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content/ta/fall2017/cpsc220/sept20.md

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+## Counting Loop
+
+Looking at the following example code
+
+```java
+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
+
+```java
+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.
+
+```java
+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.
+
+```java
+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)
+
+```java
+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)
+
+```java
+if (!(age >= 18 && age <= 39)) {
+    System.out.println(age);
+}
+```
+
+Or you can use De Morgan's Law (for the curious)
+
+```java
+if (age < 18 || age > 39) {
+    System.out.println(age);
+}
+```
+
+
+
+## For Loops (Cont.)
+
+### Backwards counting
+
+You can use the loop to count backwards
+
+```java
+for (int i = 10; i > -1; i--) {
+    System.out.println(i);
+}
+```
+
+This prints the following
+
+```java
+10
+9
+8
+7
+6
+5
+4
+3
+2
+0
+```
+
+### Rows-Columns
+
+You can make rows and columns of asterisks
+
+```java
+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
+
+```java
+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*
+