Computation Chapter 4

Chapter 4Chapter 4
ComputationComputation
Bjarne StroustrupBjarne Stroustrup
www.stroustrup.com/Programmingwww.stroustrup.com/Programming

22
AbstractAbstract
 Today, I’ll present the basics of computation. InToday, I’ll present the basics of computation. In
particular, we’ll discuss expressions, how toparticular, we’ll discuss expressions, how to
iterate over a series of values (“iteration”), anditerate over a series of values (“iteration”), and
select between two alternative actionsselect between two alternative actions
(“selection”). I’ll also show how a particular sub-(“selection”). I’ll also show how a particular sub-
computation can be named and specifiedcomputation can be named and specified
separately as a function. To be able to performseparately as a function. To be able to perform
more realistic computations, I will introduce themore realistic computations, I will introduce the
vectorvector type to hold sequences of values.type to hold sequences of values.
 Selection, Iteration, Function, VectorSelection, Iteration, Function, Vector
Stroustrup/Programming/2015Stroustrup/Programming/2015

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OverviewOverview
 ComputationComputation
 What is computable? How best to compute it?What is computable? How best to compute it?
 Abstractions, algorithms, heuristics, data structuresAbstractions, algorithms, heuristics, data structures
 Language constructs and ideasLanguage constructs and ideas
 Sequential order of executionSequential order of execution
 Expressions and StatementsExpressions and Statements
 SelectionSelection
 IterationIteration
 FunctionsFunctions
 VectorsVectors

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You already know most of thisYou already know most of this
 Note:Note:
 You know how to do arithmeticYou know how to do arithmetic
 d = a+b*cd = a+b*c
 You know how to selectYou know how to select
““if this is true, do that; otherwise do something else ”if this is true, do that; otherwise do something else ”
 You know how to “iterate”You know how to “iterate”
 ““do this until you are finished”do this until you are finished”
 ““do that 100 times”do that 100 times”
 You know how to do functionsYou know how to do functions
 ““go ask Joe and bring back the answer”go ask Joe and bring back the answer”
 ““hey Joe, calculate this for me and send me the answer”hey Joe, calculate this for me and send me the answer”
 What I will show you today is mostly just vocabulary andWhat I will show you today is mostly just vocabulary and
syntax for what you already knowsyntax for what you already know

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 Input: from keyboard, files, other input devices, other programs, otherInput: from keyboard, files, other input devices, other programs, other
parts of a programparts of a program
 Computation – what our program will do with the input to produce theComputation – what our program will do with the input to produce the
output.output.
 Output: to screen, files, other output devices, other programs, other partsOutput: to screen, files, other output devices, other programs, other parts
of a programof a program
(input) data (output) data
data
Code, often messy,
often a lot of code

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 Our job is to express computationsOur job is to express computations
 CorrectlyCorrectly
 SimplySimply
 EfficientlyEfficiently
 One tool is called Divide and ConquerOne tool is called Divide and Conquer
 to break up big computations into many little onesto break up big computations into many little ones
 Another tool is AbstractionAnother tool is Abstraction
 Provide a higher-level concept that hides detailProvide a higher-level concept that hides detail
 Organization of data is often the key to good codeOrganization of data is often the key to good code
 Input/output formatsInput/output formats
 ProtocolsProtocols
 Data structuresData structures
 Note the emphasis on structure and organizationNote the emphasis on structure and organization
 You don’t get good code just by writing a lot of statementsYou don’t get good code just by writing a lot of statements

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Language featuresLanguage features
 Each programming language feature exists to expressEach programming language feature exists to express
a fundamental ideaa fundamental idea
 For exampleFor example
 ++ : addition: addition
 ** : multiplication: multiplication
 if (if (expressionexpression)) statementstatement elseelse statement ;statement ; selectionselection
 while (while (expressionexpression)) statementstatement ;; iterationiteration
 f(x);f(x); function/operationfunction/operation
 ……
 We combine language features to create programsWe combine language features to create programs

