Introduction to While Loops

Introduction to Loops: The while Loop

• A loop is a section of code that runs more than once
• The while loop has the format: while(condition) { statement(s); }
• The curly braces can be left out if there is only one statement in the loop's body

How the while Loop Works

• The condition is evaluated first
• If the condition evaluates to true, the statement(s) are executed, then the condition is checked again
• If the condition evaluates to false, the loop is exited
• An iteration is a single execution of the loop body
• int val = 5; while (val >= 0) { c out << val << " "; val = val - 1; } produces output: 5 4 3 2 1 0

The while Loop

• A pretest loop; the loop condition is evaluated before execution
• If the loop begins as false, the loop will not execute
• If the loop begins as true, the loop will continue to execute until the condition becomes false
• The loop must contain code that will allow the condition to eventually become false; otherwise, it will loop infinitely

Common Loop Errors

• Do not put a semicolon immediately after the conditional statement
• Do not forget curly braces. This may result in unintended code to be executed and the loop may not function as expected
• Do not use = in the conditional statement, use ==

While Loop Programming Style

• The loop body statements should be indented
• Align the curly braces with the loop header and keep them on separate lines

Using the While Loop for Input Validation

• Loops are the appropriate structure for validating user input data
• Prompt for and read the data
• Use a while loop to test if data is valid
• Enter the loop only if data is not valid
• The loop body displays an error message, and prompts the user to re-enter the data
• The loop will not be exited until the user enters valid data

Increment and Decrement Operators

• The Increment operator (++) increases the value of a variable
  • val++; is the same as val = val + 1;
• The Decrement operator (--) decreases the value of a variable
  • val--; is the same as val = val - 1;
• Prefix mode can be used (before) and postfix mode (after) a variable

Prefix Mode

• With the prefix mode (++val and --val), the variable is incremented or decremented, before the value of the variable is returned
• It is the new value of the variable that is used in any other operations used in the same statement

Postfix Mode

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

Increment & Decrement Notes

• They can be used in arithmetic expressions
  • result = num1++ + --num2;
• They must be applied to a variable, not an expression or literal value
  • result = (num1 + num2)++; is illegal
• They can be used in relation expressions
  • 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 be used to control the execution of a loop as a loop control variable
• Counters must be initialized before entering a loop
• Counters can be incremented or decremented inside the loop or in the loop test

Letting the User Control the Loop

• Programs can be such that user input determines loop repetition
• This can be used when program processes a list of items and the user knows the number of items
• The user is prompted before the loop is entered, and the user input is used to control the number of iterations

Keeping a Running Total

• A running total is an accumulated sum of numbers, from iterations of a loop
• An accumulator is a variable that holds running totals

Sentinels

• A sentinel is a value in a list of values, that indicates the end of the list
• It should not be confused with a valid value
• Sentinels are used to terminate input, when the user may not know how many values will be entered

The do-while Loop

• A post test loop where the condition is evaluated after the loop executes
• The format is: do { statements; } while (condition);
• The loop body always executes at least once
• Execution continues as long as the condition is true and will exit when the condition becomes false
• The curly braces are not required if the body contains a single statement
• A semicolon after (condition) is required

The do-while Loop in Menu-Driven Programs

• do-while loops bring the user back to the menu to make another choice in menu-driven programs
• The toupper and tolower functions simplify the processing of user input and allows checking of the input regardless of case

The for Loop

• The for loop is a pretest loop that executes zero or more times
• It is useful for counter-controlled loops
• for loop format: for (initialization; test; update) { statements; }

The for Loop: Mechanics

• Perform the initialization expression
• Evaluate the test expression
  • If it is true, go to step 3
  • Otherwise, terminate the loop
• Execute the body of the loop
• Perform the update expression, then go back to step 2

The for Loop: Notes

• If the test expression 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

The for Loop: Modifications

• Variables can be defined in initialization code with a scope of the loop
• Initialization and update code can contain more than one statement, separate the statements with commas

