CSIS 113A Lecture 5 Random Numbers, while, do-while

Random Numbers rand() – Returns a pseudo-random integral number in the range 0 to RAND_MAX –number is generated by an algorithm that returns a sequence of apparently non-related numbers each time it is called. RAND_MAX is a constant defined in. –Its default value may vary between implementations but it is guaranteed to be at least

Scaling Use the mod operator % – x = rand() % 100; Generates a random number between 0 & 99 More rand() – rand() % is in the range 1 to 100 Typical use with variables –Val = (rand() % max) + min; »Returns random number between min and max

Seeding Can interject more randomness in algorithm by seeding it –srand(value); Where do you get value? –Best to seed from time of day clock Must include –Use time(0) so »srand(time(0)) Only call srand() one time –Do not include in a loop!

Example #include #include using namespace std; int main() { srand(time(0)); // See from time of day clock for(int i = 0; i < 10; i++) { cout << (rand() % 100) + 1 << endl; } }

The while Loop Syntax Parts of a while –Keyword while, a boolean test, loop body

Counted while Loops To use the while statement to build a counted loop, you must do three things: –1. Create a counter variable, and initialize it before the loop is encountered. –2. Test the counter variable inside the while loop's boolean condition expression –3. Update your counter at the end of the loop body. This takes the place of the for loop's update expression.

A Counted while Skeleton –int counter = 0; while (counter < 10 ) { // Loop body statements counter++; }

Common while Loop Errors I Several problems common to counted while loops Using an "expired" counter –Define and initialize counter in for initializer Looks empty if you forget initialization Allows you to use the same variable name in each loop –The while requires counter initialization before loop –You must remember to initialize your counter just before you enter the loop

Common while Loop Errors II Endless Loops –Not unique to while loops Change exponential for loop bounds to –Counted while loops are especially susceptible Counter update is often far removed from test Makes it easy to forget the update –Solution? Use loop-building strategy that starts with bounds Get mechanics working before attacking the goal

Common while Loop Errors III The phantom semicolon –If you put a semicolon after the test condition of a while loop, its body becomes the null statement u int counter = 0; while (counter < 10 ); { // This is unreachable counter++; }

Indefinite Loops How many times will this loop execute? int someNumber = rand() % 1000; while ( someNumber != 500 ) { someNumber = rand() % 1000; cout << someNumber << endl; } You can’t tell. That’s why it’s an indefinite loop –This is where the while loop really shines

Indefinite One way Indefinite works –integer variable someNumber is assigned a value between 0 and 999, using the rand() function –Variable someNumber is compared to 500 in while test If it is not equal to 500 then the loop body is entered In the loop body, the number of repetitions is incremented and displayed A new value for someNumber is then generated This kind of indefinite loop is called a Sentinel Loop –A specific value [500] is searched for

Sentinel Ranges I Sentinel doesn't have to be a single value –It may include a range of values Here's a problem that requires a sentinel range "Generate random integer numbers until a negative number is generated. Display the sum, count, and average of the numbers entered"

Sentinel Ranges II Questions you should ask –1. What is the loop's bounds? –2. What are the necessary preconditions? –3. What actions are required to advance the loop? –4. What is the loop's goal? –5. What are the goal preconditions? –6. What actions are necessary in the body? –7. What postconditions are necessary for the goal?

Sentinel Ranges III 1. What is the loop's bounds? –Answer: A negative number was generated –The C++ loop condition should look like this: while (num >= 0) 2. What are the necessary preconditions? –Look at the test condition; num must have a value –int num = rand();

Sentinel Ranges IV 3. What actions advance the loop? –Must change something in the test expression –Should generate another random number At this point, the "mechanics" are finished –You can test the loop to see that it works 4. What is the loop's goal? –Display the count, sum and average of positive integers generated

Sentinel Ranges V What are the goal preconditions? –Must have variables for the count and sum Sum should be a double because of overflow count and sum must both be initialized int count = 0; double sum = 0.0; 6. What actions are necessary to advance goal? –Add num to sum, incr count, before next num count++; sum += num;

Sentinel Ranges VI 7. What post-condition actions are required? –Must check for a count of zero. Means no numbers were entered Can't compute the average if there are no numbers –If count is > 0 then Compute and display the sum and the average

Primed and Inline Tests I So far, all our indefinite loops have been primed –Initialize the test condition before the loop Usually this involves reading a value This is called priming the loop –Test the value in the loop condition –Process the value in the body of the loop –Initialize the test condition at bottom of loop body Preparing for the next repetition of the loop Means "preparation" statements appear in two places

Intentional and Necessary Bounds Suppose you search a String for the letter "F" –A sentinel bounds is the obvious choice while (s[i] != 'F') i++; But what if the letter 'F' is not in the String s ? –You need to supply an additional bounds to use in case the value you are looking for is not found –This additional bounds is called a necessary bounds while (i < s.length && s[i] != 'F') i++;

Two Logical Problems The impossible condition if (age > 65 && age < 21) … –Solution? Change order if you are trying to bound value Change AND to OR The unavoidable condition if (age > 21 || age < 65) –Solution? Change OR to AND

Bottom-Testing One more loop in addition to for and while –The do-while loop is an exit-condition loop –That means the test is located after the body do { Statements; } while ( boolean condition ); Notice position of semicolon Generally not the loop of choice. [See next page]

The do-while Illustrated

C++ Jumps: break and continue Two jump statements from inside a loop –To restart the loop: use continue For while and do-while this jumps to the test With for it jumps to the update expression –To exit the loop: use break for (int i = 0; i < 7; i++) { if ( i % 2 == 0) continue; if ( i == 5 ) break; cout<< i << endl; }