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Chapter 4 Loops

Chapter 4 Loops. §4.1 The “while” Loop §4.2 The “do-while” Loop §4.3 The “for” Loop §4.4 Nested loop §4.5 “break” and “continue” §4.6 Simple File Input and Output. Objectives. To master the usage of while , do - while , and for loops statements

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Chapter 4 Loops

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  1. Chapter 4 Loops §4.1 The “while” Loop §4.2 The “do-while” Loop §4.3 The “for” Loop §4.4 Nested loop §4.5 “break” and “continue” §4.6 Simple File Input and Output

  2. Objectives • To master the usage of while, do-while, and for loops statements • To understand the flow of control in loops • To use Boolean expressions to control loops • To know the similarities and differences of three types of loops • To know the use functionality of break and continue • To know how to read and write data from/to a file

  3. §4.1 The “while” Loop while (loop-continuation-condition) { // loop-body; Statement(s); } int count = 0; while (count < 100) { cout << "Welcome to C++!\n"; count++; }

  4. (count < 2) is false since count is 2 now Print Welcome to C++ The loop exits. Execute the next statement after the loop. (count < 2) is still true since count is 1 Increase count by 1 count is 2 now Initialize count (count < 2) is true Print Welcome to C++ Increase count by 1 count is 1 now Trace while Loop int count = 0; while (count < 2) { cout << "Welcome to C++!"; count++; }

  5. Example: An Advanced Math Learning Tool The Math subtraction tutor program in Listing 3.6, SubtractionTutor.cpp, generates just one question for each run. You can use a loop to generate questions repeatedly. Listing 4.1 gives a program that generates ten questions and reports the number of the correct answers after a student answers all ten questions. The program also displays the time spent on the test and lists all the questions, as shown in sample output. SubtractionTutorLoop

  6. Controlling a Loop with User Confirmation char continueLoop = 'Y'; while (continueLoop == 'Y') { // Execute body once // Prompt the user for confirmation cout << "Enter Y to continue and N to quit: "; cin >> continueLoop; }

  7. Ending a Loop with a Sentinel Value Often the number of times a loop is executed is not predetermined. You may use an input value to signify the end of the loop. Such a value is known as a sentinel value. Write a program that reads and calculates the sum of an unspecified number of integers. The input 0 signifies the end of the input. SentinelValue

  8. Caution • Don’t use floating-point values for equality checking in a loop control. • Since floating-point values are approximations, using them could result in imprecise counter values and inaccurate results. • double data = pow(sqrt(2.0), 2) - 2; • if (data == 0) • cout << "data is zero"; • else • cout << "data is not zero";

  9. §4.2 The “do-while” Loop “while” do { // Loop body; Statement(s); } while (loop-continuation-condition); “do-while”

  10. Example • Listing 4.3: a program that reads and calculates the sum of an unspecified number of integers. • The input 0 signifies the end of the input. TestDoWhile

  11. §4.3 The “for” Loop for (initial-action; loop-continuation-condition; action-after-each-iteration) { // loop body; Statement(s); } int i; for (i = 0; i < 100; i++) { cout << "Welcome to C++!\n"; }

  12. Execute adjustment statement i now is 2 (i < 2) is true since i is 0 Print Welcome to C++! Execute adjustment statement i now is 1 (i < 2) is still true since i is 1 Print Welcome to C++ (i < 2) is false since i is 2 Exit the loop. Execute the next statement after the loop Declare i Execute initializer i is now 0 Trace for Loop int i; for (i = 0; i < 2; i++) { cout << "Welcome to C++!"; }

  13. Note • All the three control expressions in “for” loop can be omitted • But the semicolons can not be omitted sum=0; i=1; for (; i<=100;){ sum += i; i++;} sum=0; i=1; for ( ; ; ){ if (i<=100) break; sum += i; i++;} • sum=0; i=1; for (; i<=100; i++) sum += i; • sum=0;for (i=1; ; i++){ if (i<=100) break; sum += i;} • sum=0;for (i=1; i<=100; ){ sum += i; i++;} -- If the loop-continuation-condition in a for loop is omitted, it is implicitly true. -- It may result in infinite loop.

