# Assignment and Operations

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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:

Operator | Operation | Operands | Result |
---|---|---|---|

+ | 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 |

div | Integer division | integer | integer |

mod | Modulus (remainder division) | integer | integer |

** div** and

**only work on**

`mod`*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.

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