2015-03-10_for-loops.md

Objectives

1. To be able to properly initialize, terminate, and increment a for statement.
2. To be able to use continue and break.
3. To understand one reason why we use methods.

Review of the while statement

Definition: A loop is code that is run over and over again.

Remember a while loop?

while (condition) {
    // do something over and over
}

Definition: A while statement repeatedly executes a block of code while a given condition is true.

for statement

A for statement is a loop that looks like this...

for (initialization; termination; increment) {
    // do something
}

...where:

1. The initialization is executed once, beginning the loop.
2. When the termination expression is false, the loop terminates; this is analogous to the condition in a while statement.
3. The increment expression is executed after each iteration; this can also decrement.

As with if and while, the body of the loop contains a single statement.

Demo: for loop that prints 0-4 inclusive.

Exercise: Rewrite the following while loop as a for loop:

int i = 0;
while (i <= 100) {
    i++;
    System.out.println(i);
}

while and for loops can both express the exact same computation.

Definition: syntactic sugar is a syntax designed to make the code easier to read.

Common errors

What is wrong with this for loop?

for (int i = 3; i < 30; i--) {
    System.out.println("counting... " + i);
}

The hardest part about for loops is properly stopping the loop. Programmers often make mistakes when setting the termination condition for a loop. In particular, we are often off-by-one. This kind of bug it so common, it has its own Wikipedia page.

Exercise: Counting Machine: Write a program that counts from 0 to a user-specified number.

Exercise: [harder] Counting Machine Revisited: Now let the user input the initial value, the max value, and the increment.

Exercise: "FizzBuzz" is a very famous program, up there with "Hello World!". Write FizzBuzz using a for loop.

break and continue

Remember break?

Definition: break terminates the loop.

Java also has another useful keyword, continue:

Definition: continue jumps to the next iteration of the loop.

continue can be useful skip over some unnecessary computation.

for (int i = 1; i <= 20; i++) {
    if (i % 2 == 0) continue;
    System.out.println(i);
}

Is continue syntactic sugar? What could you do with the above code if Java didn't provide the continue statement?

Exercise: Simplify your code for FizzBuzz using continue.

Iterating over strings

Often, programmers need to iterate over text. Java provides two functions, String.length() and String.charAt() that make iterating over a String fun. Here is how to use them:

System.out.println("Queens!".length()); // 7

The function String.charAt() returns the char at the index provided. For example:

System.out.println("Queens!".charAt(3)); // e

In-class assignment: [hard] Below is a quote from Grace Hopper, the inventor of the first compiler. Write a program that uses a for loop to count how many times she says the word "data". You'll need two String functions in order to complete this assignment. Make sure to verify your answer.

"We must include in any language with which we hope to describe complex data-processing situations the capability for describing data. We must also include a mechanism for determining the priorities to be applied to the data. These priorities are not fixed and are indicated in many cases by the data.

"Thus we must have a language and a structure that will take care of the data descriptions and priorities, as well as the operations we wish to perform. If we think seriously about these problems, we find that we cannot work with procedures alone, since they are sequential. We need to define the problem instead of the procedures. The Language Structures Group of the Codasyl Committee has been studying the structure of languages that can be used to describe data-processing problems. The Group started out by trying to design a language for stating procedures, but soon discovered that what was really required was a description of the data and a statement of the relationships between the data sets. The Group has since begun writing an algebra of processes, the background for a theory of data processing.

"Clearly, we must break away from the sequential and not limit the computers. We must state definitions and provide for priorities and descriptions of data. We must state relationships, not procedures."