Building on the foundation. Now that we know a little about cout cin math operators boolean operators making decisions using if statements

Transcription

1 Chapter 5 Looping

2 Building on the foundation Now that we know a little about cout cin math operators boolean operators making decisions using if statements

3 Advantages of Computers Computers are really quick Computers don't get bored They can do the same thing over and over and be happy

4 Loops What is the effect of a loop? { some C++ code; some C++ code; some C++ code; some C++ code; some C++ code; some C++ code; some C++ code; as long as some condition is true

5 While loops while ( num < 10 ) { cout << num = << num << endl; num = num + 1; Bool Exp True False Code Block

6 While loops while ( num < 10 ) cout << Hello there \n ; As before if there is only one line in the body of the loop, the { are not needed. But...what is the problem? Bool Exp True Code Block False

7 While loops while ( num < 10 ) cout << Hello there \n ; As before if there is only one line in the body of the loop, the { are not needed. But...what is the problem? Infinite Loop: a loop that doesn't end Bool Exp True Code Block False

8 What is a difference between if and while Which of the following is the if (left or right)? Bool Exp Bool Exp True False True False Code Block Code Block

9 What is a difference between if and while Which of the following is the if (left or right)? Bool Exp while Bool Exp if False True Code Block False True Code Block

10 Class Exercise Ask the user for 10 integers Return to the user the total of the 10 integers and their average Things to think about: What part needs to be repeated? This will go in the loop body How many variables do you need? Please enter 10 numbers num 1: 23 num 2: num 10: -34 The total of your 10 numbers is: 345 The average for the 10 numbers is: 34

11 Answer int num = 1, total = 0, temp; cout << Please enter 10 numbers \n while ( num <= 10 ) { cout << num << num << : ; cin >> temp ; total = total + temp; num += 1; cout << The total of your 10 numbers is: << total << endl; cout << The average of your 10 number is: << ( total / 10 ) << endl;

12 Counter Controled Loop Counter Controled Loop uses a variable (num) to count and control when the loop stops int num = 1, total = 0, temp; cout << Please enter 10 numbers \n while ( num <=10 ) { cout << num << num << : ; cin >> temp ; total = total + temp; num = num + 1; cout << The total of your 10 numbers is: << total << endl;

13 Another Class Exercise Most of the time our DOS screen is 80 characters wide To help with making a pretty display, make a loop that will print 80 numbers across the screen...but always print from 1 to 10 ( 0 will represent a 10)

14 Answer int count = 1, output = 1; while ( count <= 80 ) { cout << output ; count = count + 1; output = output + 1; if (output > 9 )// reset output to 0 if over 9 output = 0;

15 Another Answer int count = 1, output = 1; while ( count <= 80 ) { cout << output ; count = count + 1; // reset output to 0 if over 9 output = ( output > 9? 0 : output + 1 );

16 Another Answer int num = 1 ; while ( num <= 80 ) { cout << ( num % 10 ) ; num = num + 1;

17 While loops You can put any kind of code in the code block... Bool Exp True False Code Block

18 While loops You can put any kind of code in the code block......even other loops Bool Exp True Code Block False Bool Exp True Code Block False

19 Embedded loops (a loop inside of another loop) In everyday life we have an embedded loop:???? Bool Exp Bool Exp True Code Block False True Code Block False

20 Embedded loops (a loop inside of another loop) In everyday life we have an embedded loop: Time minutes Bool Exp hours Bool Exp True False True False Code Block Code Block

21 Creating Some Time Output int hour = 0, min; while ( hour < 24 ) { min = 0; while ( min < 60 ) { cout << hour << ':' << min << endl; min = min + 1; // end of minute loop hour = hour + 1; // end of hour loop

22 int hour = 0, min; Now What is the Output? while ( hour < 24 ) { min = 0; while ( min < 60 ) { cout << (hour < 10? '0' : ) << hour; cout << ':' ; cout << (min < 10? '0' : ) << min << endl; min = min + 1; // end of minute loop hour = hour + 1; // end of hour loop

23 Random Numbers Generating random numbers is often useful In a game, player do not want the same behavior/moves each time In a simulation of a complex system, random numbers can be used to help generate random events Car crash in a simulation of a highway system Likelihood of a gene in cell mutation Weather simulation

24 Random numbers in real life Examples Generate a random number sequence in the range 1 to 6 Use a six-sided die ( 色子 shǎi zi) Each roll represents a new random number Generate a random number sequence in the range 1 to 2 Use a fair coin Heads: 1, Tails: 2

