Iterators and Conditionals

Medley offers a set of useful functions and operators for conditionals and loops (e.g., COND, IF, AND, OR, FOR, WHILE). It also provides us with a list of functions to compare our data and understand its type.

You can refer to the IRM's Chapter 9, "Lists and Iterative Statements," for an exhaustive list of available options. Here are a select few as examples:

(FLOATP X): Returns x if x is a floating point number. Otherwise returns NIL.

(NUMBERP X): Returns x if x is a number of any type. Otherwise returns NIL.

(STRINGP X): Returns x if x is a string. Otherwise returns NIL.

(EQP X Y): Returns T if x and y are numbers and equal in value. Otherwise returns NIL.

(EQUAL X Y): Returns T if X and Y are numbers with equal value, or strings with the same sequence of characters, or lists where CAR of X and Y and CDR of X and Y are equal. Otherwise returns NIL.

Functions ending with “P” are called predicates. They test a condition and return a true value (any non-NIL object) or NIL.

Conditionals

You may have used if/else statements in other programming languages. In Interlisp, COND lets you chain together multiple conditional branches concisely. If you're familiar with switch statements in C++ or C#, or match blocks in Python, COND serves a similar purpose- evaluating conditions in order and returning the result of the first one that succeeds.

A verbose conditional block like (if A then B else if C then D else E) can be rewritten instead as:

(COND (A B)(C D)(T E))

T marks the default value. If all previous condition checks return NIL, then the argument following T is evaluated.

The arguments after COND are referred to as clauses.

(COND CLAUSE₁ CLAUSE₂ CLAUSE₃ ... CLAUSE_N)

Each clause is evaluated in order and is a list of the form (P₁ C₁ ... C_1N). Here, P is the condition to be checked. If P₁ returns true, then C₁ to C_1N are evaluated sequentially. If P1 is not true, then we evaluate P2 and then P3, and so on. If none of the conditions are true, then COND returns NIL.

---

Do it yourself: Write a small function using COND to identify whether a value is a string or a number and print that answer.

Your function should look similar to this:

---

Loops

Similar to other languages, you can use operators like FOR and WHILE to set up loops.

(for X from 1 to 5 do (PRINT X)

Medley has a large collection of iterative statement operators. Check out the IRM's Conditionals and Iterative Statements chapter for a full list. Here's a small collection:

DO: Evaluates the form/argument for each iteration.

COLLECT: Puts the value of all iterations in a list.

JOIN: Returns a list with all values concatenated.

SUM: Returns the sum of all values.

FROM: Specifies the starting value.

TO: Specifies the end value.

IN: Steps through each element of a list, returning the first one that matches a given condition.

WHEN: Only runs when the condition is met.

---

Do it yourself: Write a loop to collect all the numbers from a list of mixed data in a new list.

Can you put a loop inside a COND statement? Give it a try!