03 Expressions Statements Control Flow

8/22/2019 03 Expressions Statements Control Flow

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Statements, Expressions and

Control Flow


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controls the sequence of execution,evaluates an expression, or does nothing(the null statement)

Basically all lines of code

What is a statement?


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Any place you can put a single statement,you can put a compound statement, alsocalled a block.

Non-strictly speaking, we can say that ablock of code acts as one statement.

Block or compound statements


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Anything that evaluates to a value.Expressions are said to return a value.

3.2

PI

SecondsPerMinute

x = a + b; return value is x

y = x = a + b; return value is y

What is an expression?


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a symbol that causes the compiler to takean action

Operators act on operands. All operands

are expressions.

Operators


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= x = a + b;

lvalue term used for an operand thatcan be on the left side of an assignmentoperator

rvalue ?

Example: x = 35; 35 = x; // error, 35 not an lvalue

Assignment operator


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+, -, *, /, %(modulo)

Some notes:

subtraction of unsigned integers that lead tonegative values can produce unpredictable results

beware of division!

23/4 5

23.0/4 5.75

Mathematical Operators


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you can put the same variable on the sameside of the assignment operator myVal = myVal + 2;

you can also use a shorthand notation(called the self-assigned additionoperator): myVal += 2;

myVal /= 2;

Notes on assignment and math operators


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c++; c = c + 1;

c +=1;

c--;

Note: do not use increment or decrementas subexpressions of a larger expression: ((n+2) * (++n)) + 5;

Increment and decrement


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postfix: x++;

prefix: ++x;

When value is not assigned to another

variable no effect. int x = 6, y = 6;

int a = ++x; // a = x = x+1

int b = y++; // b = y; y = y + 1;

The value of x and y is of course 7. a = 7, b = 6

prefix and postfix


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*, /, % +, -

=, +=, -=, *=, /=, %=

Use parentheses to control precedence

Precedence


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Not-so-good: EffortSpent = (((NumDays NumWeekends)

* 24)*(NumSupervisors + NumEngrs));

Better:

ActualDays = NumDays NumWeekends;

ActualHours = ActualDays * 24;

NumPeople = NumSupervisors + NumEngrs;

EffortSpent = ActualHours * NumPeople;

Note on Nesting parentheses


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In C++, zero is considered false, all othervalues are considered true.

Note: there is a data type bool whosevalue can only be TRUE or FALSE.

Program flow is controlled by whether acertain condition is TRUE or FALSE.

Truth


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used to compare whether two numbersare equal, or if one is greater or less thanthe other.

equals ==

not equals !=

greater than >

greater than or equals >=

less than y and y>z?

AND && (x > y) && (y > z)x>y and y>z

OR ||(x > y) || (y > z)

(x > y) or (y > z)

NOT ! !(x > y)is it NOT true that x > y?

Logical operators


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NOT Mathematical

Relational

AND OR

Assignment

More precedence


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Machine Problem #2

Deadline: June 28, 2010


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enables us to branch to different parts ofour code by testing for conditions

if (expression)

statement;

note that a block can replace anystatement:

if (expression){

statement1;

statement2;

}

IF


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Aif (expression) {

statements

}

B

if (expression)

{statements

}

C

if (expression)

{

statements}

Indentation styles


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if (expression){ statement;

}

else { statement;

}

ELSE


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if (expression1) { if (expression2)

statement1;

else {

if (expression3)

statement2; else

statement3;

}

}

else statement4;

Nested IFs


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It can cause by difficult to debug run-timeerrors

if (x > 10)

if (x > 100)

cout


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if (expression1)

statement1;

else if (expression2)

statement2;

else if (expression3)

statement3;

else

statement4;

ELSE IF


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the only ternary operator; i.e. onlyoperator with 3 terms.

(expression1) ? (expression2) :

(expression3)

this reads: if expression1 is true, returnthe value of expression2; else, return thevalue of expression3

absValue = a>b ? a-b : b-a;

Conditional Operator


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Write a program that outputs High if the value of thevariable is greater than 10, and Low if otherwise. Thevariable is of type int and is inputted by the user.

Write a program that obtains input from the user ahigh_value, a low_value and a value from the user. Yourprogram should inform the user if the value is out of range!;otherwise, output thank you!

Enter high value: 10

Enter low value: 3

Enter your choice: 2

value out of range!

Write a program similar to number 1 except that it will inform

the user of the 3 following possibilities: a) number


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Write a program that computes the absolutevalue of the difference of two user inputtedintegers.

Exercises


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similar to the IF statement. allows multi-way branches.

it is less flexible than IF-ELSE becauseswitch statements use constants.

switch


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switch (controlling_expression) { case constant1:

statement_sequence_1;

break;

case constant2:

statement_sequence_2;

break;

default:

default_statement_sequence; }

switch


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if you forget a break statement, your codewill continue until a break statement fromanother case statement is reached or untilthe end of the switch statement

case A:

case a:

cout


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each loop is called an iteration while

for

do-while

Looping

hil


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while (boolean_expression) { statement_1;

statement_2;

}

while

hil


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example:int numOfGreetings;

cin >> numOfGreetings;

while(numOfGreetings > 0) {

cout


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do { statement_1;

statement_2;

} while (boolean_expression);

do - while

d hil


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do {

cout 0) ;

cout


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for (initialization; boolean_expr; update) {

statement_1;

statement_2;

... }

for

f


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for (; numOfGreetings > 0; numOfGreetings--

){

cout


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int numOfGreetings; cin >> numOfGreetings;

for (int i=0; i


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when exit conditions are not satisfied,the program continues to loop forever

your program will hang

Infinite loops

break and continue


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used to alter the flows of while, do-whileand for statements

break

ends the nearest enclosing loop

continue

ends the current loop iteration of the nearestenclosing loop

break and continue

Practice exercises


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Write a program that finds and prints all of theprime numbers between 3 and 100.

Write a program which takes a single integer fromthe user and displays a "pyramid" of this height(between 1-30) made up of of "*" characters on thescreen.

6:**

****

******

********

**********************

Practice exercises

Exercises


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The LeVan Car Rental Company charges30PhP/km if the total mileage is less than100km. If the total mileage is over 100, thecompany charges 30PhP/km for the first 100

km, then it charges 20PhP for any additionalkm over 100. The company gives a 500PhPdiscount for total mileage more than 500km.

Write a program so that the clerk has to input

the number of kilometers and the programwill output the total price

Exercises

READING ASSIGNMENT


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Functions

READING ASSIGNMENT

03 Expressions Statements Control Flow

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