  14. Note • The initial-action and action-after-each-iteration can be a list of comma-separated expressions • rarely used in practice “,” operator • for (int i = 0, j = 0; (i + j < 10); i++, j++) { • // Do something • }

  15. “,” Operator • “,”: concatenate expressions and the value of the rightmost expression is the value of the whole comma expression • Example： int main(){ int x,y; x=50; for (int i = 0,j = 0;i + j < 10;i++,j++) { y=(x=x-5,x/5); cout<<x<<" "<<y<<endl; } }

  16. Example: Using for Loops Problem: Write a program that sums a series that starts with 0.01 and ends with 1.0. The numbers in the series will increment by 0.01, as follows: 0.01 + 0.02 + 0.03 and so on. TestSum

  17. Floating-point Numbers in Relational Expressions int main() { // Initialize sum double sum = 0; double prei=0; double i; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (i = 0.01f; i <= 1.0f; i = i + 0.01f){ sum += i; prei=i; } // Display result cout << "The sum is " << sum<<endl; cout<<setprecision(10)<<"i is increaded by "<<i-prei<<endl; cout<<"The final i is "<<i<<", and i-1.0 is " <<(i-1.0)<<endl; return 0; } int main() { // Initialize sum double sum = 0; double i=0; // Add 0.01, 0.02, ..., 0.99, 1 to sum for (i = 0.01; i <= 1.0; i = i + 0.01) sum += i; // Display result cout << "The sum is " << sum; return 0; }

  18. Which Loop to Use? • while, do-while, and for, are expressively equivalent and mutually conversable.

  19. Which Loop to Use? • “for”: • if the number of repetitions is known. • “while”: • if the number of repetitions is unknown • “do-while”: • to replace a while loop if the loop body has to be executed before testing the continuation condition.

  20. §4.4 Nested loop • Loop with one outer loop and one or more inner loops

  21. Nested Loops Listing 4.5: a program that uses nested for loops to print a multiplication table. TestMultiplicationTable

  22. Loop Example:Finding the Greatest Common Divisor Problem: Write a program that prompts the user to enter two positive integers and finds their greatest common divisor. Solution: Let the two input integers be n1 and n2. You can check whether k (for k = 2, 3, 4, and so on) is a common divisor for n1 and n2, until k is greater than n1 or n2. GreatestCommonDivisor Run

  23. Loop Example: Finding the Sales Amount • Problem: You have just started a sales job in a department store. Your pay consists of a base salary and a commission. The base salary is $5,000. The scheme shown below is used to determine the commission rate. • Sales Amount Commission Rate • $0.01–$5,000 8 percent • $5,000.01–$10,000 10 percent • $10,000.01 and above 12 percent • Your goal is to earn $30,000 in a year. Write a program that will find out the minimum amount of sales you have to generate in order to make $30,000. FindSalesAmount Run

  24. Loop Example: Displaying a Pyramid of Numbers Problem: Write a program that prompts the user to enter an integer from 1 to 15 and displays a pyramid. For example, if the input integer is 12, the output is shown below. Run PrintPyramid

  25. §4.5 “break” and “continue” • break: • May only be used inside a loop or a switch statement. • To terminate the current loop/switch immediately and transfer control to the statement immediately following that loop/switch. sum=0; mark=1; for ( i=0; ; ){ if (i>100) break; sum += i; i++;} if(i>0) avgnum = sum/I; …

  26. “continue” • “continue”: • May only be used inside a loop. • To terminate the current iteration of the loop and proceeds directly to the next. • In the case of a for loop it jumps to its action-after-each-iteration. sum=0; mark=1; for ( i=0; ; ){ if (i>100) continue; sum += i; i++;} if(i>0) avgnum = sum/I; …

  27. Examples • Use of “break” TestBreak • Use of “continue” TestContinue

  28. Loop Example: Displaying Prime Numbers • Problem: Write a program that displays the first 50 prime numbers in five lines, each of which contains 10 numbers. • An integer greater than 1 is prime if its only positive divisor is 1 or itself. For example, 2, 3, 5, and 7 are prime numbers, but 4, 6, 8, and 9 are not. • Solution: The problem can be broken into the following tasks: • For number = 2, 3, 4, 5, 6, ..., test whether the number is prime. • Determine whether a given number is prime. • Count the prime numbers. • Print each prime number, and print 10 numbers per line. PrimeNumber Run

  29. §4.6 Simple File Input and Output • To read input from the keyboard and write output to the screen/console? • “cin” and “cout” • To read and write data from/to a file? • ifstream and ofstream

  30. Simple File Input and Output • To write to a file SimpleFileOutput Run • To read from a file SimpleFileInput Run

  31. Testing End of a File • Listing 4.3 read only three lines • How to determine when to stop reading if you don’t know how many numbers are in a file? • Use the eof() function to detect it. Listing 4.14 revises Listing 4.13 to read all lines from the file numbers.txt. TestEndOfFile Run

  32. Summary • “while” and “do-while” loops • “for” loops • Difference between loop stements • Use of “break” and “continue” • Simple file I/O • Testing of end-of-file

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