Formatting Output/ If

Formatting Output (not required for assignment 1)

This code :          i=15; j=2; k=6;

Cout << "answer is: " << I << j << k;

Produces output:

Answers: 1526

We could hardcode the spacing as:

Cout << "answer is: " << I << j << k;

Giving output

Answer is: 15 2 6

Set Width (setw)

• For integer and string data types, not char

• Takes an integer parameter specifying the width

• Output is right-justified (blanks on left)

• Setw only folds for the very next output item

Ex)

Ans =33;

Num=7132;

Cout << setw(4) << ans << setw(5) << num << setw(6) << "Hi";

__33_7132____Hi

Cout << setw(1) << ans <<setw(5) << num;

33_7132

• Expands automatically to fit the output item

• You can not under specify the value

• Ans is being printed regardless of setw parameters

Cout << "Hi" << setw(5) << ans << num;

Hi___337132

• Setw only applies to the next instruction

Floating-Point numbers

• Setw still works but remember 3.14 requires FOUR positions

• Setprecision means show n significant digits

• Fixed: show output in decimal from (not scientific notation)

• Setprecision(2), with fixed, says to display two decimal places

• Setprecision, unlike setw, remains in effect until you specify another setprecision call

• Showpoint means always show the decimal point (even if .0)

Ex)

Float x=4.88;

Cout << setw(10) << setprecision(3) << x;

_____4.880

Cout << setw(5) << setprecision(1) << x;

__4.9

• Round off last displayed digit

The computer does not know what setw is so…

^^ are defined in the iomanip header file.

• Thus add the header to the top of our C++ program:

#include <iomanip>

• Want to include common math functions (sqrt)

• Add the cmath header file, #include <cmath>

• Things like this can be found in the text book

Flow of Control

• The order of which program statements are executed

• Before now we have focused on sequential flow of control

  • Execute the first instruction then the second ect.

  • If we want it to loop and run another block of code again

What is a control structure?

• Special instructions that transfer control to a statement other than the one that physically comes next

The selection Control Structure (if)

• Commonly called "if"

• Forces the computer to choose between alternate actions

• We make an assertion (a claim that is either true of false)

• If the assertion is true, the computer executes one statement otherwise if executes a different instruction

Ex)

If (age > 40)

Cout << "You are old";

• Why is there no ; after the id statement

• It is a control structure, it determines whether to execute the following line

The Bool Data Type

• Only has two values, the constants true and false

• Logical data type

• Way simpler than int

Ex)

Bool done; //set to true when work is finished

Done = true; // statement somewhere in my program

If statements

What is undesirable about the following code?

If (mark >= 80)

Cout << "Good Work!";

If (mark < 80)

Cout << "Try Harder!";

• If the first expression is true then the second must be false, and vise versa

  • It makes no sense to check the mark twice

• We also forcing the computer to look up the value of mark twice

• The two ifs are dependent

• The proper approach is to use an if-Else, also called a Two-Way If.

If(mark >=80)

Cout << "Good Work!";

Else

Cout << "Try Harder!";

Rational Operators

• We can compare expressions using the following relational operators: == (equal to), != (not equal to), <, >, <=, =>.

  • We always use == when checking a quality

Ex)

Int age;

Age=20;

Age 10*2;

Ex)

Bool lessThan;

lessThan = I < j; //I is compared to j using the < operator. If expression is true, then lessThan is assigned true.

Relational Expressions

• A relational expression is an expression followed by a relational operator followed by an expression

• The result of a relational expression is of type bool.

Ex)

What if I need more than one statement in each block?

• Then you need to add {} to delimit the instructions

Ex)

If (mark>=80)

{

Cout << "Good Work!";

Cout << "Thought about grad school?";

}

Else

{

Cout << "Try Harder!";

Cout << "come to my office hours.";

}

Without brackets, what is the difference between:

If (mark>=80)

Cout << "Good Work!";

Cout << "Thought about grad school?";

And

If (mark >=80)

Cout << "Good Work!";

Cout << "Thought about grad school?";

Nothing is different! The computer does not care about spacing, its for readability for programmers

• If someone's mark was 50 the printout would be "thought about grad school?"

Nested if statements

• There is no restriction on what can appear in a block ({…}) in an if statements

• If could even be another if

Ex)

If (month == 1)

Cout << "January";

Else

If (month == 2)

Cout << "February";

Else

If (month ==3)

…

Nested if (commonly displayed as:)

If (month ==1)

Cout << "January";

Else if (month ==2)

Cout << "February";

Else if (month ==3)

…

Else

Cout << "error" //to check bad data (#>12)

The Dangling Else Problem

• To which if does Else belong?

• In the absence of braces, an else is always paired with the closest preceding if that doesn't already have an else paired with it

Ex)

If (average >= 50.0)

{

If (average < 60.0)

Cout << "Barley Passing";

}

Else

Cout << "failing"; // else is paired with the closed if

• Without braces, the else is paired with the inner if

• Hence, if Mary's

Boolean Operators (logical Operators)

• In addition to relational operators, we can express Boolean operators in C++ as:

The truth tables for these operators are:

&& - both x and y must be true

|| - at least one of x and y

! - the opposite truth value of x

Ex)

Int finalScoare = 45;

Int midtermScore =95;

Independent learning

• Construct a truth table to show !(a==b || a==c) is equivalent to a !=b && a!=c

To summarize, an assertion in an IF statement is made using Boolean expression such as:

1. A Boolean variable or constant

2. A relational expression

3. A Boolean expression

Ex)

1. If (done ==false)

2. If (mark > 50)

3. If (mark > 50) && (done ==false))