Operations and Expressions. Computer Programming 2 Objectives Detailed look at numeric operations Work with bool expressions Consider the processing of

Operations and Expressions

Computer Programming2

Objectives

Detailed look at numeric operations Work with bool expressions Consider the processing of characters Study assignment operator Use increment, decrement operators User-defined operators


Expressions

Definition: any sequence of objects (E.g. literals and variables) and operations that combine them to produce a value is called an expression.

Example:

double pressure = ((depth / FEET_PER_ATM) + 1) * LBS_PER_SQ_IN_PER_ATM;


Operations

Arithmetic/numeric operations (for real numbers and/or integers)– +, -, /, %, *, --, ++

Logic operations (for Booleans)– &&, ||, !

Relational/comparison operations (for comparing expressions)– ==, !=, <, >, <=, >=

Bitwise operations (for manipulating bits)– &, |, >>, <<


Arithmetic/Numeric Expressions

C++ provides four familiar arithmetic operators:– + for performing addition– - for performing subtraction– * for performing multiplication– / for performing division

Each of these four can be applied to either real (double) or integer (int) operands.


Division

Division behaves differently for int and double operands

– Note the results of the following

3/40 3.0/4.00.753.0/4 0.75 3/4.00.75

If both operands are integers– Integer division performed– Otherwise real division performed


Integer vs. Real Division

Recall division from grade school.– Teacher: “4 goes into 3 how many times?” – Pupils: “0 times with a remainder 4”

The expression 3 / 4 returns the quotient.– This is integer division

The expression 3 % 4 returns the remainder.– This is read, "3 mod 4"


Type Conversions

Combining an int and a real in the same expression– 2 + 3.0 5.0

C++ automatically converts narrow values into wider values

– An integer is converted to a real– Result of the expression is a real

Often called "promotion"


Precedence

Consider the possible value of the expression 2 + 3 * 4

(2 + 3) * 4 24 or 2 + (3 * 4) 14

Operator precedence governs evaluation order. * has higher precedence than + * is applied first


Operator Precedence

() HIGHER+ (positive), - (negative), ! (NOT)

*, /, %

<, <=, >, >=

==, !=

&&

|| LOWER

See Appendix C for a complete list...


Associativity

Consider the possible value of the expression 8 - 4 - 2

(8 - 4) - 2 4

or 8 - (4 - 2) 6

Precedence doesn’t help us Associativity tells us.

– Subtraction is left-associative, the left - is evaluated first, giving us 4.

Most (but not all) C++ operators associate left.– See Appendix C in the text for a complete list...


Numeric Functions

The library <cmath> contains a variety of mathematical

functions, including:sin(x) asin(x)cos(x) acos(x)tan(x) atan(x)sqrt(x) log10(x)log(x) pow(x, y)floor(x) ceiling(x)abs(x)


Using <cmath> functions

#include <iostream>#include <cmath>using namespace std;int main() { cout << “\nEnter base and exponent: “; double base, exponent; cin >> base >> exponent; double result = pow(base, exponent); cout << base << “ raised to the power “ << exponent << “ is “ << result << endl; return 0;

}


Type Conversions

Possible to explicitly convert a value from one type to another

Syntax to use:type (expression) or (type) expression

Conversion can cause loss of datadouble x = 3.456;cout << (int) x;

What is the output?3


Boolean/logic Expressions

C++ type bool has two literals– false and true

Relational operators produce boolean expressions


Relational Operations

Use operators for comparisons– Each takes two operands – Produces a bool value (true or false):

x == y x != y

x < y x >= y

x > y x <= y

Warning:– Do NOT confuse = (assignment) – With == (equality).


Logic Expressions

Logical operators


Logic Expressions

More complex boolean expressions can be built using the logical operators:a && b // true iff a, b are both true

a || b // true iff a or b is true

!a // true iff a is false

Example:cin >> score;assert (0 <= score && score <= 100);


Short-Circuit Evaluation

Consider the boolean expression( n != 0 ) && ( x < 1.0 / n )

– If n == 0 the right hand expression causes a program crash

C++ will evaluate the original expression from left to right

– If n == 0, the left expression evaluates false– Since it is an &&, this makes the whole expression false– So, it does not proceed to the right expression


Preconditions

Definition: When a program makes assumptions about its input values

– Example: that they’re positive

Preconditions are boolean expressions – Must be true in order for the program to work correctly.

To check preconditions, C++ provides the assert() mechanism...


Assertions

#include <iostream>#include <cassert>using namespace std;int main() {cout << “\nEnter your age: “;

int age; cin >> age; assert(age > 0); // ...}


Character Expressions

Character variables can be …– Declared and initialized

char middleInitial = 'Q';

– Assigned middleInitial = 'Z';

– Used for I/O cout << middleInitial; cin >> middleInitial;

– Compared assert (middleInitial != 'X');


Character Functions

Boolean character-processing functions found in <ctype> library:isalpha(ch) isalnum(ch)isdigit(ch) iscntrl(ch)islower(ch) isupper(ch)isspace(ch) ispunct(ch)isprint(ch) isgraph(ch)

Case-conversion functions:toupper(ch) tolower(ch)


Assignment

Syntax:variable = expression;

– Expression is evaluated– Value placed in memory location associated with variable

Example:xCoord = 4.56;code = 'T';


Given the sequence of three assignment statements, note the results

Note that previous variable values are gone after execution of assignment

Assignment


Assignment

The assignment operator =– Right-associative, – Supports expressions like:

int w, x, y, z;

w = x = y = z = 0; The rightmost = is applied first,

– assigning z zero, – then y is assigned the value of z (0), – then x is assigned the value of y (0)– finally w is assigned the value of x (0).