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ExpressionsExpressions
//// compute area:compute area:
int length = 20;int length = 20; //// the simplest expression: a literal (here, 20)the simplest expression: a literal (here, 20)
//// (here used to initialize a variable)(here used to initialize a variable)
int width = 40;int width = 40;
int area = length*width;int area = length*width; //// a multiplicationa multiplication
int average = (length+width)/2;int average = (length+width)/2; //// addition and divisionaddition and division
The usual rules of precedence apply:The usual rules of precedence apply:
a*b+c/da*b+c/d meansmeans (a*b)+(c/d)(a*b)+(c/d) and notand not a*(b+c)/da*(b+c)/d..
If in doubt, parenthesize. If complicated, parenthesize.If in doubt, parenthesize. If complicated, parenthesize.
Don’t write “absurdly complicated” expressions:Don’t write “absurdly complicated” expressions:
a*b+c/d*(e-f/g)/h+7a*b+c/d*(e-f/g)/h+7 //// too complicatedtoo complicated
Choose meaningful names.Choose meaningful names.

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ExpressionsExpressions
 Expressions are made out of operators and operandsExpressions are made out of operators and operands
 Operators specify what is to be doneOperators specify what is to be done
 Operands specify the data for the operators to work withOperands specify the data for the operators to work with
 Boolean type:Boolean type: boolbool ((truetrue andand falsefalse))
 Equality operators:Equality operators: = == = (equal),(equal), !=!= (not equal)(not equal)
 Logical operators:Logical operators: &&&& (and),(and), |||| (or),(or), !! (not)(not)
 Relational operators:Relational operators: << (less than),(less than), >> (greater than),(greater than), <=<=,, >=>=
 Character type:Character type: charchar (e.g.,(e.g., 'a''a',, '7''7', and, and '@''@'))
 Integer types:Integer types: short, int, longshort, int, long
 arithmetic operators:arithmetic operators: +, -, *, /, %+, -, *, /, % (remainder)(remainder)
 Floating-point types: e.g.,Floating-point types: e.g., float, doublefloat, double (e.g.,(e.g., 12.4512.45 andand 1.234e31.234e3))
 arithmetic operators:arithmetic operators: +, -, *, /+, -, *, /

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Concise OperatorsConcise Operators
 For many binary operators, there are (roughly) equivalentFor many binary operators, there are (roughly) equivalent
more concise operatorsmore concise operators
 a += ca += c meansmeans a = a+ca = a+c
 a *= scalea *= scale meansmeans a = a*scalea = a*scale
 ++a++a meansmeans a += 1a += 1
oror a = a+1a = a+1
 ““Concise operators” are generally better to useConcise operators” are generally better to use
(clearer, express an idea more directly)(clearer, express an idea more directly)

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StatementsStatements
 A statement isA statement is
 an expression followed by a semicolon, oran expression followed by a semicolon, or
 a declaration, ora declaration, or
 a “control statement” that determines the flow of controla “control statement” that determines the flow of control
 a = b;a = b;
 double d2 = 2.5;double d2 = 2.5;
 if (x == 2) y = 4;if (x == 2) y = 4;
 while (cin >> number) numbers.push_back(number);while (cin >> number) numbers.push_back(number);
 int average = (length+width)/2;int average = (length+width)/2;
 return x;return x;
 You may not understand all of these just now, but you will …You may not understand all of these just now, but you will …

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SelectionSelection
 Sometimes we must select between alternativesSometimes we must select between alternatives
 For example, suppose we want to identify the larger of twoFor example, suppose we want to identify the larger of two
values. We can do this with anvalues. We can do this with an ifif statementstatement
if (a<b)if (a<b) //// Note: No semicolon hereNote: No semicolon here
max = b;max = b;
elseelse //// Note: No semicolon hereNote: No semicolon here
max = a;max = a;
 The syntax isThe syntax is
if (condition)if (condition)
statement-1statement-1 //// if the condition is true, do statement-1if the condition is true, do statement-1
elseelse
statement-2statement-2 //// if not, do statement-2if not, do statement-2

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Iteration (while loop)Iteration (while loop)
 The world’s first “real program” running on a stored-programThe world’s first “real program” running on a stored-program
computer (David Wheeler, Cambridge, May 6, 1949)computer (David Wheeler, Cambridge, May 6, 1949)
//// calculate and print a table of squares 0-99:calculate and print a table of squares 0-99:
int main()int main()
{{
int i = 0;int i = 0;
while (i<100) {while (i<100) {
cout << i <<cout << i << ''tt'' << square(i) <<<< square(i) << ''nn'';;
++i ;++i ; //// incrementincrement ii
}}
}}
//// (No, it wasn’t actually written in C++(No, it wasn’t actually written in C++ .).)

