1. Programming Patterns
Many problems fit a “pattern” that experienced software developers recognize
And hence can easily code, from experience
Previously, we saw these patterns:
Swap
Absolute value
Maximum of two
Select from cases
Increment
Today, we will see these looping patterns:
Do N times
Count
Sum
Loop through a list
Loop followed by another loop
Later, we will see other looping patterns, including:
Max/min
Loops inside loops
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2. Programming patterns (non-looping)
absolute value
maximum of two
swap
if (y < 0) { x = -y; } else { x = y; }
if (x > y) { z = x; } else { z = y; }
temp = x;
x = y;
y = temp;
if (x >= 90) { z = ‘A’; } else if (x >= 80) { z = ‘B’;
} else if (x >= 70) {
z = ‘C’;
} else if (x >= 60) {
z = ‘D’;
} else {
z = ‘F’; }
Increment (for counting)
Select from cases
x = x + 1;
Keep these patterns in mind, to help you with similar problems that you encounter
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3. Loop pattern: “do N times”
Problem 1: Display the sine of 0.01, sine of 0.02, … sine of 4.00
for (int k = 1; k <= 400; ++k) {
System.out.println(Math.sin(k/100.0)); }
Problem 2: Display the 1st, 2nd and 3rd roots of 8 through 11, inclusive
for (int k = 8; k <= 11; k++) {
System.out.print(k + ″:\t″;
System.out.print(Math.pow((double) k, 0.5) + ″\t″);
System.out.print(Math.pow((double) k, 0.333) + ″\t″);
System.out.print(Math.pow((double) k, 0.25);
System.out.println(); }
Note the “cast” of k to type double
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4. Loop pattern: “do N times”
Problem 3: Display the 1st, 2nd and 3rd roots of start through stop, inclusive, where start and stop are determined at run time by the user
Scanner inputStream = new Scanner(System.in);
int start, stop;
System.out.print(″Start at? ″);
start = inputStream.nextInt();
System.out.print(″Stop at? ″);
stop = inputStream.nextInt();
for (int k = start; k <= stop; k++) {
Same code as in the loop in the previous example };
This example also shows how to do console input
Note that start and stop are both variables here
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5. Loop pattern: “count”
Problem: How many integers from to 1000, inclusive, have cosines that are negative? Display the count.
int count;
count = 0;
for (int k = -1000; k <= 1000; ++k) {
if (Math.sin((double) k) < 0 {
++ count; }
System.out.println(count);
Start count at 0
Increment count whenever it is time to “count”
Exercise: Sum the cosines of the integers from -20 to Display the sum.
Answer on next slide.
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6. Loop pattern: “sum”
Problem: Sum the cosines of the integers from -20 to Display the sum.
sum has type double here, because the sum is of double’s
double sum;
sum = 0.0;
for (int k = -20; k <= 2000; ++k) {
sum = sum + Math.cos((double) k); }
System.out.println(count);
Start sum at 0.0
Increment sum whenever it is time to “sum”, by whatever amount should be summed
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7. Loop pattern: “loop through a list”
Problem: Write a method that returns how many times a given character appears in a given string
A static method is one that does not use any of the fields of the class
public static int charCount(char c, String s) {
int count;
count = 0;
for (int k = 0; k < s.length(); ++k) {
if (s.charAt(k) == c) {
++ count; }
return count;
This example includes the counting pattern
FYI: Use == only to compare primitives. To compare objects (like Strings), use their equals method, e.g. s.substring(k, k+1).equals(c + ””)
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8. Loop pattern: “sum” Sums the digits in a string of digits: int digit;
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;
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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;
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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; }
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11. Loop Pattern: “do-forever”
Our instruction-followers often go “forever”:
while (true) { System.out.println("hi"); }
for (; true;) {
System.out.println("hi"); }
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12. Loop pattern: “break in middle”
while (true) {
...
if (...) {
break; }
continue;
would continue the loop, skipping the rest of the current iteration
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