Loops (in Java)

“History doesn’t repeat itself but it often rhymes”

-Attributed mistakenly to Mark Twain

Overview
While Loops
For Loops
Some Common Tasks
Conclusions

Overview

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.

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.

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.

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!

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!

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.

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

// 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

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.

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.

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);

Conclusions

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

Thank you. Bye.