Alternate Version of STARTING OUT WITH C++ 4th Edition.ppt

1、Copyright 2004 Scott/Jones Publishing,Alternate Version of STARTING OUT WITH C+ 4th Edition,Chapter 5 Looping,Chapter 5 slide 2,Topics,5.1 The Increment and Decrement Operators 5.2 Introduction to Loops: The while Loop 5.3 Counters 5.4 Letting the User Control the Loop 5.5 Keeping a Running Total 5.

2、6 Sentinels 5.7 Using a Loop to Read Data From a File,Chapter 5 slide 3,5.1 The Increment and Decrement Operators,+ adds one to a variableval+; is the same as val = val + 1; - subtracts one from a variableval-; is the same as val = val 1;,Chapter 5 slide 4,Postfix Mode,val+ and val- return the old v

3、alue of the variable, then increment or decrement the variable It is this returned old value of the variable that is used in any other operations within the same statement,Chapter 5 slide 5,5.2 Introduction to Loops: The while Loop,Loop: part of program that may execute 0 or more times (i.e., it rep

4、eats)while loop formatwhile (expression) statement(s); If there is only one statement in the body of the loop, the can be omitted,Chapter 5 slide 6,How the while Loop Works,while (expression) statement(s); expression is evaluated if it is true, the statement(s) are executed, and then expression is e

5、valuated again if it is false, the loop is exited,Chapter 5 slide 7,while Loop Flow of Control,Chapter 5 slide 8,while Loop Example,int val = 5; while (val = 0) cout val “ “;val-; produces output:5 4 3 2 1 0,Chapter 5 slide 9,while Loop is a Pretest Loop,while is a pretest loop (expression is evalua

6、ted before the loop executes)If the expression is initially false, the statement(s) in the body of the loop are never executedIf the expression is initially true, the statement(s) in the body continue to be executed until the expression becomes false,Chapter 5 slide 10,Exiting the Loop,The loop must

7、 contain code to allow expression to eventually become false so the loop can be exited Otherwise, you have an infinite loop (i.e., a loop that does not stop) Example infinite loopx = 5;while (x 0) / infinite loop becausecout 0,Chapter 5 slide 11,5.3 Counters,Counter: variable that is incremented or

8、decremented each time a loop repeats Can be used to control execution of the loop (loop control variable) Must be initialized before entering loop May be incremented/decremented either inside the loop or in the loop test,Chapter 5 slide 12,5.4 Letting the User Control the Loop,Program can be written

9、 so that user input determines loop repetition Used when program processes a list of items, and user knows the number of items User is prompted before loop. Their input is used to control number of repetitions,Chapter 5 slide 13,User Controls the Loop Example,int num, limit; cout limit; cout “nnnumb

10、er squaren“; num = 1; while (num = limit) cout setw(5) num setw(6) num*num endl;num+; ,Chapter 5 slide 14,5.5 Keeping a Running Total,running total: accumulated sum of numbers from each repetition of loop accumulator: variable that holds running total int sum = 0, num = 1; / sum is the while (num 10

11、) / accumulator sum += num;num+; cout “Sum of numbers 1 10 is “ sum endl;,Chapter 5 slide 15,5.6 Sentinels,sentinel: value in a list of values that indicates end of data Special value that cannot be confused with a valid value, e.g., -999 for a test score Used to terminate input when user may not kn

12、ow how many values will be entered,Chapter 5 slide 16,Sentinel Example,int total = 0; cout points; while (points != -1) / -1 is the sentinel total += points; cout points; ,Chapter 5 slide 17,5.7 Using a Loop to Read Data From a File,A Loop can be used to read in each piece of data from a file It is

13、not necessary to know how much data is in the file Several methods exist to test for the end of the file,Chapter 5 slide 18,Using the eof() Function to Test for the End of a File,eof() member function returns true when the previous read encountered the end of file; returns false otherwise Example:da

14、tafile score;while (!datafile.eof() sum += score;datafile score;,Chapter 5 slide 19,Problems Using eof(),For the eof() function to work correctly using this method, there must be a whitespace (space, tab, or Enter ) after the last piece of data Otherwise the end of file will be encountered when read

15、ing the final data value and it will not be processed,Chapter 5 slide 20,Using the Operation,The stream extraction operator () returns a value indicating if a read is successful This can be tested to find the end of file since the read “fails” when there is no more data Example: while (datafile score)sum += score;,Copyright 2004 Scott/Jones Publishing,Alternate Version of STARTING OUT WITH C+ 4th Edition,Chapter 5 Looping,