Repetition

• Function Stubs
• Random Numbers
• Switching values using temporary variables
• Repetition
• While
• For statement
• Increment and Decrement
• Do While
• Grade program

Function Stubs

• Function header, prototype and call
• No real code in the function body
• Allows testing of main without writing all code
• Skeleton of function
• Message in function body to let you know you were there

Random Numbers

• Random number generator
• stdlib.h header
• srand and rand

Switching values using temporary variables

• To switch firstNum to secondNum
• temp = 0;
• temp = firstNum;
• firstNum = secondNum;
• secondNum = temp;
• see page 255

Repetition

• Rob the mechanical man
• Repeating one or more instructions until a condition is met
• Brownies
• Paying bills
• Other names - looping, iteration
• Repeat a specified number of times or until condition is met
• while, do while, for

While

• pseudocode
• Characteristics of the While
• Flowcharts
• Coding C++ while loops
• Examples
• Using Counters and Accumulators
• Position of statements
• Problems
• Nested Repetition

pseudocode

• while (money in acct > 0 or no more bills)
• repeat while (numStudents <30 )

while (money in acct > 0 or no more bills)

• write check
• subtract check amount from balance
• subtract one from bills to pay
• end while
• What happens if we forget to subtract bills to pay or money?

repeat while (numStudents <30 )

• register
• add 1 to numStudents
• end

Characteristics of the While

• Loop body
• Condition
• How does it process?

Loop body

• One statement
• Block of statements

Condition

• Evaluates to true or false
• Boolean expression
• Where does processing go if statement is false?

How does it process?

• Evaluates conditions before executing loop body
• Pre-test
• May never execute loop body depending on condition
• Top driven loop
• After each execution of loop body, condition is re-evaluated

Flowcharts

• Diamond symbol
• 2 flowlines leaving
• 2 flowlines coming in

Coding C++ while loops

• Page 269 - syntax and examples
• while (loop condition)
• one or statements or block
• // end while comment
• No semicolon at end of while
• Most times will use a block
• Good to get in habit of {} even with one statement
• Easier to modify
• Will the instructions in the loop body always execute?

Examples

• LaProg01 - what is happening?
• What happens if condition is initially false?
• LaProg02 - Game - what is happening?

Using Counters and Accumulators

• Counter - counting something
• Accumulator - adding values together, accumulating a total
• Updating - changing values of accumulator or counter

Position of statements

• Priming read, lead read - before entering loop
• Another read within loop
• Values to end loops - sentinel values, trip values, trailer values
• Makes a difference where initial values are, where increments take place
• LaProg03
• Calculate number of sales numSales = numSales + 1
• Calculate total sales sumSales = sumSales + sales

Problems

• Endless Loops
• Divide by Zero

Endless Loops

• What is it?
• CTRL c (press both at same time) to end endless loop
• Close DOS window
• What if never ask for new values in a loop and are testing the value?

Divide by Zero

• Not allowed
• Try with Sales Express program
• Number of sales never gets updated, never enters loop
• How to correct problem with divide by zero?
• Why do we increment number sales and total before request new values?

Nested Repetition

• One loop inside another loop
• Inner loop, outer loop
• LaProg04 -- collecting sales from two regions
• Loop for sales, Loop for Region 1 and Region 2

For statement

• Counter controlled loop
• Accomplishes same as while, but more efficient coding
• for (short =1; count <+ 3; count="count" + 1)
• *****
• // end for
• for (type initial value; condition; update counter)
• Statement or block
• // end
• Lbprog01 - what is happening?
• Lbprog02 - replace a while statement with a for statement

Increment and Decrement

• Short hand
• count = count + 1;
• count++;
• count = count - 1;
• count--;

Do While

• do
• {}
• while (condition);
• Post test (where while is a pre test)
• Tests condition after completing loop first time
• Always executes a loop at least once
• Laprog03 - what is happening?

Maintained by Pat Hathaway
Instructor, Information Systems Technology
Paul D. Camp Community College
Pages last updated on Tuesday, October 13, 1998.