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Understanding Loops in Java Programming - Examples & Syntax

Explore how loops work in Java programming with syntax examples for while and for loops, including explanations and code snippets. Enhance your coding skills!

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Understanding Loops in Java Programming - Examples & Syntax

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  1. CMSC 150Loops CS 150: Fri 20 Jan 2012

  2. Representing DNA AGTCCAGTGTCAA

  3. Start Codon: ATG

  4. Start Codon: ATG

  5. Consider in Java String dna = “AGTCCAGTGTCAA”;

  6. Consider in Java String dna = “AGTCCAGTGTCAA”; if ( dna.substring(0,3).equals(“ATG”) )

  7. Consider in Java String dna = “AGTCCAGTGTCAA”; if ( dna.substring(0,3).equals(“ATG”) ) if ( dna.substring(1,4).equals(“ATG”) )

  8. Consider in Java String dna = “AGTCCAGTGTCAA”; if ( dna.substring(0,3).equals(“ATG”) ) if ( dna.substring(1,4).equals(“ATG”) ) if ( dna.substring(2,5).equals(“ATG”) )

  9. Consider in Java String dna = “AGTCCAGTGTCAA”; if ( dna.substring(0,3).equals(“ATG”) ) if ( dna.substring(1,4).equals(“ATG”) ) if ( dna.substring(2,5).equals(“ATG”) ) if ( dna.substring(3,6).equals(“ATG”) ) if ( dna.substring(4,7).equals(“ATG”) ) ... if ( dna.substring(10,12).equals(“ATG”) )

  10. Loops

  11. Loop Syntax • while ( condition) { statement; } • for ( initialization; condition; update) { statement; }

  12. Loop Syntax Use while when you don’t know in advance the # of times to loop • while ( condition) { statement; } • for ( initialization; condition; update) { statement; }

  13. Loop Syntax • while ( condition) { statement; } Use for when you know in advance the # of times to loop • for ( initialization; condition; update) { statement; }

  14. While Loop Syntax • while ( condition ) { statement_to_execute; } • build a condition that eventually becomes false • need statement w/in body to advance toward false • condition evaluated each timebefore executing statement

  15. While Loop Example • while ( condition ) { statement_to_execute; } • intcount = 0; while ( count < 3 ) { System.out.println( “count = “ + count); count++; }

  16. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0

  17. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so…

  18. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0

  19. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1

  20. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so…

  21. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1

  22. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1 • execute statement: increment count to 2

  23. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1 • execute statement: increment count to 2 • evaluate condition: count < 3 is true, so…

  24. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1 • execute statement: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 2

  25. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1 • execute statement: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 2 • execute statement: increment count to 3

  26. While Loop Action intcount = 0; while ( count < 3) { System.out.println( “count = “ + count ); count++; } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 0 • execute statement: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 1 • execute statement: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints count = 2 • execute statement: increment count to 3 • evaluate condition: count < 3 is false, so exit the loop

  27. For Loop Syntax • for ( initialization; condition; update ) { statement_to_execute; } typically declare and initialize a variable for the loop: int count = 0; executed exactly once, as loop starts

  28. For Loop Syntax • for ( initialization; condition; update ) { statement_to_execute; } build a condition based on the loop variable that eventually becomes false count < 3; evaluated each timebefore executing statement

  29. For Loop Syntax • for ( initialization; condition; update ) { statement_to_execute; } statement that advances the condition toward failure count = count + 1 executed each timeafter executing statement

  30. For Loop Example • for ( initialization; condition; update ) { statement_to_execute; } • for ( int count = 0; count < 3; count++ ) { System.out.println( "Count = " + count ); }

  31. For Loop Example • for ( initialization; condition; update ) { statement_to_execute; } • for ( int count = 0; count < 3; count++ ) { System.out.println( "Count = " + count ); }

  32. For Loop Example • for ( initialization; condition; update ) { statement_to_execute; } • for ( int count = 0; count < 3; count++ ) { System.out.println( "Count = " + count ); }

  33. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0

  34. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so…

  35. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0

  36. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1

  37. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so…

  38. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1

  39. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1 • update: increment count to 2

  40. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1 • update: increment count to 2 • evaluate condition: count < 3 is true, so…

  41. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1 • update: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 2

  42. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1 • update: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 2 • update: increment count to 3

  43. For Loop Action for ( int count = 0; count < 3; count++ ) System.out.println( "Count = " + count ); { } • initialize count to 0 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 0 • update: increment count to 1 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 1 • update: increment count to 2 • evaluate condition: count < 3 is true, so… • execute statement: prints Count = 2 • update: increment count to 3 • evaluate condition: count < 3 is false, so exit the loop

  44. What happens? for ( int count = 0; count > 3; count++ ) System.out.println( "Count = " + count ); { }

  45. What happens? for ( int count = 0; count > 3; count++ ) System.out.println( "Count = " + count ); { }

  46. What happens? for ( int count = 0; count < 10; count = count++) System.out.println( "Count = " + count ); { }

  47. What happens? for ( int count = 0; count < 10; count = count++) System.out.println( "Count = " + count ); { } infinite loop!! because count++ returns value of count before incrementing

  48. What happens? for ( int count = 0; count < 10;) System.out.println( "Count = " + count ); { }

  49. What happens? for ( int count = 0; count < 10;) System.out.println( "Count = " + count ); { } infinite loop!! no update, so count is always 0

  50. What happens? for ( int count = 1; count != 10; count += 2) System.out.println( "Count = " + count ); { }

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