C++ while Loop

Introduction to Loops: The while Loop

• Loop represents a part of a program that can execute more than once.
• The basic structure of while loop includes checking a condition; if it is true then a statement is run.
• The format for a while loop is while (condition){ statement(s);}
• If the loop contains only one statement, the curly braces {} can be omitted.

How while Loop Works

• First the condition is evaluated; If true, the statement(s) are executed, and then the condition is evaluated again.
• If the condition is false, the loop will exit.
• A loop iteration is when the body of the loop is executed

while Loop Example

• Example:

int val = 5;
while (val >= 0)
{
    cout << val << " ";
    val = val - 1;
}

• The above produces the output: 5 4 3 2 1 0.
• val is called a loop control variable.
• while loop is a pretest loop and condition is evaluated before the loop executes.

Exiting The Loop

• The loop must have code that allows the condition to eventually become false.
• Otherwise, can result in an infinite loop, which does not stop.
• Example:

x = 5;
while (x > 0)
  cout << x; // infinite loop because x is always > 0 is always true

Common Loop Errors

• Don't put ; immediately after (condition)
• The curly braces {} should not be forgotten.

 int numEntries = 1;
 while (numEntries <=3)
    cout << "Still working ... ";
    numEntries++; // not in the loop body

• The = should not be used when == is required

while (numEntries = 3)   // always true
{
    cout << "Still working ... ";
    numEntries++;
}

while Loop Programming Style

• The loop body statements should be indented
• The braces{ and } should align with the loop header and placed ​​on lines by themselves.
• The above conventions make the source code more understandable.

Using The while Loop for Input Validation

• Loops are useful for validating user input data.
• Prompt for and read data
• Use a while loop tests if the data is valid.
• It will enter the loop only if the data is not valid.
• In the loop body, display an error message and asks the user to re-enter the data.
• The loop is exited when the user inputs valid data.

Input Validation Loop Example

• Example:

cout << "Enter a number (1-100) and" << "I will guess it. ";
cin >> number;
While ((number < 1) || (number > 100))
{
    cout << "Number must be between 1 and 100." << "Re-enter your number. ";
    cin >> number;
}
//Code to use the valid number follows

Increment and Decrement Operators

• The increment operator increases the value of a variable. I.e. val++; is the same as val = val + 1;
• The decrement operator reduces the value of a variable. I.e. val--; is the same as val = val – 1;
• The operators can be used in the prefix or postfix mode.

Prefix Mode

• ++val and --val increment or decrement the variable, then return the new value of the variable.
• The new value of the variable is used in other operations within the same statement

Prefix Mode Example

• Example:

int x = 1, y = 1;
x = ++y; // y is incremented to 2, then 2 is assigned to x
cout <<  x<< " " << y; // Displays 2 2

x = --y;  // y is decremented to 1, then 1 is assigned to x
cout << x << " " << y; // Displays 1 1

Postfix Mode

• val++ and val-- return the current value of the variable, then increment or decrement the variable
• It is the returned current value of the variable that is used in any other operations within the same statement

Postfix Mode Example

• Example:

int x = 1, y = 1;

x = y++;  // y++ returns a 1, The 1 is assigned to x and y is incremented to 2
cout << x << " " << y;       // Displays 1 2

 x = y--; // y-- returns a 2, The 2 is assigned to x and y is decremented to 1
cout << x << " " << y; // Displays 2 1

Increment & Decrement Notes

• They can be used in arithmetic expressions; result = num1++ + --num2;
• They must be applied to a variable, not an expression or a literal value; result = (num1 + num2)++; // Illegal
• They can be used in relational expression, for example if (++num > limit)
• Pre- and post-operations will cause different comparisons

Counters

• A counter is a variable that is incremented or decremented each time a loop iterates.
• Counters can control the loop execution.
• They must be initialized before entering the loop.
• They can be incremented/decremented when inside the loop or in the loop test.

Letting the User Control the Loop

• Program can be written so that user input determines loop repetition
• This can be applied when a program processes a list of items, and the user knows the number of items
• The user is prompted before the loop is entered
• User input is used here to control the number of loops

User Controls the Loop Example

• Example:

int num, limit;

cout << "Table of squares\n";
cout << "How high to go? ";
cin >> limit;

cout << "\n\nnumber square\n";
num = 1;
while (num <= limit)
{
	cout << setw(5) << num << setw(6) << num*num << endl;
num++;
}

Keeping a Running Total

• Represents an accumulated sum of numbers from loop iterations.
• When finding a running total, can be found by using an accumulator
• Example:

int sum = 0, num = 1;  // This represents the sum; known as the accumulator
while (num <= 10)  // the accumulator
{
sum += num;
num++;
}
cout << "Sum of numbers 1 – 10 is " << sum << endl;

Sentinels

• A sentinel is a value in a list of values that indicates the end of the list.
• Special value that cannot be confused with a valid value, -999 can be used for the test score value.
• It is used to terminate the input when a user may not know how many values.

