Assignment and Operations
1E - Assignment and Operations (author: Tao Yue, state: unchanged)
Once you have declared a variable, you can store values in it. This is called assignment.
To assign a value to a variable, follow this syntax:
variable_name := expression;
Note that unlike other languages, whose assignment operator is just an equals sign, Pascal uses a colon followed by an equals sign, similarly to how it's done in most computer algebra systems.
The expression can either be a single value:
some_real := 385.385837;
or it can be an arithmetic sequence:
some_real := 37573.5 * 37593 + 385.8 / 367.1;
The arithmetic operators in Pascal are:
|+||Addition or unary positive||real or integer||real or integer|
|-||Subtraction or unary negative||real or integer||real or integer|
|*||Multiplication||real or integer||real or integer|
|/||Real division||real or integer||real|
|mod||Modulus (remainder division)||integer||integer|
div and mod only work on integers. / works on both reals and integers but will always yield a real answer. The other operations work on both reals and integers. When mixing integers and reals, the result will always be a real since data loss would result otherwise. This is why Pascal uses two different operations for division and integer division. 7 / 2 = 3.5 (real), but 7 div 2 = 3 (and 7 mod 2 = 1 since that's the remainder).
Each variable can only be assigned a value that is of the same data type. Thus, you cannot assign a real value to an integer variable. However, certain data types will convert to a higher data type. This is most often done when assigning integer values to real variables. Suppose you had this variable declaration section:
var some_int : integer; some_real : real;
When the following block of statements executes:
some_int := 375; some_real := some_int;
some_real will have a value of 375.0.
Changing one data type to another is referred to as typecasting. Modern Pascal compilers support explicit typecasting in the manner of C, with a slightly different syntax. However, typecasting is usually used in low-level situations and in connection with object-oriented programming, and a beginning programming student will not need to use it. Here is information on typecasting from the GNU Pascal manual.
In Pascal, the minus sign can be used to make a value negative. The plus sign can also be used to make a value positive, but is typically left out since values default to positive.
Do not attempt to use two operators side by side, like in:
some_real := 37.5 * -2;
This may make perfect sense to you, since you're trying to multiply by negative-2. However, Pascal will be confused — it won't know whether to multiply or subtract. You can avoid this by using parentheses to clarify:
some_real := 37.5 * (-2);
The computer follows an order of operations similar to the one that you follow when you do arithmetic. Multiplication and division (* / div mod) come before addition and subtraction (+ -), and parentheses always take precedence. So, for example, the value of: 3.5*(2+3) will be 17.5.
Pascal cannot perform standard arithmetic operations on Booleans. There is a special set of Boolean operations. Also, you should not perform arithmetic operations on characters.