4.2 - for Loops

for Loops

• Iterative statement that checks for condition
• Repeatedly execute a a block of code as long as the condition is met
• Condition specifies amount of times

for Loops vs. while Loops

• while Loops: use when number of iterations is unknown
• for Loops: use when number of iterations is known

int count = 1;
while (count <= 5) {
    System.out.println(count);
    count++;
}

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5

for (int count = 1; count <= 5; count++) {
    System.out.println(count);
}

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5

Syntax

• Three parts in for loop header: variable initialization, Boolean (conditional) expression, and increment/decrement statement

for (int i = 0; i < numTimesToRepeat; i++) { 
    // whatever to repeat here
}

Question: Which part is which? (In reference to above)

• variable initialization:
• Boolean (conditional) expression:
• variable can be used in the code block for other various reasons besides specifying how many times the loop will repeat
• Boolean (conditional) expression and increment/decrement statement together determine how many times the loop will repeat

Other syntax for increment/decrement statement:

Operator	Example	Same As
++	i ++	i = i + 1
–	i –	i = i - 1
+=	i += y	i = i + y
-=	i -= y	i = i - y
*=	i *= y	i = i * y
/=	i /= y	i = i / y
%=	i %= y	i = i % y

// play around with this 
for (int i = 0; i < 4; i++) { 
    System.out.println(i);
}

Popcorn Hacks

• Calculate and print the sum of all even numbers from 1 to a given positive integer ‘n’ (user input n)

// CODE CODE CODE HERE
// No need to finish code now, just share ideas

// SOLUTION
import java.util.Scanner;

public class evenNumSum {
    public static void main(String[] args) {
        // Scanner object for user input
        Scanner scanner = new Scanner(System.in);

        // user enter positive integer
        System.out.print("Enter a positive integer (n): ");
        int n = scanner.nextInt();
        System.out.println(n);

        // initialize sum to store
        int sum = 0;

        // for loop 1 to n
        for (int i = 1; i <= n; i++) { // note the <= since start at 1
            // check if n is even
            if (i % 2 == 0) {
                // if even, add to sum
                sum += i;
            }
        }

        // print sum
        System.out.println("\nSum of even numbers from 1 to " + n + " is: " + sum);
    }
}
evenNumSum.main(null)

Enter a positive integer (n): 10

Sum of even numbers from 1 to 10 is: 30

Java for-each Loop

• for-each loop used to iterate through elements of arrays and collections (e.g. ArrayList)
• AKA enhanced for loop

  Syntax

for (dataType item : array) {
    // whatever here
}

• array: array/collection
• item: variable each item of array/collection assigned is assigned to
• dataType - array/collection data type

int[] numbers = {1, 7, -9, 4, 89};
for (int num: numbers) {
    System.out.println(num);
}

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7
-9
4
89