Foundations

Conditionals

Make Decisions

You're building a login system. If the password matches, show the dashboard. Otherwise, show "Invalid password". Programs need to take different paths based on conditions - that's what conditionals do.

Simple if

Execute code only when a condition is true.

Simple.java 
public class Simple {
    public static void main(String[] args) {
        int x = ;
        
        if (x > 5) {
            System.out.println("x > 5");
        }
        
        if (x > 20) {
            System.out.println("x > 20");
        }
    }
}

public class Simple {
    public static void main(String[] args) {
        int x = ;
        
        if (x > 5) {
            System.out.println("x > 5");
        }
        
        if (x > 20) {
            System.out.println("x > 20");
        }
    }
}

public class Simple {
    public static void main(String[] args) {
        int x = ;
        
        if (x > 5) {
            System.out.println("x > 5");
        }
        
        if (x > 20) {
            System.out.println("x > 20");
        }
    }
}

The code inside if only runs when the condition is true.

if `if (condition)` - executes the block only when condition is true.
comparison `>` greater, `<` less, `>=` greater or equal, `<=` less or equal, `==` equal.

Find the maximum in an array

Scan through all elements, keep track of the largest one found.

Max.java 
public class Max {
    public static void main(String[] args) {
        int[] nums = {23, 45, 12, 67, 34, 89, 41};
        
        System.out.println("nums=" + java.util.Arrays.toString(nums));
        int max = nums[0];
        
        for (int i = 1; i < nums.length; i++) {
            if (nums[i] > max) {
                max = nums[i];
            }
        }
    }
}

Compare each element to our current max. If bigger, update max. This is the find maximum pattern.

Find the minimum in an array

Same idea, but track the smallest.

Min.java 
public class Min {
    public static void main(String[] args) {
        int[] nums = {23, 45, 12, 67, 34, 89, 41};
        
        System.out.println("nums=" + java.util.Arrays.toString(nums));
        int min = nums[0];
        
        for (int i = 1; i < nums.length; i++) {
            if (nums[i] < min) {
                min = nums[i];
            }
        }
    }
}

Same pattern, opposite comparison: update when you find something smaller.

else `else` - executes when the if condition is false.

First N Fibonacci numbers

Generate a variable-length Fibonacci sequence.

FibonacciN.java 
public class FibonacciN {
    public static void main(String[] args) {
        int n = ;
        int[] fib = new int[n];
        
        fib[0] = 0;
        fib[1] = 1;
        
        for (int i = 2; i < n; i++) {
            fib[i] = fib[i - 1] + fib[i - 2];
        }
        
        System.out.println("fib=" + java.util.Arrays.toString(fib));
    }
}

public class FibonacciN {
    public static void main(String[] args) {
        int n = ;
        int[] fib = new int[n];
        
        fib[0] = 0;
        fib[1] = 1;
        
        for (int i = 2; i < n; i++) {
            fib[i] = fib[i - 1] + fib[i - 2];
        }
        
        System.out.println("fib=" + java.util.Arrays.toString(fib));
    }
}

public class FibonacciN {
    public static void main(String[] args) {
        int n = ;
        int[] fib = new int[n];
        
        fib[0] = 0;
        fib[1] = 1;
        
        for (int i = 2; i < n; i++) {
            fib[i] = fib[i - 1] + fib[i - 2];
        }
        
        System.out.println("fib=" + java.util.Arrays.toString(fib));
    }
}

The user specifies how many Fibonacci numbers they want. The condition i < n controls how many iterations run.

Search for a value in array

Check if a specific number exists.

Search.java 
public class Search {
    public static void main(String[] args) {
        int[] nums = {23, 45, 12, 67, 34, 89, 41};
        int target = ;
        
        System.out.println("nums=" + java.util.Arrays.toString(nums));
        int found = 0;
        int position = -1;
        
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                found = 1;
                position = i;
            }
        }
    }
}

public class Search {
    public static void main(String[] args) {
        int[] nums = {23, 45, 12, 67, 34, 89, 41};
        int target = ;
        
        System.out.println("nums=" + java.util.Arrays.toString(nums));
        int found = 0;
        int position = -1;
        
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                found = 1;
                position = i;
            }
        }
    }
}

public class Search {
    public static void main(String[] args) {
        int[] nums = {23, 45, 12, 67, 34, 89, 41};
        int target = ;
        
        System.out.println("nums=" + java.util.Arrays.toString(nums));
        int found = 0;
        int position = -1;
        
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] == target) {
                found = 1;
                position = i;
            }
        }
    }
}

Loop through, check each element against the target. When found, we can stop early or record the position.