Loops: Handling Infinite Processes CS 21a: Introduction to Computing I First Semester,
What’s a Loop? ► A way to express a constant number of repetitions ► A way to express an arbitrary number of repetitions, usually based on some input value or condition
Loops ► Java structures for loops: ► for statement ► while statement ► do-while statement
Example: Compound Interest public class BankAccount {... public void yearlyInterest() { double interest = balance*intRate/100; balance += interest; years++; println( "Interest:" + interest + ", balance is now" + balance ); }... }
Repeatedly Applying Interest ► Suppose you want to apply interest on the balance for three years public void applyThreeYearInterest( ) { double interest; interest = balance*intRate/100; balance += interest; years++; println( "Year 1 - interest:" + interest + ", balance:" + balance ); interest = balance*intRate/100; balance += interest; years++; println( "Year 2 - interest:" + interest + ", balance:" + balance ); interest = balance*intRate/100; balance += interest; years++; println( "Year 3 - interest:" + interest + ", balance:" + balance ); }
Repeated Execution
The Power of Loops ► Allow an arbitrarily long process to be described with a short algorithm
The for Statement ► Syntax for ( expr1; expr2; expr3 ) statement ► Notes ► expr1: initialization or setup (e.g., i = 1 ) ► expr2: condition (e.g., i <= 3 ) ► expr3: increment (e.g., i++ ) ► statement means any valid statement in Java (including blocks)
Applying Yearly Interest 3 Times public void applyThreeYearInterest( ) { double interest; int i; for( i = 1; i <= 3; i++) { interest = balance*intRate/100; balance += interest; print( "Year " + i + "- interest: " + interest ); println( ", balance:" + balance ); years++; }
First Use of Loops ► Express a repetitive process, where the number of repetitions is constant. ► Problem can still be solved without loops, but coding is too tedious without it. ► Becomes more useful as the constant grows larger.
Applying Yearly Interest An Arbitrary Number of Times public void applyYearlyInterest( int numYears ) { double interest; int i; for( i = 1; i <= numYears; i++) { interest = balance*intRate/100; balance += interest; print( "Year " + i + "- interest: " + interest ); println( ", balance:" + balance ); years++; }
Second (More Important) Use of Loops
Practice Programming Problem
public int factorial( int n ) { int result = 1; int i = 1; while ( i <= n ) { result = result * i; i = i + 1; } return result; } The while Statement ► Syntax while ( condition ) statement Setup Condition Loop Body Increment / go to next step
public int factorial( int n ) { int result = 1; int i = 1; do { result = result * i; i = i + 1; } while ( i <= n ); return result; } The do-while Statement ► Syntax do statement while ( condition ); Setup Loop Body Increment / go to next step Condition
Components of a Loop ► Setup/Initialization ► Terminating/Continuing condition ► Incrementing step ► not necessarily an increment, but something that moves the program further on – i.e., to a state possibly closer to a terminating condition ► e.g., decrementing, dividing, multiplitying, getting another input, etc. ► Loop body
Using a for Loop for Factorial public int factorial( int n ) { int result = 1; int i; for ( i = 1; i <= n; i++ ) { result = result * i; } return result; } Setup Condition Loop Body Increment / go to next step
for Loop (version 2) public int factorial( int n ) { int result; result = 1; for ( int i = 1; i <= n; i++ ) { result = result * i; } return result; } You can declare the "counter" variable inside the for scope is within the loop's block good when i is not used outside the loop
for Loop (version 3) public int result( int n ) { int i, result; for ( result = 1, i = 1; i <= n; i++ ) { result = result * i; } return result; } You can have multiple statements in the "setup" part of for separated by commas need to declare the variables before the for, since we can't have declarations here also works for the increment part NOT RECOMMENDED! generally bad style to put several statements on one line if possible, choose a single counter variable, and just use that
for Loop (version 3b) public int factorial( int n ) { int i, result; for ( result = 1, i = 1; i <= n; result *= i, i++ ) { } return result; } Shorthand for: result = result * i; This is legal, but BAD! cryptic The for loop has no body!
for Loop (version 3w) public int factorial( int n ) { int i, result; for ( result = 1, i = 1; i <= n; result *= i++ ) { } return result; } ("w" for worse!) even more cryptic Some C programmers actually like writing like this! DON'T!
