()‏; else if(x <3)‏ DoSomething();

ENGI 1020: Control Statements We can do this if (x < 1)‏ dothis(); else if(x < 2)‏ doThat(); else if(x < 3)‏ doSomthing(); else doNothing();

ENGI 1020: Control Statements Grading Examples