Introduction to Loops

IterationRepetitionCounting

Loops Also known as

Purpose

To apply the same steps again and again to a block of statements.Recall a block of statement is one or more statement, block usually defined by braces { and }

Most Common Uses of Loops

For countingFor accumulatingFor searchingFor SortingFor displaying tablesFor data entryFor menu processingFor list processingFor simulations.

Types of loops

for (start condition; stop condition; change expression)while (stop condition)do..while(stop condition)

C/C++ Loop Structures

Pre-testfor loops (most common)

When you know how many times (fixed condition)

while loopsWhen you don’t know how many times

Event controlled (variable condition)

Post-testdo … while loops

When you don’t know how many times, but you know you need at least one pass.

What you need to think about to construct a loop

What you are going to control the loop with?A counter

Keep repeating a number of times

An eventKeep repeating until controller takes a particular value.More generally:Keep repeating until something happens (You define the event).

Usually a controller variable is needed.What you are going to initialise your controller with?How does your controller change?What is the condition of controller that stops loop?

The for Statement Syntax

for (init controller; Ctrl expression; Alter controller){

statements;}

Example:for (count=1; count < 7; count++) {

cout << “count = ” << count << endl;}next statement;

start condition change expressionstop condition

The for StatementUsed as a counting loopUsed when we can work out in advance the number of iterations.Semicolons separate the itemsYou may declare variable and initialize it

example:

for (int count = 1; count < 7; count++){statement(s);

}

int num;cout << "NUMBER\tSQUARE\tCUBE\n“; cout << "------\t------\t----\n";

for (num = 1; num < 11; num++) {cout << num << “\t“;cout << num * num << “\t“;cout << num * num * num<<“\n";

}

A Simple ExampleCreate a table with a for loop

NUMBER SQUARE CUBE---------- ---------- ------ 1 1 1 2 4 8 . . . . . . 10 100 1000

for and if Statements working together.

Simple search for divisorsGiven an integer number find all the numbers that divide exactly into it (except 1 and itself).

e.g. if number = 12, divisors are 2,3,4,6

Thinks I canuse %

operator to find divisors

Solution Design

Get the number for userBy starting with check number= 2 and finish with check number 1 less than number (is this efficient?)

find the remainder of dividing number with current check numberif remainder is 0 display current check number as a divisor.otherwise don’t display anything

Program segment for finding divisors of an integer

cout << “Enter an integer :”;

cin >> number;

for (j = 2; j < number; j = j + 1){ if (number % j == 0){

cout << j << “ is a divisor of “;

cout << number << ‘\n’;

}sum = sum + j;

}

for (j = 0, j < n, j = j + 3)

// commas used when semicolons needed

for (j = 0; j < n)

// three parts needed

for (j = 0; j >= 0; j++)

?????what is wrong here ?????

Common errors in constructing for Statements

Infinite loops

for (j=0; j>=0; j++) {cout << i << endl;

}

DEMO1 ChrisBe careful with terminating

conditions!

Programming convention

Use of integers called i,j,kYou will see them all the time.Most commonly used as loop controller variablesConventionally used for counters

We will see later that counters often have a dual use as array indices.

When you see i,j,k declared expect to see a loop!

for -- Null (Empty ) ExpressionsExample 1:

j = 1;sum = 0;for ( ; j <= 10; j = j + 1){

sum = sum + j;

}

Empty/Null expression

Start condition outside for control area

We could do this

Example 2:

j = 1;sum = 0;for ( ; j <= 10; ){

sum = sum + j;

j = j + 1;

}

Empty/Null expressionEmpty/Null expression

change expression outside for control area

Still legal!- Nothing but two semi-colons!

Example 3:

j = 1;sum = 0;for ( ; ; ){

sum = sum + j;

j++;if (j > 10) break;

}

stop condition outside for control area

Nested LoopsRecall when a control structure is contained within another control structure, the inner one is said to be nested.

for ...if ...

for ...for...

You may have repetition within decision and vice versa.

Example

int row,col;

for (row = 0; row < 5; row++) {

cout << "\n" <<row;

for (col = 1; col <= 3; col++){

cout <<"\t" << col;

}

cout << "\t**";

}

Nested Loops - Ex.1for within for

Output0 1 2 3 **

1 1 2 3 **

2 1 2 3 **

3 1 2 3 **

4 1 2 3 **

Nested Loops – Example 1

variable row changes from 0 to 4

variable col changes from 1 to 3 for every

time row changes

Tracing Execution through Loops

1 for (row = 0; row < 5; row++) {

2 cout << "\n" <<row;

3 for (col = 1; col <= 3; col++){

4 cout <<"\t" << col;

5 }

6 cout << "\t**";

7 }

Line row col

1 0 ?