Deciding Which Loop to Use

• while: A pretest loop; the loop body may not be executed at all
• do-while: A post-test loop; the loop body will always be executed at least once
• for: A pretest loop; the loop body may not be executed at all; has initialization and update code; it is useful with counters or if the precise number of iterations is known

Nested Loops

• A nested loop is a loop that is inside the body of another loop
• 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 a nested loop is the product of number of iterations of the two loops

Breaking Out of a Loop

• break is used to terminate the execution of a loop iteration
• Its use should be sparing as it can make code harder to understand
• When used in an inner loop, break terminates that loop only, and will return to the outer loop

The continue Statement

• continue prepares for the next iteration after the end of the current one
  • while and do-while loops iterate after the expression is true and repeats if the test is true
  • for loop goes to the update step, then test, and repeats the loop if the test condition is true after
• Its use should be sparingly as it can make code harder to understand

Using Files for Data Storage

• Files can be used instead of the computer screen for program output
• Files are stored on secondary storage media, such as a disk
• Files retain data between program executions
• Previously, the file can be used instead of a keyboard for program input

File Types

• A text file contains information encoded as text, such as letters, digits, and punctuation, and can be viewed with a text editor, such as Notepad
• A binary file contains binary information that has not been encoded as text and cannot be viewed with a text editor

File Access - Ways to Use the Data in a File

• With Sequential Access, data is read in order
• It is required retrieve the preceding n-1 pieces first to access the nth piece of data
• With Random Access, any piece of data can be accessed directly

What is Needed to Use Files

• The ifstream and/or ofstream header files are needed
• A file stream object should also be defined
  • ifstream for input (read data) from a file: ifstream inFile;
  • ofstream for output (write data) to a file: ofstream outFile;
• The file should be opened using the open member function
  • inFile.open("inventory.dat");
  • outFile.open("report.txt");
• The filename may include drive and path info. The filename must include the full name, included extensions
• The output file will be created if necessary, but an existing output will be erased first
• The input file must exist for open to work
• Creating a filestream object and opening a file can be accomplished in a single statement, like this:
  • ifstream inFile("inventory.dat");
  • ofstream outFile("report.txt");
• The file is then used, where an output file object uses << to send data to a file: outFile << "Inventory report";
• An input file object uses >> to copy data from the file to variables: inFile >> partNum; inFile >> qtyInStock >> qtyOnOrder;
• Finally the file is closed using the close member function:
  • inFile.close();
  • outFile.close();
• It is important to close files, and not wait for the operating system at program end
  • There may be buffered output data waiting to be sent to the file that could be lost
  • There may be a limit on the number of open files
• The read position is the location of the next piece of data in an input file:
  • Initially set to the first byte in the file
  • Advancing for each data item that is read
  • Successive reads will retrieve successive data items

User-Specified Filenames

• Programs can prompt the user to enter the names of input and/or output files, which makes the program more versatile
• Filenames can be read into string objects. In C++ prior to C++11, the C-string representation of the string object can be passed to the open function:
  • cout << "Which input file?";
  • cin >> inputFileName;
  • inFile.open(inputFileName.c_str());
• In C++11, the string object can be passed to the open() function directly

Using the >> Operator to Test for End of File (EOF) on an Input File

• The stream extraction operator (>>) returns a "true" or "false" value indicating if a read is successful
• This is tested to find the end of file since the read "fails" (the read expression is false) when no more data is present

File Open Errors

• An error will occur if an attempt to open a file for input fails if the file does not exist, is mispelled, or is in a different place
• The file stream object is set to true if the open operation succeeded, which can be tested to see if the file can be used

Creating Good Test Data

• The quality of test data is more important than the quantity
• Test data should show how different parts of the program execute
• Test data that is evaluated should describe how the program handles normal data, data that is at the limits, and invalid data