Loops

nyu-java-programming / course-materials / loops

# Loops

> "History doesn’t repeat itself but it often rhymes"
>
> -Attributed mistakenly to [Mark Twain](https://en.wikipedia.org/wiki/Mark_Twain)

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# Agenda

1. [Overview](#concept)
1. [While Loops](#while)
1. [For Loops](#for)
1. [Some Common Tasks](#common)
1. [Conclusions](#conclusions)

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# Overview

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## Concept

It is often useful to repeat a block of code more than once.

- Rather than copy/paste the same code multiple times in sequence, loops provide a way of writing the code only once and indicating the instructions for how many iterations it should be repeated.

---

# While Loops

## Concept

While loops give us ultimate flexibility in controlling repetition.

- We can easily set up counter-based loops known as 'accumulators', or loops based on the value in flag variables.

- We can also write infinite loops that never stop repeating.

---

## Accumulator-based loop

One of the classic patterns, where a counter stops the loop after a certain number of iterations.

```java
int i = 0; // set the starting value of the counter

while (i < 10) {
    // iterate as long as the counter is below some target value
    System.out.println("The value of i is" + i);
    i++; // increment the counter
}
```

---

## Flag-based loop

Another of the classic patterns, where the value of a boolean variable causes the loop to stop iterating at some point.

```java
boolean keepGoing = true; // by default, iterate!

while (keepGoing) {
    // iterate as long as the boolean value in the flag is true
    System.out.println("Iterating!");

// stop the loop if a random number between 1-100 is equal to 22
    int rand = (int) (Math.random() * 100) + 1;

// use the ternary operator to flip the value of the flag, if necessary
    keepGoing = (rand == 22) ? false : true; // set to false if the rand value is 22, true otherwise
}
```

---

## Infinite loop

Usually undesireable, but while loops can iterate infinitely.

```java

while (true) {
    // iterate forever... until the user quits the program or the computer shuts it down
    System.out.println("Iterating!");
}
```

---

# For Loops

## Concept

For loops allow us to simplify the syntax for writing an 'accumulator'- or 'counter'-based loop.

- The accumulator pattern is baked into the `for` loop syntax.

- A `for` loop can be set to terminate based on a flag's value, but its syntax is not designed for that.

- A `for` loop can be set to run infinitely, but its syntax is not designed for that.

---

## Accumulator-based loop

One of the classic patterns, where a counter stops the loop after a certain number of iterations.

- this is what `for` loops were designed to do!

```java
for (int i = 0; i < 10; i++) {
    // iterate as long as the counter is below some target value
    System.out.println("The value of i is" + i);
}
```

---

## Flag-based loop

Another of the classic patterns, where the value of a boolean variable causes the loop to stop iterating at some point.

- `for` loops are almost never used for this... but here we go!

```java
boolean keepGoing = true; // by default, iterate!

for (; keepGoing ;) {
    // iterate as long as the boolean value in the flag is true
    System.out.println("Iterating!");

// stop the loop if a random number between 1-100 is equal to 22
    int rand = (int) (Math.random() * 100) + 1;

// use the ternary operator to flip the value of the flag, if necessary
    keepGoing = (rand == 22) ? false : true; // set to false if the rand value is 22, true otherwise
}
```

---

## Infinite loop

Usually undesireable, but for loops can iterate infinitely.

```java

for (; ;) {
    // iterate forever... until the user quits the program or the computer shuts it down
    System.out.println("Iterating!");
}
```

---

# Some Common Tasks

## Iterating through values in an array

```java
// an array of strings
String[] messages = {
        "hi",
        "how are you?",
        "how's it going?",
        "good to see you!",
        "have a great day!",
        "hope to see you around :)"
};

// classic for loop that iterates as many times as there are elements in the array
for (int i=0; i<messages.length; i++) {
    // print out the value at each position in the array
    System.out.println("The value in the array at index " + i + " is: " + messages[i]);
}
```

---

## Iterating until a particular response is received from a user

```java
Scanner scn = new Scanner(System.in); // open the scanner outside the loop, since we could potentially need to get more than one int of input
keepGoing = true; // set the flag to true so this loop will at least iterate once
while (keepGoing) {
    // get user input
    System.out.print("Please enter an integer between 1 and 10: ");
    int num = scn.nextInt();

// validate the user's input
    if (num >= 1 && num <= 10) {
        keepGoing = false; // change the value of the flag to cause the loop to terminate
        System.out.println("Thanks for the good input!... quitting loop");
    }

// if the user's input is invalid, x is still true, so no need to set it to that!
}
scn.close(); // close the scanner resource to be nice to the processor
```

---

## Iterating through the characters in a string

In Java, strings contain arrays of `char` values. Note that the length of a string is obtained using the `length()` function, not the `length` property used with arrays.

```java
String s = "asparagus";
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
    System.out.println(c);
}
```

---

## Calculating a running total

A 'running total' is a specific sub-variety of an accumulator pattern, where a value is added to with each iteration.

```java
Scanner scn = new Scanner(System.in);

int total = 0; // start the total at zero
String stop_response = "stop"; // what the user must type in order to stop the loop
String users_response = ""; // will hold what the user types in each iteration

// keep looping until the user types 'stop'
while (!users_response.equals(stop_response)) {
    System.out.println("Enter a number: "); // ask for a number
    users_response = scn.nextLine(); // get the user's response as a string
    try {
        int val = Integer.parseInt(users_response); // convert the respose to an int
        total = total + val; // add it to the running total
    }
    catch (Exception e) {
        // if the user types a non-integer, an error will be caught here
        System.out.println("Sorry, that's not a valid number");
    }
}
System.out.println("The total is " + total);
```

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# Conclusions

You now have a basic understanding of for loops, while loops, and a few common tasks performed by each.

- Thank you. Bye.