# Special Ways to Input Expressions

*Mathematica* allows you to use special notation for many common operators. For example, although internally *Mathematica* represents a sum of two terms as Plus[x, y], you can enter this expression in the much more convenient form .

The *Mathematica* language has a definite grammar that specifies how your input should be converted to internal form. One aspect of the grammar is that it specifies how pieces of your input should be grouped. For example, if you enter an expression such as , the *Mathematica* grammar specifies that this should be considered, following standard mathematical notation, as rather than . *Mathematica* chooses this grouping because it treats the operator as having a higher *precedence* than . In general, the arguments of operators with higher precedence are grouped before those of operators with lower precedence.

You should realize that absolutely every special input form in *Mathematica* is assigned a definite precedence. This includes not only the traditional mathematical operators, but also forms such as , or the semicolons used to separate expressions in a *Mathematica* program.

The table in "Operator Input Forms" gives all the operators of *Mathematica* in order of decreasing precedence. The precedence is arranged, where possible, to follow standard mathematical usage, and to minimize the number of parentheses that are usually needed.

You will find, for example, that relational operators such as have lower precedence than arithmetic operators such as . This means that you can write expressions such as without using parentheses.

There are nevertheless many cases where you do have to use parentheses. For example, since has a lower precedence than , you need to use parentheses to write . *Mathematica* interprets the expression as . In general, it can never hurt to include extra parentheses, but it can cause a great deal of trouble if you leave parentheses out, and *Mathematica* interprets your input in a way you do not expect.

Four ways to write expressions in *Mathematica*.

There are several common types of operators in *Mathematica*. The in is an "infix" operator. The in -p is a "prefix" operator. Even when you enter an expression such as *Mathematica* allows you to do it in ways that mimic infix, prefix and postfix forms.

In[1]:= |

Out[1]= |

In[2]:= |

Out[2]= |

In[3]:= |

Out[3]= |

You should notice that has very low precedence. If you put //f at the end of any expression containing arithmetic or logical operators, the f is applied to the *whole expression*. So, for example, means , not .

The prefix form has a much higher precedence. is equivalent to , not . You can write in prefix form as .