website/content/ta/fall2017/cpsc220/sept20.md

Counting Loop

Looking at the following example code

int i;
for (i = 0; i < 3; i++) { //begin for
  System.out.println("i = " + i); //body
} //end for
System.out.println("After loop, i = " + i);

i = 0 is the initializing statement

i < 3 is the conditional, that is when the loop ends

i++ is the increment/decrement

i++ is synonymous with i = i + 1

The initialization statement only occurs once at the beginning of the loop.

Execution Example

Let us go through this for loop example

• Let us set i = 0
• Is i < 3? Yes execute the body
  • The body executes an output of "i = 0"
• Now we increment i ++, i is now 1
• Is i < 3? Yes, 1 is less than 3. Execute body
  • The computer prints out "i = 1"
• Increment i++ i is now 2
• Is i < 3? Yes 2 is less than 3. Execute body
  • The computer prints out "i = 2"
• Increment i++, i is now 3
• Is i < 3? No 3 is not less than 3
  • Don't execute body of loop

Exit loop. Print "After loop, i = 3"

Condensing Syntax

You can also do the declaration in the initialization statement

for (int i = 0; i < 3; i++) {
    System.out.println("i = " + i);
}

This now runs like above without the "After loop, i = 3" print. You cannot access the variable i outside the for loop since in this example, it belongs to the for loop's scope.

Logic Expressions

And Statements

With the AND operator && both the left and right side needs to be true for the expression to be true.

true && true // true
true && false // false
false && true // false
false && false // false

Or Statements

With the OR operator || either the left or right side needs to be true for the expression to be true.

true || true // true
true || false // true
false || true // true
false || false // false

Examples

Example: Print out the number n if it is between 10 and 20 (inclusive)

if (n >= 10 && n <= 20) {
    System.out.println(n);
}

Example: Print out the age if it is not of young adult age. Young adult range is from 18 to 39 (inclusive)

if (!(age >= 18 && age <= 39)) {
    System.out.println(age);
}

Or you can use De Morgan's Law (for the curious)

if (age < 18 || age > 39) {
    System.out.println(age);
}

For Loops (Cont.)

Backwards counting

You can use the loop to count backwards

for (int i = 10; i > -1; i--) {
    System.out.println(i);
}

This prints the following

10
9
8
7
6
5
4
3
2
0

Rows-Columns

You can make rows and columns of asterisks

for (int j = 0; j < someNumber; j++) { // Corresponds to rows
  for (int i = 0; i < someNumber2; i++) { // Corresponds to columns
    System.out.print("*"); 
  }
  System.out.println(""); // Goes to the next row
}

If someNumber equals someNumber2, then we have the same amount of rows as columns.

Let someNumber equal to 2 and someNumber2 equal to 2

Output:

**
**

Right Triangles

You can make a right triangle of Tilda with the following code

for (int i = 1; i <= num; i++) { // Corresponds to the row
  for (int j = 0; j < i; j++) { // Corresponds to the column and stops at the current row number
      System.out.print("~");
  }
  System.out.println(""); // Moves to next row
}

What are for-loops used for? Reusing code