Programming patterns involving iteration

1. Programming patterns involving iteration • Overview Loops • We’ll also see how loops are often combined with arrays (officially your next topic), e.g., -- Programming patterns involving iteration

2. Programming patterns involving iteration • We’ll review loops – • some of the important variations, and • ways you can use them toward certain ends. • We’ll also see how loops are often combined with arrays (officially your next topic), e.g., -- • Processing every member of an array • Searching for things in an array • Loops within loops for multi-dimensional tables Programming patterns involving iteration

3. Loops while (condition) { statement; … statement; } • A loop is: • a block of code that executes repeatedly while some condition holds true. • Java provides three forms for explicit loops: • while • for • do..while • The conditions in loops are written as in if statements • The break statement breaks out of the loop that it is within • Some loop examples follow for (start; condition; step) { statement; … statement; } do { statement; … statement; } while (condition); Programming patterns involving iteration

4. for loops versus while loops • for (int i = 0; i < 7; i++) { System.out.println (i + " " + i*i);} is equivalent to • int i = 0;while (i < 7) { System.out.println (i + " " + i*i);i++;} • Typically we use: • for when we know in advance how many times the loop will execute • while when something that happens in the loop determines when the loop exits • do..while when we want a while loop that always does at least one iteration Programming patterns involving iteration

5. Loop patterns • The next few slides present loop patterns that are often useful, including: • Do N times • Do N times, changing per loop variable • Count • Sum, minimum, maximum • Find-first • Do-forever • Wait-until Programming patterns involving iteration

6. Loop pattern: “do N times” for (int k = 0; k < 5; k++) { System.out.print(k + ″ ″); System.out.println( Math.pow((double) k, 5.0) ); Example: print k and k5 to the console window a given number of times: String s; int start, stop; s = JOptionPane.showInputDialog(″Start at?”) start = Integer.parseInt(s); s = JOptionPane.showInputDialog(″Stop after?”) stop = Integer.parseInt(s); for (int k = start; k <= stop; k++) { System.out.print(k + ″ ″); System.out.println( Math.pow((double) k, 5.0)) }; Programming patterns involving iteration

7. Loop pattern: “count” • Counts how many times a given character appears in a string: public static int charCount(String s, char c) { int count = 0; for (k = 0; k < s.length(); k++) { if (s.charAt(k) == c) { ++ count; } } return count; } Programming patterns involving iteration

8. Loop pattern: “sum” • Sums the digits in a string of digits: public static int digitSum(String s) { int digit; int sum = 0; for (k = 0; k < s.length(); k++) { digit = Integer.parseInt(s.substring(k, k+1)); sum = sum + digit; } return sum; } Programming patterns involving iteration

9. Loop pattern: “maximum” • Finds the largest digit in a string of digits: public static int digitMax(String s) { int digit; int max = Integer.parseInt(s.substring(0, 1)); for (k = 1; k < s.length(); k++) { digit = Integer.parseInt(s.substring(k, k+1)); if (digit > max) { max = digit; } } return max; } Programming patterns involving iteration

10. Loop Pattern: “find-first” • Find the first place where a given character appears in a string.Return -1 if the character does not appear. • public int findChar(String s, char c) { int i = 0; while (i < s.length()) { if (s.charAt(i) == c) { return i; } i++; } return -1; // Not found } for (i=0; i < s.length(); i++) { if (s.charAt(i) == c) { return i; } } Programming patterns involving iteration

11. Loop Pattern: “do-forever” • Our instruction-followers often go “forever”: while (true) { System.out.println("hi"); ... } for (; true;) { System.out.println("hi"); } Programming patterns involving iteration

12. Loop pattern: “break in middle” while (true) { ... if (...) { break; } ... } Programming patterns involving iteration

13. How loops are combined with arrays • This is a preview of what you’ll really be studying next! • Example: Processing every member of an array: for (int k = 0; k < maxDepth; k++) { myArray[k] = 0; // Clear out whole array! } Programming patterns involving iteration

14. How loops are combined with arrays • Preview, cntd: • Searching for things in an array int k; for (k = 0; k < maxDepth; k++) { if (myArray[k] = 0) { break; // Find an empty slot in array! } } Programming patterns involving iteration

15. How loops are combined with arrays • Preview, cntd: • Loops within loops for multi-dimensional tables for (int k = 0; k < maxDepth; k++) { for (int m=0; m<maxWidth; m++) { myTable[k][m] = 0; // Clear out 2-dim table! } } Programming patterns involving iteration