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Loops: Handling Infinite
ProcessesCS 21a: Introduction to
Computing IFirst Semester, 2013-2014
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
► We want a way to indicate that the following should be executed three times:
interest = balance*intRate/100;
balance += interest;
years++;
println( "Yearly-interest:" + interest + ", balance:" + balance );
► Better still, we want the following executed where takes the value 1, 2, 3:
interest = balance*intRate/100;
balance += interest;
years++;
print( "Year " + i + "- interest: " + interest );
println( ", balance:" + balance );
The Power of Loops
►Allow an arbitrarily long process to be described with a short algorithm
The for Statement
► Syntaxfor ( 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
►Express a repetitive process whose length is based on some condition. Two possibilities…►Bounded iteration: Simply repeat
times, where is given►Conditional iteration: The number
of repetitions is not known in advance, but the length will always depend on some condition
Practice Programming Problem
►Write a procedure/function that computes given a non-negative integer . Try not to cheat from the next slides!
and ►Bored? Try an alternate
implementation based on this definition:
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
►Syntaxwhile ( condition ) statement
SetupCondition
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
► Syntaxdo statementwhile ( 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 commasneed 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 Input Sample Output1 2 3 1
4 5 2
0 3
4
5
Practice Programming Problem
► Write a program that reads a positive integer and prints out all the positive integers, separated by spaces, up to an including . Is this a bounded or a conditional iteration?
► Sample Input
16► Sample Output
2 4 6 8 10 12 14 16
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 + 0.3333333f;
}
// 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 <= or >= 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
► Print out all 16 pairs of numbers from the set ► What if the numbers have to be distinct?► What if the the order does not matter (i.e.,
the pair 1,2 is the same as the pair 2,1)
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 rowsfor ( int i = 1; i <= n; i++ )
{ // make triangle row }
Printing a Shape (continued)
► The following loop creates a row of i starsfor ( int j = 1; j <= i; j++ ) r = r + "*";
r = r + "\n";
► Put loops together → Nested loopsString r = "" ;
for ( int i = 1; i <= n; i++ )
{
// make triangle row
}
Triangle Classpublic 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:* * * *
* * * ** * * ** * * *
* * * ** * ** **
* * * * * ** * * *
* * * * * * * * * *
