Iteration with Loops

A for loop executes a block of statements multiple times.

Press the F8 key to run the program, or click the Run button.

A statement block is a group of statements enclosed inside braces {}. Braces are sometimes called “curly brackets”.

A for loop repeats, or iterates, the execution of the statement block.

In this example, the for loop will iterate 7 times.

Additionally, the statement sleepMs(600) is used to slow down execution of the program.

Exercises:

1. In the source code, the number 600 is the argument of the sleepMs function. Change the number 600 to number 150 to see how it affects the program speed.
[Run the program]

2. Now, change the argument of the sleepMs function to 900.
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This example prints on the screen the value of the loop control variable i, in each iteration.
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A for statement should not be ended by a semicolon because a for statement is not a plain statement. A for statement is a block-ended statement.

The statement sleepMs(500) pauses the execution for exactly one half of a second.

Exercise:

1. Modify the program so that it prints out 30 numbers.

The loop control variable, named m in this example, holds the number of current iteration.

The program completes execution almost instantly, because it contains no pauses.
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Exercise:

1. Change the println statement as follows: println "m*3 = ", m*3;

In this example, the body of the for loop consists of three statements inside a statement block.
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The argument of sleepMs function equals 500 in this program; it specifies the sleep time in milliseconds. 500 milliseconds equals exactly one half of a second.

Exercise:

1. Inside the body of the for loop, insert a new statement: println i;

In this example, the loop control variable is named k.
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The variable k is also a part of the compound expression k*3.

Name of a variable is a primary expression. Primary expressions can be combined to form more complex expressions.

Numbers are also primary expressions. In this example, the number 3 is a primary expression, which is a part of the compound expression k*3.

Exercises:

1. Add the following statement at the end of the source code (below the body of the loop).: println "Program ends"; [Run the program]

2. Insert the following statement in a new line inside the body of the for loop: sleepMs(200); [Run the program]

This example prints out first 10 multiples of number 3.
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A string is a text composed of characters. Kinds of characters include letters, digits, spaces, punctuation, and many others.

A single-quotes mark is actually a character called “an apostrophe”. A double-quotes character is more precisely called “a quotation mark”.

Strings are allowed to be enclosed inside single-quotes. Such strings are allowed to contain double-quotes.

Exercise:

1. Modify the program so that it prints out first ten multiples of number 7.
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2. Change the name of the variable i to z.
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Name of a variable can be composed of any letters, but it cannot contain spaces.
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A println statement scrolls the screen up when the screen is full.

This program prints out the squares of first 50 natural numbers. A square of a number is the number multiplied by itself.

Exercise:

(Challenging exercise!) Change the program so that it outputs results of expression 1000/number.

Remember that arguments of println statement must be separated by commas (,).

Peek at the solution if you can't solve this exercise.

Many differens solutions are possible. Here is one of them:

    println 1000, "/", number, " = ", 1000/number;

In this example, the for loop counts backwards. The initial value (50) of the loop is greater than the bounding value (1).
[Run the program]

Exercise:

1. Change the argument 55 of the sleepMs function into expression n*4 .

In this example, value of the loop control variable n is increased by 3 in each iteration. In other words, the step of the for loop equals 3.
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The step of the for loop is specified after the symbols += .

If a loop is counting downwards, then the step can be specified after the symbols -= .

Exercise:

1. Change the loop so that the variable n starts from value 50 and then decreases by 3 in each iteration.

for #n= 50 .. 1  -=3

In this example, some advanced features of colors and println statements will remain unexplained.
[Run the program]

Exercise:

1. Change the string “ZedLX” displayed on the screen. A string is a text composed of characters.

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