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cs180 Handout - Programming Fundamentals Fall'09
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Handout 5 Loops.
Loops implement repetitive computation, a k a iteration. Java loop statements: while do-while for 1. Start with the while-loop. Syntax
while(condition) Statement; //body of loop
or
while(condition) { //body of loop First_Statement; ... Last_Statement; }
condition
true
Body of loop
false
Several logical loop organizations 1. counting loops know exactly how many times to repeat a set of actions usually done with a use of a counter variable counter is initialized before the loop starts executing counter is updated after each iteration of the loop Example: print numbers from 0 to m.
public class SimpleCountingLoop { public static void main(String[] args) { int i = 0; // the counter variable - to keep track of // number of times gone through the loop int m = 5; while (i < m){ System.out.println("i is " + i + " now."); i++; } System.out.println("Done. i is " + i ); } }
Question:
What will happen if m = -5? If the i++; in the loop body is omitted?
Programming and Debugging Pitfalls: infinite loops , off by one
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Handout 5
cs180 - Programming Fundamentals Fall'09
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Practice problems: for all the problems, assume Scanner kb = new Scanner (System.in); 1. Write a code segment that allows the user to enter 10 numbers and computes and prints out their sum, average, product.
2. Write a code segment that asks the user how many numbers there are in a list, then allows the user to enter the numbers and computes and prints out the smallest and the largest number.
3. Write a code segment that prints out all characters of a string one character per line.
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Handout 5
cs180 - Programming Fundamentals Fall'09
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2.condition controlled loops repeat while a certain condition is true
o
sentinel-controlled loops stop repeating when a certain sentinel value is encountered
Examples: 1. Let user input numbers until user enters 0. 0 is the sentinel in this case. Scanner kbrd = new Scanner (System.in); aNum = kbrd.nextInt(); while ( aNum != 0 ) { System.out.print(aNum); aNum = kbrd.nextInt() ; } Practice problems: 1. Write a code segment that continues to read a number from its user, until an even number is entered.
2. Write a code segment that continues to read user input until the user enters a number between 0 and 9.
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Handout 5
cs180 - Programming Fundamentals Fall'09
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o
boolean flag controlled loops stop when a boolean variable that reflects a certain state is false Example: describe what happens, when the following code segment executed.
int is prevPos = 0; int currPos = 1;
Scanner kbrd =
new Scanner (System.in);
String str = kbrd.nextLine(); boolean foundDouble = false; // boolean flag int len = str.length(); while (!foundDouble && currPos <= len - 1) { if (str.charAt(prevPos) == str.charAt(currPos)) foundDouble = true; else{ prevPos++; currPos++; } }
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Handout 5
cs180 - Programming Fundamentals Fall'09
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2. do-while loop: similar to while-loop, but condition is checked at the end of each iteration. The body of the loop is going to be executed at least once.
do Statement; while(condition); Body of loop //body of loop
do { //body of loop First_Statement; ... Last_Statement; } while(condition);
true
condition
false
Practice problem: 1. Write a code segment that continues to read a number from the user, until the user enters an even number. Use a do-while loop.
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Handout 5
cs180 - Programming Fundamentals Fall'09
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3. for loops especially useful for counting loops. Syntax: for (init-statements; loop-condition; update-statements) { //body of loop First_Statement; init-statements ... Last_Statement; }
Loopcondition
true
Body of loop
false
Example: count down from 9 to 0 for( count = 9; count >= 0; count--) { System.out.print("T = " + count); System.out.println(" and counting"); } System.out.println("Blast off!");
Update-statements
Practice problems. 1. Write a code segment that prints out a reverse of the string entered by the user, e.g. given "Walter" it would print "r...