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177 lines
3.3 KiB
Markdown
177 lines
3.3 KiB
Markdown
## Counting Loop
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Looking at the following example code
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```java
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int i;
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for (i = 0; i < 3; i++) { //begin for
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System.out.println("i = " + i); //body
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} //end for
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System.out.println("After loop, i = " + i);
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```
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`i = 0` is the initializing statement
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`i < 3` is the conditional, that is when the loop ends
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`i++` is the increment/decrement
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`i++` is synonymous with `i = i + 1`
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The initialization statement only occurs once at the beginning of the loop.
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### Execution Example
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Let us go through this for loop example
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- Let us set `i = 0`
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- Is `i < 3`? Yes execute the body
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- The body executes an output of `"i = 0"`
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- Now we increment `i ++`, i is now 1
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- Is `i < 3`? Yes, 1 is less than 3. Execute body
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- The computer prints out `"i = 1"`
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- Increment `i++` i is now 2
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- Is `i < 3`? Yes 2 is less than 3. Execute body
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- The computer prints out `"i = 2"`
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- Increment `i++`, i is now 3
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- Is `i < 3`? No 3 is not less than 3
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- Don't execute body of loop
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Exit loop. Print `"After loop, i = 3"`
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### Condensing Syntax
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You can also do the declaration in the initialization statement
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```java
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for (int i = 0; i < 3; i++) {
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System.out.println("i = " + i);
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}
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```
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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.
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## Logic Expressions
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### And Statements
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With the AND operator `&&` both the left and right side needs to be true for the expression to be true.
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```java
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true && true // true
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true && false // false
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false && true // false
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false && false // false
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```
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### Or Statements
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With the OR operator `||` either the left or right side needs to be true for the expression to be true.
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```java
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true || true // true
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true || false // true
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false || true // true
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false || false // false
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```
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### Examples
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**Example**: Print out the number `n` if it is between 10 and 20 (inclusive)
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```java
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if (n >= 10 && n <= 20) {
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System.out.println(n);
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}
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```
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**Example**: Print out the `age` if it is not of young adult age. Young adult range is from 18 to 39 (inclusive)
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```java
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if (!(age >= 18 && age <= 39)) {
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System.out.println(age);
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}
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```
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Or you can use De Morgan's Law (for the curious)
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```java
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if (age < 18 || age > 39) {
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System.out.println(age);
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}
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```
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## For Loops (Cont.)
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### Backwards counting
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You can use the loop to count backwards
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```java
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for (int i = 10; i > -1; i--) {
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System.out.println(i);
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}
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```
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This prints the following
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```java
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10
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9
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8
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7
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6
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5
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4
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3
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2
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0
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```
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### Rows-Columns
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You can make rows and columns of asterisks
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```java
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for (int j = 0; j < someNumber; j++) { // Corresponds to rows
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for (int i = 0; i < someNumber2; i++) { // Corresponds to columns
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System.out.print("*");
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}
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System.out.println(""); // Goes to the next row
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}
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```
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If `someNumber` equals `someNumber2`, then we have the same amount of rows as columns.
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Let `someNumber` equal to 2 and `someNumber2` equal to 2
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Output:
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```
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**
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**
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```
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### Right Triangles
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You can make a right triangle of Tilda with the following code
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```java
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for (int i = 1; i <= num; i++) { // Corresponds to the row
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for (int j = 0; j < i; j++) { // Corresponds to the column and stops at the current row number
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System.out.print("~");
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}
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System.out.println(""); // Moves to next row
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}
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```
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##### What are for-loops used for? *Reusing code*
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