for and while are equivalent! public int factorial( int n ) { int result; result = 1; for ( int i = 1; i <= n; i++ ) { result = result * i; } return result; } public int factorial( int n ) { int result; result = 1; { int i = 1; while ( i <= n ) { result = result * i; i++; } return result; } Increment Setup Condition Braces here are shown because in Java (not in C), scope of variables declared in setup of for is limited
Deciding which Statement to Use ► Using a for statement ► most appropriate when the number of iterations is easily known (e.g., factorial) ► Difference between while and do-while ► loop condition is performed at the top (while) or at the bottom (do-while) of the loop ► In do-while, body is executed at least once ► in while, we check first before executing
Practice Programming Problem ► Write a program that reads numbers from console input and prints them out, one on each line, until a zero is encountered (don’t print out the zero). Identify the setup, condition, and increment. Is this a bounded or a conditional iteration? ► Sample InputSample Output
Practice Programming Problem
What’s Wrong with This Code // assume grandmasFavoriteNumber is a positive integer for(int i = 0; i != grandmasFavoriteNumber; i++) ; println("It took " + i + " iterations to guess grandma's fave number."); // What’s a better way to write this?
What’s Wrong with This Code? int years = 0; while ( years < 20 ) { double interest = balance * rate / 100; balance = balance + interest; }
What’s Wrong with This Code? int years = 20; while ( years > 0 ) { years++; double interest = balance * rate / 100; balance = balance + interest; }
What’s Wrong with This Code? float count = 0.0f; while ( count != 1.0f ) { count = count f; } // How would you fix it?
What’s Wrong with This Code? void derpNTimes(int n) { for(int i = 1; i < n; i++) println("derp"); }
What’s Wrong with This Code? void derpNTimes(int n) { for(int i = 0; i <= n; i++) println("derp"); }
Avoiding Pitfalls ► Infinite Loop ► loop body or increment statement should contain a statement that eventually leads to termination ► Real Numbers ► Avoid using == or != when using real numbers ► Use = depending on direction of loop
Avoiding Pitfalls ► OBOE (Off-By-One-Error) aka "Fencepost error" ► To execute the loop body N times … ► if counter starts at 0, check for counter < N ► if counter starts at 1, check for counter <= N ► Remember that after the loop, the counter (if still in scope) will be beyond the limit of your condition ► if we started from 0, counter would be N, ► if we started from 1, counter would be N+1
Nested Loops ► It is possible to have a loop within a loop ► Example (What gets printed out?) for ( int i = 0; i < 5; i++ ) { println( i ); for ( int j = 0; j < 5; j++ ) { print(" " + j ); } println(); }
Nested Loops ► The condition can vary depending on the outer loop ► Example (What gets printed out?) for ( int i = 0; i < 5; i++ ) { println( i ); for ( int j = 0; j < i; j++ ) { print( " " + j ); } println( ); }
Practice Programming Problem
How to Avoid Confusion with Nested Loops ► Focus on one loop at a time. ► Abstract away the inner loop.
Printing a Shape ► Create a triangle pattern * ** *** ► Loop through rows for ( int i = 1; i <= n; i++ ) { // make triangle row }
Printing a Shape (continued) ► The following loop creates a row of i stars for ( int j = 1; j <= i; j++ ) r = r + "*"; r = r + "\n"; ► Put loops together → Nested loops String r = "" ; for ( int i = 1; i <= n; i++ ) { // make triangle row }
Triangle Class public class Triangle { private int width; public Triangle( int aWidth ) { width = aWidth; } public String toString() { String r = ""; for ( int i = 1; i <= width; i++ ) { for ( int j = 1; j <= i; j++ ) r = r + "*"; r = r + "\n"; } return r; }
TriangleRunner Class public class TriangleRunner { public static void main(String[] args) { Triangle small = new Triangle(3); System.out.println(small.toString()); Triangle large = new Triangle(15); System.out.println(large); } By the way, the built-in print methods calls the toString method automatically
Practice Programming Problem ► Create classes to print these shapes: * * * * * * * * * * * * * * * * * * *