Programming Forums > Application Development > Java

namsu · Jan 3rd, 2008, 4:26 AM

I have a problem understanding how arrays are working when using the for loop. I am confused as to how lines 23-24 are working, how the stars are being printed. Because the statement says if the stars are less than the value of the one in the index using the counter variable, then increment (stars++). Does that not increment stars by one every time, how does the star get printed when a number like 4 gets read out of the index in the array. I just cant seem to get my head round on what variable it is applying it to. If someone is kind of enough to help me out in understanding please help. Thank you very much.

Jan 3rd, 2008, 4:26 AM	#1
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	Array with For Loop I have a problem understanding how arrays are working when using the for loop. I am confused as to how lines 23-24 are working, how the stars are being printed. Because the statement says if the stars are less than the value of the one in the index using the counter variable, then increment (stars++). Does that not increment stars by one every time, how does the star get printed when a number like 4 gets read out of the index in the array. I just cant seem to get my head round on what variable it is applying it to. If someone is kind of enough to help me out in understanding please help. Thank you very much. (Toggle Plain Text) Figure 7.6. Bar chart printing program. 1 // Fig. 7.6: BarChart.java 2 // Bar chart printing program. 3 4 public class BarChart 5 { 6 public static void main( String args[] ) 7 { 8 int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; 9 10 System.out.println( "Grade distribution:" ); 11 12 // for each array element, output a bar of the chart 13 for ( int counter = 0; counter < array.length; counter++ ) 14 { 15 // output bar label ( "00-09: ", ..., "90-99: ", "100: " ) 16 if ( counter == 10 ) 17 System.out.printf( "%5d: ", 100 ); 18 else 19 System.out.printf( "%02d-%02d: ", 20 counter * 10, counter * 10 + 9 ); 21 22 // print bar of asterisks 23 for ( int stars = 0; stars < array[ counter ]; stars++ ) 24 System.out.print( "" ); 25 26 System.out.println(); // start a new line of output 27 } // end outer for 28 } // end main 29 } // end class BarChart Figure 7.6. Bar chart printing program. 1 // Fig. 7.6: BarChart.java 2 // Bar chart printing program. 3 4 public class BarChart 5 { 6 public static void main( String args[] ) 7 { 8 int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; 9 10 System.out.println( "Grade distribution:" ); 11 12 // for each array element, output a bar of the chart 13 for ( int counter = 0; counter < array.length; counter++ ) 14 { 15 // output bar label ( "00-09: ", ..., "90-99: ", "100: " ) 16 if ( counter == 10 ) 17 System.out.printf( "%5d: ", 100 ); 18 else 19 System.out.printf( "%02d-%02d: ", 20 counter 10, counter * 10 + 9 ); 21 22 // print bar of asterisks 23 for ( int stars = 0; stars < array[ counter ]; stars++ ) 24 System.out.print( "*" ); 25 26 System.out.println(); // start a new line of output 27 } // end outer for 28 } // end main 29 } // end class BarChart

Jan 3rd, 2008, 5:14 AM	#2
Klipt Hobbyist Programmer Join Date: Dec 2005 Posts: 118 Rep Power: 0	Re: Array with For Loop (Toggle Plain Text) for (int stars = 0; stars < array[counter]; stars++) System.out.print(""); for (int stars = 0; stars < array[counter]; stars++) System.out.print(""); is equivalent to (Toggle Plain Text) int stars = 0; while (stars < array[counter]) { System.out.print(""); stars++; }; int stars = 0; while (stars < array[counter]) { System.out.print(""); stars++; }; (except that the scope of 'stars' is limited to the inside of the for loop in the first case). 'stars' is just a counter that keeps track of how many stars you've printed. Once it reaches array[counter], you print a newline and move to the next index in the array. __________________ Get PLT Scheme\|SB Common Lisp\|Haskell\|wxDev-C++\|Python

Jan 3rd, 2008, 8:14 AM	#4
Sane Programming Guru Join Date: Apr 2005 Location: Waterloo, Ontario Posts: 1,890 Rep Power: 5	Re: Array with For Loop I see where your difficulty lies. I had to blink a couple times before seeing what was happening. I'll try to explain. (Toggle Plain Text) int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; As you know, that's the input of the grade distribution. In the first for loop, the variable "counter" starts at 0, and loops until it's through to the end of array[]. (Toggle Plain Text) for ( int counter = 0; counter < array.length; counter++ ) for ( int counter = 0; counter < array.length; counter++ ) So let's try to calculate the grade distribution of the 9th item. That means we're on the for loop's 9th iteration... also known as the 8th index. Which contains the value, 4. The counter will equal 8 at this point. So what happens when we get to this code? (Toggle Plain Text) for (int stars = 0; stars < array[counter]; stars++) System.out.print(""); for (int stars = 0; stars < array[counter]; stars++) System.out.print(""); If you go back to the first line I posted: (Toggle Plain Text) array[counter] = array[8] = 4 array[counter] = array[8] = 4 So array[counter] = 4 at this point in time, and the code becomes interpreted as: (Toggle Plain Text) for (int stars = 0; stars < 4; stars++) System.out.print(""); for (int stars = 0; stars < 4; stars++) System.out.print(""); The variable "stars", starts at 0, and iterates as long as its less than 4. Therefore, the second for loop will iterate 4 times, printing out a star each time. (Toggle Plain Text) ****** ** Voila. Does that helps you understand what's happening? Last edited by Sane; Jan 3rd, 2008 at 8:31 AM.

