2.2. Expressions

So far we have dealt with statements which can be executed and properties which can hold values. Often we will want to perform a series of mathematical operations on some set of values. In programming, a sequence of values and mathematical symbols is known as an expression. Expressions and blocks that "fit" into expressions have two shapes: rectangles with rounded ends and rectangles with pointed ends:

The difference between the rounded rectangles and the pointed rectangles is that rounded rectangles return numbers and text strings. Number values are exactly what you would expect, anything from integers (0, 1, 2, etc) to real numbers (0.1, 3.5, etc). String values are sequences of symbols (a, b, A, B, 1, 2, @, *, etc) which are also known as characters in most programming languages. Pointed rectangles return Boolean values. Boolean values can be only 1 of 2 possibilities: True or False.

While single blocks that return values can be quite useful, they are often combined into expressions using operators. A value that is passed to an operator is called an operand. If an operator takes two operands then it is called a binary operator. The following are the binary numeric operators in Scratch:

Most of these operators like + and - are obviously familiar. * stands for multiply and / stands for divide. The mod operator is less common but often very useful. It returns the remainder of a division. In other words 11 mod 4 is 3 because 11 is divisible by 4 twice with remainder of 3. You can see this in Scratch by placing two numbers in the less than operator's blank boxes and then double clicking on the operator.

The following blocks also "operate" on numbers in a sense but we have separated them into a different category because in most programming languages these are called functions:

The random function picks a number randomly between two values (1 and 10 at the moment). The round function rounds a number to the nearest integer. The sqrt takes the square root of a number as it is specified now, but it has a drop-down menu that allows the selection of several possible mathematical functions like sine, cosine, natural log, etc.

Another subtle difference between operators and functions is that the blank boxes in operator blocks are operands while the blank boxes in function blocks are arguments. There is absolutely no difference in how operands and arguments behave in Scratch, but you should be familiar with these terms because they are used extensively in discussing programming languages.

The following are the 3 string functions in Scratch.

The first function joins two strings together. To join multiple strings, simply place more joins inside of joins:

The second function returns a single character from a certain position in a string. The first character is given the index 1 (although this may seem obvious, in some languages the first character is given the index 0). The third function returns the length of a string.

Relational operators link numbers and strings to Boolean values (values that are only True or False). The following are the relational operators equals, less than, and greater than:

Relational operators can take number arguments or string arguments. For numbers, the operator returns True if the relation is valid for two numbers and False if it is not. For example, 2 < 3 returns True but 4 < 3 returns False. You can see this in Scratch by placing two numbers in the less than operator's blank boxes and then double clicking on the operator. The Boolean operators in scratch are AND, OR, and NOT.

The AND operator returns True if and only if both operands are True:

True AND True => True
True AND False => False
False AND True => False
False AND False => False

The OR operator returns True if at least one operand is True:

True OR True => True
True OR False => True
False OR True => True
False OR False => False

The NOT operator simply negates the Boolean value:

NOT True => False
NOT False => True