Nested For Loops, Random, and the Math Class

Nested For Loops¶

(loops inside loops, can be used to produce complex text patterns)

Nested For Loop Example

for (int i = 1; i <= 5; i++>) {
    for (int j = 1; j <= (5 - i); j++) {
        System.out.print(".");  // print dots
    }
    System.out.println(i);  // print line number
}

Nested For Loop Output

....1
...2
..3
.4
5

Update Statements¶

(common update statements in for loops)

Statement	Description
`i++`	increment `i` by `1`; same as `i = i + 1`
`i += 1`	increment `i` by `1`; same as `i = i + 1`
`i--`	decrement `i` by `1`; same as `i = i - 1`
`i -= 1`	decrement `i` by `1`; same as `i = i - 1`

Fenceposting¶

(pulling one iteration out of the loop)

Fenceposting Example

System.out.print(1);               // one number to start with
for (int i = 2; i <= 5; i++) {
    System.out.print(", " + i);    // commas between numbers
}
System.out.println();              // end the line

Fenceposting Output

1, 2, 3, 4, 5

Random¶

(produce random numbers)

import java.util.*;  // We must include this in order to use Random!

// Create a Random object
Random rand = new Random();

// Generating values
int a = rand.nextInt(10); // generating an integer in the range of 0 to 9
int b = rand.nextInt(10) + 1; // generating an integer in the range of 1 to 10
int c = rand.nextInt(7) + 4; // generating an integer in the range of 4 to 10

Methods¶

Method	Description
`nextInt()`	random integer
`nextInt(max)`	random integer between `0` (inclusive) and `max` (exclusive)
`nextDouble()`	random double between `0.0` (inclusive) and `1.0` (exclusive)

Formula for Generating a Random Number between min - max (inclusive)¶

Steps	Example
1. What is the desired min?	4
2. What is the desired max?	10
3. Calculate the range = max - min + 1	10 - 4 + 1 = 7
4. Substitute into: `rand.nextInt(range) + min`	`rand.nextInt(7) + 4`

Math Class¶

(a set of useful methods for performing mathematical operations)

Method	Returns
`Math.abs(value)`	absolute value (make nonnegative)
`Math.max(value1, value2)`	larger of two values
`Math.min(value1, value2)`	smaller of two values
`Math.pow(base, exp)`	`base` to the `exp` power
`Math.sqrt(num)`	square root of `num`

Example

// Evaluates to 8.0
double num = Math.pow(2, 3);
// Evaluates to 9.0
double big = Math.max(num, Math.sqrt(81));
System.out.println(big + " is big!");

Output

9.0 is big!