25 Random Numbers We can write an algorithm for generating what looks like random numbers Because it s an algorithm, we know the rules for generating the next number The generated numbers are not really random They are properly called pseudo-random numbers

26 Stdlib Library Provides functions for generating pseudo-random numbers rand() Returns a pseudo-random unsigned int in the range of 0 to RAND_MAX (32,767) #include <iostream> #include <cstdlib> using namespace std; int main() { int i = 1; while( i <= 5) { cout << rand() << endl; ++i; return 0;

27 Program Output Output

28 Output # Output # Program Output for two runs Every time the program is run we will get the same random numbers

29 Different Sequences To produce a different sequence, use another function (seed random) void srand(unsigned int); Example srand(10); cout << rand() << endl;

30 Different Sequences Consider seed.cpp int main() { cout << "Enter a seed: "; unsigned int seed; cin >> seed; srand(seed); int i = 1; while(i <= 5) { cout << rand() << endl; ++i; return 0;

31 Different Sequences To automatically get a different sequence each time Need a method of setting the seed to a random value The standard method is to use the computer's clock as the value of the seed The function time() can be used Returns an integer value time(0) returns a good value for generating a random sequence

32 Randseed.cpp #include <iostream> #include <cstdlib> #include <ctime> using namespace std; int main() { srand( time(0) ); int i = 1; while(i <= 5) { cout << rand() << endl; ++i; return 0;

33 Output #1 Program Output for two runs Output # Every time the program is run we will get the different random numbers

34 Sentinel Controlled Loops Continue doing the loop until a special value (the sentinel) is encounter

35 Sentinel Controlled Loops Continue doing the loop until a special value (the sentinel) is encounter Example: Before we asked the user for 10 numbers and then we told the user total of the 10 numbers average of the 10 numbers

36 Sentinel Controlled Loops Continue doing the loop until a special value (the sentinel) is encounter Example: Before we asked the user for 10 numbers and then we told the user total of the 10 numbers average of the 10 numbers Now we want the user to enter as many numbers as they want a negative number is the signal that they are done ( the sentinel )

37 int count = 1, total = 0, input = 0; Will this work? cout << Please enter some positive numbers that I will average \n ; cout << Enter a negative number to signal that you are done \n ; while ( input >= 0 ) { cout << # << count << : ; cin >> input; total = total + input; count = count + 1; count = count 1; try try it it with: with: if ( count > 0 ) { cout << The total of your << count ; cout << numbers is: << total << endl; cout << The average of your << count cout << number is: << ( total / count ) << endl;

38 int count = 1, total = 0, input = 0; Will this work? No cout << Please enter some positive numbers that I will average \n ; cout << Enter a negative number to signal that you are done \n ; while ( input >= 0 ) { cout << # << count << : ; cin >> input; total = total + input; count = count + 1; count = count 1; try try it it with: with: if ( count > 0 ) { cout << The total of your << count ; cout << numbers is: << total << endl; cout << The average of your << count cout << number is: << ( total / count ) << endl;

39 int count = 0, total = 0, input = 0; Will this work? cout << Please enter some positive numbers that I will average \n ; cout << Enter a negative number to signal that you are done \n ; while ( input >= 0 ) { total = total + input; count = count + 1; cout << # << count << : ; cin >> input; count = count 1; try try it it again again with: with: if ( count > 0 ) { cout << The total of your << count ; cout << numbers is: << total << endl; cout << The average of your << count cout << number is: << ( total / count ) << endl;

40 int count = 0, total = 0, input = 0; Will this work? cout << Please enter some positive numbers that I will average \n ; cout << Enter a negative number to signal that you are done \n ; while ( input >= 0 ) { total = total + input; count = count + 1; cout << # << count << : ; cin >> input; try try it it again again with: with: -5-5 count = count 1; if ( count > 0 ) { cout << The total of your << count ; cout << numbers is: << total << endl; cout << The average of your << count cout << number is: << ( total / count ) << endl;

41 Infinite loops ( 无限循环 wú xiàn xún huán) Common Infinite Loops while ( ch = y ) { Remember: non-zero numbers true zero false

42 Infinite loops ( 无限循环 wú xiàn xún huán) Common Infinite Loops while ( ch = y ) { or int i = 0 while ( i < 10 ) { // forgot i++; Remember: non-zero numbers true zero false

