Intro to CS

These are practice programs to help you get familiar with loops in Java. They are ordered from easiest to hardest.

Loop refresher

Recall the general structure of a for loop:

for(int i = 0; i < 15; i++) {
    // some code...
}

The loop above runs for 15 iterations. You can parse the meaning of a for loop from the first line:

Create a counter i that starts at 0.
Every iteration of the for loop, check if i < 15.
If i < 15, run the code inside the loop. While running the loop code, you can use the value of i. So for example, the first time the loop runs, the value of i is zero.
At the end of running the loop code, increment i++.
If !(i < 15), exit the loop. So the last time the loop code runs, the value of i is 14, and i is incremented to 15. Then it fails the check for i < 15 and the loop code doesn't run.

Caution! It is very difficult to catch programming bugs in the loop condition! So we always suggest that you design the most basic loop, and increment the loop counter by one (that is, i++). And then don't change the value of the loop counter within the loop itself!

The following code will only run four times!!! Do you see why?

// Don't do this! This is hard to debug!
for(int i = 0; i < 10; i ++) {
    i += 2; // bad!!!
    println(i);
}

A better way to do the above code:

// Uses another variable to keep the loop counter safe
for(int i = 0; i < 10; i ++) {
    int j = i + 2; // does not change the value of i
    println(j);
}

The following exercises are all about using the for-loop counter i.

Looping Fun 1

BonusConsole: LoopingFun1.java

Try to produce the following output with a loop. Print the first ten odd numbers, starting from 1.

Extension: Now try to print the first ten even numbers, starting from 0.

Looping Fun 2

BonusConsole: LoopingFun2.java

Try to produce the following output with a loop. Print the first ten even numbers in decreasing order.

Can you solve this problem without changing the loop condition from the previous problem? That is, start from the following for loop code:

for(int i = 0; i < 10; i++) {
    // some code...
}

How would you solve the problem by using the following loop condition?

for(int i = 0; i < 20; i++) {
    // some code...
}

Looping Fun 3

BonusConsole: LoopingFun3.java

Use a loop, but print out everything on a single line. Recall that println() adds a line break/new line, whereas print() does not.

Print out the squares, starting from 1.

Try avoiding a comma at the end of the last square!

Looping Fun 4

BonusConsole: LoopingFun4.java

Time for nested for loops! Print out the coordinates of a 5x5 grid.

Hint: try the following code structure:

for(int i = 0; i < 5; i++) {
    for(int j = 0; j < 5; j++) {
        // some code...
    }
}

Additional practice: While loops

Want more practice with while loops? Do all the above exercises with a while loop instead of a for loop. Good luck!