Basic Conditionals¶

This Lesson is estimated to be completed in 20 minutes.

You should have first completed the Lessons:

Description¶

Flow control is an essential aspect of most programming languages.

Here we cover in M the IF / ELSE commands.

The IF Command in M has the familiar form

if  condition  command

in its simplest form the command can be placed in front of the condition

if a<5 write "a is smaller than 5",!

however when the if body has long commands or has multiple commands, the vertical form below if better suited for clarity

if a<5 do
. write "a is smaller than 5",!

note again the use of the ”.” to indicate indentation in the commands that are part of the IF body.

As you may expect, the ELSE command indicates the alternative outcome of the logical condition.

if a<5 write "a is smaller than 5",!
else  write "a is not smaller than 5",!

note the double space in front of the ELSE command.

one can also place both the positive and the negative logical outcomes into indented bodies, as in

if a<5 do
. write "a is smaller than 5",!
else  do
. write "a is not smaller than 5",!

or for with larger bodies

if a<5 do
. write "a is smaller than 5",!
. set b=a*2
else  do
. write "a is not smaller than 5",!
. set b=a/2

Note again that there are two spaces after the ELSE command.