Sentinel Example

Example:

int total = 0;
cout << "Enter points earned "  << "(or -1 to finish): ";
cin >> points;

while (points != -1) // -1 is the sentinel
{
 total += points;
 cout << "Enter points earned: ";
 cin >> points;
}

The do-while Loop

• The do-while loop is a post-test loop, and the condition is evaluated after the loop executes.
• The format is do{ statement; } while (condition);
• The {} are not required if the body contains a single statement.
• There is a semi-colon ; after the (condition) is required

do-while Loop Notes

• The loop body always executes at least once.
• Execution continues until true and the loop is exited when condition is false.

do-while and Menu Driven Programs

• do-while can be used in a menu-driven program to bring the user back to the menu to make another choice.
• To simplify processing user input, use the toupper ('to uppercase') or tolower ('to lowercase') function. This allows you to check user input regardless of the case of the input. NOTE: cctype needs to be included

Menu-Driven Program Example

• Example:

do{
   // code to display menu
   // and perform actions
   cout << "Another choice? (Y/N) ";
} while ((choice =='Y')||(choice=='y'));   //The condition could be written as(toupper(choice) == 'Y');or as (tolower(choice) == 'y');

The for Loop

• The loop is a pretest loop that can execute zero or more times as needed.
• Loop format represents the following

for( initialization; test; update )
{
    statement;
}

The for Loop Notes

• If test is false the first time it is evaluated, the body of the loop will not be executed.
• The update expression can increment or decrement by any amount.
• Variables used in the initialization section should not be modified in the body of the loop.

For Loop Modifications

• You can define variables in initialization code
• Their scope is the for loop
• Initialization and update code can contain more than one statement
• Separate the statements with commas
• Example:

for (int sum = 0, num = 1; num <= 10; num++)
    sum += num;

More for Loop Modifications

• Can omit initialization if it’s already done

int sum = 0, num = 1;
for (; num <= 10; num++)
    sum += num;
- Can omit update if it’s done in loop body
for (sum = 0, num = 1; num <= 10;)
        sum += num++;
- Can omit the loop body if all of the work is done in the header

Deciding Each Loop to use

• while: pretest loop (this loops body may not be executed).
• do-while: post test loop (this loop body will always be executed at least once).
• for: pretest loop (this loop body may not be executed); has initialization and update code; is most useful for counters or if the number can be precisely calculated.

Nested Loops

• The nested loop is when one loop exists inside of another loop’s body
• Example:

for (row = 1; row <= 3; row++)  //This is called the outer loop
{
    for (col = 1; col <= 3; col++)        //This is called the inner Inner loop
        {
         cout << row * col << endl;
        }
}

Notes on Nested Loops

• The inner loop goes through all of its iterations for each iteration of the outer loop
• The inner loop completes its iterations faster than the outer loop
• The total number of iterations for inner loop is product of number of iterations of the two loops. In previous example, inner loop iterates 9 times in total.

Breaking Out of A Loop

• Use it sparingly . It’s used to terminate the execution of a loop iteration, making the code harder to understand
• When used in an inner loop, break terminates that loop only and returns to the outer loop

The continue Statement

• The continue statement can go to the end of the loop and prepare for next iteration.
• while and do-while loops go to the test expression and repeat if test is true.
• for loop goes to the update step and then test, and repeats the loop if test condition is true.
• Use continue sparingly - like break, it can make program logic hard to understand

Using Files for Data Storage

• The file can be used instead so it doesn’t appear as computer screen output for your program.
• Files are stored on secondary storage media such as a disk which allows data to be retained between program execution.
• A file can be used instead of a keyboard for program input.

File Types

• A text file has encoded letters such as text, digit and punctuation. A text editor such as notepad is able to view.
• A binary file is binary that cannot be encoded as text, a regular text editor cannot read this.

File Access-Ways to Use the Data in a File

• Sequential access starts reading the first piece of data and continues until last piece of data.
• Random (direct) access to retrieve any piece of data directly, without the need to retrieve preceding data items

What is Needed to Use Files

• First, one must include the ifstream or the ofstream header file.
• Then, define a file stream object for either input/output.
• The input reads data from a file using ifstream inFile;
• The output writes data to a file usingofstream outFile;

Opening the File

• Use the member function.

inFile.open("inventory.dat");
outFile.open("report.txt");

• The files name may include its location on the disc, or the path. It must also include its extensions and the output file is created while an existing file, will be erased first. Also, the input file must exist for the open to work.

Using The File

• To use the file: use output file object and << to send data to a file with outFile << "Inventory report";
• Then use the input file object and >> to copy data from the file to variables with the example below:

    inFile >> partNum;
    inFile >> qtyInStock >> qty0n0rder;

Closing the File

• Use the close member function

    inFile.close();
    outFile.close();

• Ensure that you are not waiting for the operating system due to there being a limit to how many open files, and there is the cause for any missing data.

Input File - The Read Position

• The next piece of data of an input file is known as the read position.
• It is set to the first byte of the file.
• As any data gets read, it advances

User- Specified Filenames

• In a program, users can specify the inputs and outputs required; C++ before 11 required the C-string reprentation.

cout << "Which input file? ";
cin >> inputFileName;
inFile.open(inputFileName.c_str());

• For anything past it, can just pass it to open without the function call.

Using the >> Operator to Test End of File

• This operation indicates whether you get a true or false if the file has been successfully read.
• The read expression fails(false) if you read passed the file and that should always be tested.
• while (inFile >> score)` sum += score; for example;

File Open Errors

• Errors can occur when a file is attempted to be opened due to the file not existing, there is a misspelled filename or in a separate location.
• if (inFile) if the open succeeds, is set to true. and in else there is an error with code cout << "Error on file open\n";

Creating Good Test Data

• If testing a specific program, data is more important than the quantity of the test. There should also be tests to show how different parts execute.
• Normal ,invalid data and those that are at valid limits all need evaluations.