43 Other equality notes == is great for comparing integers, but should not be used for floating-point numbers instead use comparison operators ( <, <=, >, >= ) float f; if ( f == 100 ) // f: // do something if ( f >= 100 ) // do something // instead do this

44 Looping Control Structures Three different commands while statement do-while statement...

45 The Do-While Statement Syntax do code block while (bool expr); True Code Block Bool Exp False

46 Using a do while loop char reply; do {... // some code... cout << "Do you want to continue?(y):"; cin >> reply; while(reply == 'y');

47 Why use a...? What is the advantage of a while: do...while:

48 Why use a...? What is the advantage of a while: do...while: body will execute 0 N times body will execute 1 N times

49 Looping Control Structures Three different commands while statement do-while statement for statement

50 Common use of loop int cnt = 0; // initialize while ( cnt < 10 ) { // check...; // do something...body of loop...; cnt++; // update

51 The for loop Syntax for (initialization ; bool expression ; update action) body of the loop Example for (int i = 0; i < 3; i++) { cout << "i is " << i << endl;

52 Executed once at the beginning of the for loop's execution Initialization Bool Exp true false The Bool Exp is evaluated at the start of each iteration of the loop Body Update Action

53 Execution Example i 0 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl;

54 Execution Example i 0 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl;

55 Execution Example i 0 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0

56 Execution Example i 0 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0

57 Execution Example i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0

58 Execution Example i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0

59 Execution Example i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1

60 Execution Example i 1 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1

61 Execution Example i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1

62 Execution Example i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1

63 Execution Example i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1 i is 2

64 Execution Example i 2 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1 i is 2

65 Execution Example i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1 i is 2

66 Execution Example i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1 i is 2

67 Execution Example i 3 for (int i = 0; i < 3; ++i) { cout << "i is " << i << endl; cout << "all done" << endl; i is 0 i is 1 i is 2 all done

68 Embedding for loops Just like while loops, for loops can also be embedded inside other loops in fact, due to there compactness (information being easier to read) many embedded loops will be for loops

69 int cntr1 =0, cntr2 =0, cntr3 =0,cntr4 =0, cntr5 =0; cntr1++; for (int i = 1; i <= 10; ++i) { cntr2++; for (int j = 1; j <= 20; ++j) { cntr3++; cntr4++; cntr5++; What is the output? cout << cntr1: << cntr1 << endl << cntr2: << cntr2 << endl << cntr3: << cntr3 << endl << cntr4: << cntr4 << endl << cntr5: << cntr5 << endl;

70 Answer: cntr1: 1 cntr2: 10 cntr3: 200 cntr4: 10 cntr5: 1 Press any key to continue

71 Class Exercise Write code to display a grid. For each position, display the location s row and column number. Put two blanks between each location. Ex: 1,1 1,2 1,3... 1,9 2,1 2,2... 2,9... 8,1 8,2... 8,9 9,1 9,2 9,3...9,9

72 One solution to grid exercise for(int row = 1; row < 10; row++ ){ for(int col = 1; col < 10; col++) { cout << row <<, << col << ; // end of col loop cout << endl; // end of row loop

73 Another use of break break can be used to exit a loop // in some game while ( life > 0 ) {... // play game cout << Do you want to quit(q): ; cin >> userresponse; if (userresponse == q ) break; // end while loop not the best example

74 A better solution // in some game userresponse = c ; while ( life > 0 && userresponse!= q ) { // play game... cin >> userresponse; // end while loop

75 The best solution // in some game do { // play game... cin >> userresponse; while ( life > 0 && userresponse!= q );

76 Another use of break break will only exit out of one loop (if embedded) for ( int row = 1; row < 10; row++ ) { for ( int col = 1; col < 10; col++) { // do something if (???? ) break; // col loop // this will stop the col loop // row loop

77 Continue, the brother of break the key word, continue, will cause execution to skip to the boolean expression while( something == true ) { // do something if ( x > 0 ) continue; // skip to end of block // do some more... // end of while

78 Continue, the brother of break the key word, continue, will cause execution to skip to the boolean expression while( something == true ) { // do something if ( x > 0 ) continue; // skip to end of block // do some more... // end of while

79 Using break and continue in loops First look for a solution that does not use break or continue Only use break and continue if it is the only solution or it makes the code easier to understand Note: always avoid using goto... there is almost always a better way

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