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Iteration (while loop)Iteration (while loop)
 What it takesWhat it takes
 A loop variable (control variable);A loop variable (control variable); here:here: ii
 Initialize the control variable;Initialize the control variable; here:here: int i = 0int i = 0
 AA termination criterion;termination criterion; here: ifhere: if i<100i<100 is false, terminateis false, terminate
 Increment the control variable;Increment the control variable; here:here: ++i++i
 Something to do for each iteration;Something to do for each iteration; here:here: cout << …cout << …
int i = 0;int i = 0;
while (i<100) {while (i<100) {
++i ;++i ; //// incrementincrement ii
}}

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Iteration (for loop)Iteration (for loop)
 Another iteration form: theAnother iteration form: the forfor looploop
 You can collect all the control information in oneYou can collect all the control information in one
place, at the top, where it’s easy to seeplace, at the top, where it’s easy to see
for (int i = 0; i<100; ++i) {for (int i = 0; i<100; ++i) {
}}
That is,That is,
for (for (initializeinitialize;; conditioncondition ;; incrementincrement ))
controlled statementcontrolled statement
Note: what isNote: what is square(i)square(i)??

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FunctionsFunctions
 But what wasBut what was square(i)?square(i)?
 A call of the functionA call of the function square()square()
int square(int x)int square(int x)
{{
return x*x;return x*x;
}}
 We define a function when we want to separate aWe define a function when we want to separate a
computation because itcomputation because it
 is logically separateis logically separate
 makes the program text clearer (by naming the computation)makes the program text clearer (by naming the computation)
 is useful in more than one place in our programis useful in more than one place in our program
 eases testing, distribution of labor, and maintenanceeases testing, distribution of labor, and maintenance

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Control FlowControl Flow
{{
i=0;i=0;
while (i<100)while (i<100)
{{
square(i);square(i);
}}
}}
{{
//// compute square rootcompute square root
return x * x;return x * x;
}}i<100
i==100

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FunctionsFunctions
 Our functionOur function
{{
return x*x;return x*x;
}}
is an example ofis an example of
Return_type function_nameReturn_type function_name (( Parameter listParameter list ))
//// (type name, etc.)(type name, etc.)
{{
//// use each parameter in codeuse each parameter in code
returnreturn some_valuesome_value;; //// ofof Return_typeReturn_type
}}

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Another ExampleAnother Example
 Earlier we looked at code to find the larger of two values.Earlier we looked at code to find the larger of two values.
Here is a function that compares the two values and returnsHere is a function that compares the two values and returns
the larger value.the larger value.
int max(int a, int b)int max(int a, int b) //// this function takes 2 parametersthis function takes 2 parameters
{{
if (a<b)if (a<b)
return b;return b;
elseelse
return a;return a;
}}
int x = max(7, 9);int x = max(7, 9); //// x becomes 9x becomes 9
int y = max(19, -27);int y = max(19, -27); //// y becomes 19y becomes 19
int z = max(20, 20);int z = max(20, 20); //// z becomes 20z becomes 20

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Data for Iteration - VectorData for Iteration - Vector
 To do just about anything of interest, we need a collection ofTo do just about anything of interest, we need a collection of
data to work on. We can store this data in adata to work on. We can store this data in a vectorvector. For example:. For example:
//// read some temperatures into a vector:read some temperatures into a vector:
{{
vector<double> temps;vector<double> temps; //// declare a vector of type double to storedeclare a vector of type double to store
//// temperatures – like 62.4temperatures – like 62.4
double temp;double temp; //// a variable for a single temperature valuea variable for a single temperature value
while (cin>>temp)while (cin>>temp) //// cin reads a value and stores it in tempcin reads a value and stores it in temp
temps.push_back(temp);temps.push_back(temp); //// store the value of temp in the vectorstore the value of temp in the vector
//// … do something …… do something …
}}
//// cin>>tempcin>>temp will return true until we reach the end of file or encounterwill return true until we reach the end of file or encounter
//// something that isn’t a double: like the word “end”something that isn’t a double: like the word “end”