2 0 ?

3 0 1

4 0 1

5 0 1

3 0 2

4 0 2

5 0 2

3 0 3

4 0 3

5 0 3

6 0 4

7 0 4

1 1 4

2 1 4

3 1 0

4 1 0

5 1 0

for (exper =1; exper<=4; exper=exper +1) { cout << "\tScores for experiment "<< exper <<":\n";

total = 0.0; for (trial = 1; trial <=6; trial = trial +1) {

cout << "Enter result of trial " << trial <<" : ";cin >> score;total = total + score;

} avg = total/(trial-1); cout << "Avg for experiment "<<exper<< " is “<<

avg<< endl; }

Nested Loops - Ex. 2One off problem

Why trial – 1 ?

Recall Hand trace of nested loop

Inner loop terminated with value 4 one unit greater than terminating condition col <=3Same situation here, inner loop terminates when value of trial becomes 7A value one more than items entered!Need to correct by subtracting one

The while Statement

Syntaxstart condition;while (stop condition{

statements;change statement;

}

Example of while

count = 1;while (count <= 10) {

cout << “count = ” << count << endl;count = count + 1;

}next statement;

While vs. Forwhile is less rigid than forwhile more suitable for event control (clearer syntax) i.e . Keep processing until something happens. E.g. the control variable is assigned a specific value.For is more structuredFor is more suitable for fixed length loops (clearer)For is more common when used to process arrays (see in later lecture on arrays)Can choose either. Many do and just use one or other all the time!

for = while?for (cnt = 1cnt = 1; cnt < 7; cnt++cnt++){cout << …

}

cnt = 1;while (cnt < 7){ cout << … cnt++;

}

The while and for Statement

The loop control expression is tested before any of the statements are executed

Possible never to execute any statement; while needs a separate control variable

initialization before while begins.

The body may contain any number of statements, including branches and other loops

The control variable is changed any time during loop execution for the while (usual to be last statement though.

The control variable is changed after the last statement in the for structure.

The statement immediately after the while or for is executed upon exiting

Example List processingFinding the Largest Value

Get the number of items in the list. Checks to see if that number is positive.

If not positive ask for the number to be entered again Get the first number and use it to initialise the variable that

hold the max value Increment a counter while we have more number to enter

get user to input a number. increment counter compare number with max - Assigns the largest to max.

end loop display max

Design: Needs a counter and a variable to hold maxvalue and a variable to hold number entered by user

int count = 0, n = 0;double max = 0, x = 0;

cout << "The maximum value will be computed.\n";

cout << "How many numbers do you wish to enter? ";cin >> n;

while (n <= 0) {cout << "\nERROR: Positive integer required.\n\n”;cout << "How many numbers to enter? ";cin >> n;

}

cout << “Enter a real number: “;cin >> x;max = x; // first value to max

count++; //entered a number so increase count by one

while (count < n) {cout << “Enter a real number: “;cin >> x;count++; //entered a number so increase count by oneif (max < x) max = x;

}cout << “Maximum value: “ << max << “\n”;}

could read from a file here

Output

The maximum value will be computed.How many numbers do you wish to enter? 4Enter a real number: 1.01 Enter a real number: -3 Enter a real number: 2.2Enter a real number: 7.07000 Maximum value: 7.07

CountersCounters

cout << “Enter “ << n cout << “Enter “ << n << … << … cin >> x;cin >> x;max = x;max = x;

count = count = 11

while (count < nwhile (count < n {{

cout << “LOOP cout << “LOOP number: “number: “

cin >> x;cin >> x; if (max < x)if (max < x) max = x;max = x;

count++;count++;

}}

count n loop executed

1 5 yes

2 yes

3 yes

4 yes

5 no

Running Totalstotal = 0;

count =0while (count <4) {

cout << “Enter a number: “;cin >> num;count++; total = total + num;cout << “The total is “ << total;

}cout << “Grand total is “ << total << endl;

Choice for or while

Personal preferenceUse for for fixed length loopsUse while for variable length loops

Common Errors

using = rather than == in test expressions can cause infinite loops.using == with floating point numbersplacing a semi colon at end of for statement.

for (i=0;i<=10;i++);statement; //only executed once.

using comas as separators in a forfor (i=0, i<=10, i++)

one off problems

SummaryA loop is a section of repeating code

loop normally controlled by control variable, a condition, an alter statement.

Three types of loop, 2 pre-test 1 post test.Conditions may be fixed count or variable count.Always use indentation and braces with loops.Loops are frequently nested.

Demos

Infinite LoopsFunctions of one variable use debugger TablesCounting and accumulatingInteractive whileInteractive forMenu systemsList processingdata validation