Jan 3rd, 2008, 9:55 AM	#7
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	Re: Array with For Loop I have one quick question, when the prefix increment operator is used before accessing the index of that array, what is it supposed to do, shift the index by one? Thanks for the help. (Toggle Plain Text) 3 import java.util.Random; 4 5 public class RollDie 6 { 7 public static void main( String args[] ) 8 { 9 Random randomNumbers = new Random(); // random number generator 10 int frequency[] = new int[ 7 ]; // array of frequency counters 11 12 // roll die 6000 times; use die value as frequency index 13 for ( int roll = 1; roll <= 6000; roll++ ) 14 ++frequency[ 1 + randomNumbers.nextInt( 6 ) ]; 15 16 System.out.printf( "%s%10s\n", "Face", "Frequency" ); 17 18 // output each array element's value 19 for ( int face = 1; face < frequency.length; face++ ) 20 System.out.printf( "%4d%10d\n", face, frequency[ face ] ); 21 } // end main 22 } // end class RollDie 3 import java.util.Random; 4 5 public class RollDie 6 { 7 public static void main( String args[] ) 8 { 9 Random randomNumbers = new Random(); // random number generator 10 int frequency[] = new int[ 7 ]; // array of frequency counters 11 12 // roll die 6000 times; use die value as frequency index 13 for ( int roll = 1; roll <= 6000; roll++ ) 14 ++frequency[ 1 + randomNumbers.nextInt( 6 ) ]; 15 16 System.out.printf( "%s%10s\n", "Face", "Frequency" ); 17 18 // output each array element's value 19 for ( int face = 1; face < frequency.length; face++ ) 20 System.out.printf( "%4d%10d\n", face, frequency[ face ] ); 21 } // end main 22 } // end class RollDie

Jan 3rd, 2008, 9:58 AM	#8
Jessehk The Oblivious One Join Date: May 2005 Location: Ontario, Canada Posts: 644 Rep Power: 4	Re: Array with For Loop I've annotated the source. I don't know if will be helpful at all (Sane seems to have covered it fine), but it couldn't hurt. java Syntax (Toggle Plain Text) public class BarChart { public static void main( String[] args ) { int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; System.out.println( "Grade distribution: " ); // For every element in the array... for ( int counter = 0; counter < array.length; counter++ ) { if ( counter == 10 ) System.out.printf( "%5d: ", 100 ); else System.out.printf( "%02d-%02d: ", counter * 10, counter * 10 + 9 ); // A helper variable to make things easier to understand. int gradeFreq = array[counter]; // print a star "gradeFreq" times for ( int i = 0; i < gradeFreq; i++ ) System.out.print( "" ); // Print a new line to seperate the bars System.out.println(); } } } public class BarChart { public static void main( String[] args ) { int array[] = { 0, 0, 0, 0, 0, 0, 1, 2, 4, 2, 1 }; System.out.println( "Grade distribution: " ); // For every element in the array... for ( int counter = 0; counter < array.length; counter++ ) { if ( counter == 10 ) System.out.printf( "%5d: ", 100 ); else System.out.printf( "%02d-%02d: ", counter 10, counter * 10 + 9 ); // A helper variable to make things easier to understand. int gradeFreq = array[counter]; // print a star "gradeFreq" times for ( int i = 0; i < gradeFreq; i++ ) System.out.print( "" ); // Print a new line to seperate the bars System.out.println(); } } } Alternatively, depending on the version of Java you have installed, you can iterate through an array like this: java Syntax* (Toggle Plain Text) public class Foreach { public static void main( String[] args ) { int[] array = { 1, 2, 3, 4, 5 }; // Print out every element of the array for ( int element : array ) { System.out.println( element ); } } } public class Foreach { public static void main( String[] args ) { int[] array = { 1, 2, 3, 4, 5 }; // Print out every element of the array for ( int element : array ) { System.out.println( element ); } } } EDIT: In reply to your second question, given: java Syntax (Toggle Plain Text) int array[] = { 1, 1, 1 }; int array[] = { 1, 1, 1 }; Doing this: java Syntax (Toggle Plain Text) ++array[0]; ++array[0]; Will result in an array that looks like this: java Syntax (Toggle Plain Text) { 2, 1, 1 } { 2, 1, 1 } __________________ Dr. Zoidberg: [ecstatic] I'm going to a movie... with FRIENDS!

Jan 3rd, 2008, 10:32 AM	#9
namsu Newbie Join Date: Jan 2008 Posts: 15 Rep Power: 0	Re: Array with For Loop I wasn't even looking at the inner for loops properly. Thanks everyone that really helped alot for my understanding in arrays.

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Programming Forums > Application Development > Java

Array with For Loop