Assignment Shortcuts

Some assignments are so common:var = var + x; // add x to var

var = var - y; // sub y from var

C++ provides shortcuts for them:var += x; // add x to var

var -= y; // sub y from var


In General

Most arithmetic expressions of the form:var = var value;

can be written in the “shortcut” form:

var = value; Examples:

double x, y;cin >> x >> y;x *= 2.0; // double x’s valuey /= 2.0; // decrease y by half


Increment and Decrement

Other common assignments include:var = var + 1; // add 1 to var

var = var - 1; // sub 1 from var C++ provides shortcuts for them, too:

var++; // add 1 to var

var--; // sub 1 from var


Prefix Increment

The prefix form of increment produces the final (incremented) value as its result:int x, y = 0;

x = ++y;

cout << x; // 1 is displayed The prefix decrement behaves similarly...


Postfix Increment

The postfix form of increment produces the original (unincremented) value as its result:int x, y = 0;

x = y++;

cout << x; // 0 is displayed

// y now holds value of 1

The prefix decrement behaves similarly...


Prefix vs. Postfix

So long as the increment (or decrement) operator is used as a separate statement:int y = 0, x = 0;

++x; // x == 1

y++; // y == 1

… it makes no difference which version is used.


An expression followed by a semicolon becomes an expression statement

– x = y + z;– 'A';– cos (z);

Expressions into Statements – Semicolons

This expression statement has the added side effect of changing the value of x


I/O Streams

C++ has no I/O as part of the language– I/O streams are provided by istream and ostream

cout cin


Input Expressions

Form– input_stream >> variable;

May be chained together– cin >> x >> y;

Adapts to whatever type variable is– User must enter correct type

Best to prompt user for input expected– cout << "Enter choice (1 – 10) : "


Output Expressions

Form– output_stream << expression;

May be chained – cout << "The sum = " << sum;

The expression can be a variable, a constant or a combination using operators– cout << "Sum = " << v1 + v2 + v3;

The expression adapts to whatever types are used


Output Formatting

Possible to specify appearance of output– From iostream:

showpoint Display decimal point and trailing zeros for all real numbers. noshowpoint Hide decimal point and trailing zeros for whole real numbers

(default). fixed Use fixed-point notation for real values. scientific Use scientific notation for real values. boolalpha Display boolean values as strings “true” and “false”. left Display values left justified within a field. right Display values right justified within a field (default).

– From iomanip: setw(w) Display the next value in a field of size w (default 1). setprecision(p) Display p fractional digits for all subsequent output of

real values (common default is 6).

Must specify the proper include files to use these.

#include <iostream>#include <iomanip>using namespace std;


User-defined operators

Implement the member function operatorXXX– cout << … is the same as cout.operator<<(…)– E.g. operator++, operator--, etc..

class Person {…

//instead of say string operator<<(string s) { return s; } };int main () {

Person belaid; cout << (belaid<<“Hello Everyone!”)<<endl; return 0;}


General

Behavior

Problem

For energy released, enter mass

(Must be non-negative): 123

Energy released = 999.99 units

Use Einstein's equation to calculate the amount of energy released by a quantity of mass given in the problem


Objects

DescriptionSoftware Objects

Type Kind Name

screen ostream varying cout

prompt string constant

quantity of matter double varying mass

keyboard istream varying cin

qty of energy double varying energy

descriptive label string constant


Operations

i.Display a string (the prompt) on the screen

ii.Read a nonnegative number (mass) from the keyboard

iii.Compute energy from mass

iv.Display a number (energy) and a string on the screen


Additional Elements

We note that step iii. requires Einstein's equation

This implies a few more operationsAnd additional objects

2e m c

v. Exponentiationvi. Multiplication of realsvii. Storage of a real

DescriptionSoftware Objects

Type Kind Name

Speed of light double constant SPEED_OF_LIGHT

2 int constant


Algorithm

We organize the objects and operations into an algorithm:

1.Declare the constant SPEED_OF_LIGHT.

2.Display to cout a prompt for the mass to be converted into energy.

3.Read a nonnegative number from cin into mass.

4.Compute

5.Display to cout a descriptive label and energy.

2energy = mass SPEED_OF_LIGHT


Coding, Execution, Testing

Code the algorithm– Put the algorithm into “C++ words”

Compile– Fix syntax errors

Execute – Fix runtime errors

Test– Fix logic and runtime errors

Documents

Operations and Expressions. Computer Programming 2 Objectives Detailed look at numeric operations Work with bool expressions Consider the processing of