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VectorVector
 Vector is the most useful standard library data typeVector is the most useful standard library data type
 aa vector<T>vector<T> holds an sequence of values of typeholds an sequence of values of type TT
 Think of a vector this wayThink of a vector this way
A vector namedA vector named vv contains 5 elements: {1, 4, 2, 3, 5}:contains 5 elements: {1, 4, 2, 3, 5}:
1 4 2 3 5
5v:
v’s elements:
v[0] v[1] v[2] v[3] v[4]
size()

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VectorsVectors
vector<int> v;vector<int> v; //// start off emptystart off empty
v.push_back(1);v.push_back(1); //// add an element with the valueadd an element with the value 11
v.push_back(4);v.push_back(4); //// add an element with the valueadd an element with the value 44 at end (“the back”)at end (“the back”)
v.push_back(3);v.push_back(3); //// add an element with the valueadd an element with the value 33 at end (“the back”)at end (“the back”)
v[0]v[0] v[1] v[2]v[1] v[2]
0v:
3
2
1 1
41
341
v:
v:
v:

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VectorsVectors
 Once you get your data into a vector you can easily manipulate itOnce you get your data into a vector you can easily manipulate it
//// compute mean (average) and median temperatures:compute mean (average) and median temperatures:
{{
vector<double> temps;vector<double> temps; //// temperatures in Fahrenheit, e.g. 64.6temperatures in Fahrenheit, e.g. 64.6
double temp;double temp;
while (cin>>temp) temps.push_back(temp); //while (cin>>temp) temps.push_back(temp); // read and put into vectorread and put into vector
double sum = 0;double sum = 0;
for (int i = 0; i< temps.size(); ++i) sum += temps[i]; //for (int i = 0; i< temps.size(); ++i) sum += temps[i]; // sums temperaturessums temperatures
cout << "Mean temperature: " << sum/temps.size() << 'n';cout << "Mean temperature: " << sum/temps.size() << 'n';
sort(temps);sort(temps); //// from std_lib_facilities.hfrom std_lib_facilities.h
//// or sort(temps.begin(), temps.end();or sort(temps.begin(), temps.end();
cout << "Median temperature: " << temps[temps.size()/2] << 'n';cout << "Median temperature: " << temps[temps.size()/2] << 'n';
}}

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Traversing a vectorTraversing a vector
 Once you get your data into a vector you can easily manipulate itOnce you get your data into a vector you can easily manipulate it
 Initialize with a listInitialize with a list
 vector<int> v = { 1, 2, 3, 5, 8, 13 }; //vector<int> v = { 1, 2, 3, 5, 8, 13 }; // initialize with a listinitialize with a list
 often we want to look at each element of a vector in turn:often we want to look at each element of a vector in turn:
for (int i = 0; i< v.size(); ++i) cout << v[i] << 'n'; //for (int i = 0; i< v.size(); ++i) cout << v[i] << 'n'; // list all elementslist all elements
//// there is a simpler kind of loop for that (a range-for loop):there is a simpler kind of loop for that (a range-for loop):
for (int i : v) cout << x << 'n'; //for (int i : v) cout << x << 'n'; // list all elementslist all elements
//// for each x in v …for each x in v …

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Combining Language FeaturesCombining Language Features
 You can write many new programs by combiningYou can write many new programs by combining
language features, built-in types, and user-definedlanguage features, built-in types, and user-defined
types in new and interesting ways.types in new and interesting ways.
 So far, we haveSo far, we have
 Variables and literals of typesVariables and literals of types bool, char, int, doublebool, char, int, double

vector, push_back(), [ ]vector, push_back(), [ ] (subscripting)(subscripting)
 !=, ==, =, +, -, +=, <, &&, ||, !!=, ==, =, +, -, +=, <, &&, ||, !
 max( ), sort( ), cin>>, cout<<max( ), sort( ), cin>>, cout<<
 if, for, whileif, for, while
 You can write a lot of different programs with theseYou can write a lot of different programs with these
language features! Let’s try to use them in a slightlylanguage features! Let’s try to use them in a slightly
different way…different way…

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Example – Word ListExample – Word List
//// “boilerplate” left out“boilerplate” left out
vector<string> words;vector<string> words;
for (string s; cin>>s && s != "quit“; )for (string s; cin>>s && s != "quit“; ) //// && means AND&& means AND
words.push_back(s);words.push_back(s);
sort(words);sort(words); //// sort the words we readsort the words we read
for (string s : words)for (string s : words)
cout << s << 'n';cout << s << 'n';
/*/*
read a bunch of strings into a vector of strings, sortread a bunch of strings into a vector of strings, sort
them into lexicographical order (alphabetical order),them into lexicographical order (alphabetical order),
and print the strings from the vector to see what we have.and print the strings from the vector to see what we have.
*/*/

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Word list – Eliminate DuplicatesWord list – Eliminate Duplicates
//// Note that duplicate words were printed multiple times. ForNote that duplicate words were printed multiple times. For
//// example “the the the”. That’s tedious, let’s eliminate duplicates:example “the the the”. That’s tedious, let’s eliminate duplicates:
for (string s; cin>>s && s!= "quit"; )for (string s; cin>>s && s!= "quit"; )
sort(words);sort(words);
for (int i=1; i<words.size(); ++i)for (int i=1; i<words.size(); ++i)
if(words[i-1]==words[i])if(words[i-1]==words[i])
““get rid of words[i]” // (pseudocode)get rid of words[i]” // (pseudocode)
for (string s : words)for (string s : words)
cout << s << 'n';cout << s << 'n';
//// there are many ways to “get rid of words[i]”; many of them are messythere are many ways to “get rid of words[i]”; many of them are messy
//// (that’s typical). Our job as programmers is to choose a simple clean(that’s typical). Our job as programmers is to choose a simple clean
//// solution – given constraints – time, run-time, memory.solution – given constraints – time, run-time, memory.

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Example (cont.) Eliminate Words!Example (cont.) Eliminate Words!
//// Eliminate the duplicate words by copying only unique words:Eliminate the duplicate words by copying only unique words:
for (string s; cin>>s && s!= "quit"; )for (string s; cin>>s && s!= "quit"; )
sort(words);sort(words);
vector<string>w2;vector<string>w2;
if (0<words.size()) {if (0<words.size()) { //// note style { }note style { }
w2.push_back(words[0]);w2.push_back(words[0]);
for (int i=1; i<words.size(); ++i) //for (int i=1; i<words.size(); ++i) // note: not a range-note: not a range-forfor
if(words[i-1]!=words[i])if(words[i-1]!=words[i])
w2.push_back(words[i]);w2.push_back(words[i]);
}}
cout<< "found " << words.size()-w2.size() << " duplicatesn";cout<< "found " << words.size()-w2.size() << " duplicatesn";
for (string s : w2)for (string s : w2)
cout << s << "n";cout << s << "n";

2929
AlgorithmAlgorithm
 We just used a simple algorithmWe just used a simple algorithm
 An algorithm is (from Google search)An algorithm is (from Google search)
 ““a logical arithmetical or computational procedure that, if correctlya logical arithmetical or computational procedure that, if correctly
applied, ensures the solution of a problem.” –applied, ensures the solution of a problem.” – Harper CollinsHarper Collins
 ““a set of rules for solving a problem in a finite number of steps, as fora set of rules for solving a problem in a finite number of steps, as for
finding the greatest common divisor.” –finding the greatest common divisor.” – Random HouseRandom House
 ““a detailed sequence of actions to perform or accomplish some task.a detailed sequence of actions to perform or accomplish some task.
Named after an Iranian mathematician, Al-Khawarizmi. Technically, anNamed after an Iranian mathematician, Al-Khawarizmi. Technically, an
algorithm must reach a result after a finite number of steps, …The term isalgorithm must reach a result after a finite number of steps, …The term is
also used loosely for any sequence of actions (which may or may notalso used loosely for any sequence of actions (which may or may not
terminate).” –terminate).” – Webster’sWebster’s
 We eliminated the duplicates by first sorting the vectorWe eliminated the duplicates by first sorting the vector
(so that duplicates are adjacent), and then copying only(so that duplicates are adjacent), and then copying only
strings that differ from their predecessor into anotherstrings that differ from their predecessor into another
vector.vector.

3030
IdealIdeal
 Basic language features and libraries should beBasic language features and libraries should be
usable in essentially arbitrary combinations.usable in essentially arbitrary combinations.
 We are not too far from that ideal.We are not too far from that ideal.
 If a combination of features and types make sense,If a combination of features and types make sense,
it will probably work.it will probably work.
 The compiler helps by rejecting some absurdities.The compiler helps by rejecting some absurdities.

3131
The next lectureThe next lecture
 How to deal with errorsHow to deal with errors

Computation Chapter 4

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