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In this session, I'd like you to focus

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on the use of parentheses and brackets.

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Don't mix them up.

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Parentheses change the priority
of an execution

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in expressions or in code,

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while brackets define blocks.

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When the expressions in a block
are defined, they are not executed.

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You control the execution: you must send
the message value to the block

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to trigger the execution
of its expressions.

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Let's talk again
about when you must use brackets.

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You use them around expressions

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if you don't know
whether they will be executed:

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for instance, do they use if? Do they
belong to the branch of a deviation?

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You also use brackets

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if you don't know how many times
you must execute

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the expressions a block contains.

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For instance:
n timesRepeat: [self doSomething]

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self doSomething
is encapsulated in a block.

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Why? Because you must
execute its expressions several times.

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You must put them in a block to send it
the message value several times.

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Another example: this time,
I send the message ifTrue:.

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Depending on the receiver,

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depending on whether this expression
is true or false,

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you execute or don't execute
the expression self doSomething.

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I can't use parentheses since if needed,
it won't be executed at all.

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If you put it in a block,

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you can decide
whether you send it the message value.

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This is the same thing
but with whileTrue:.

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It's the same with the receiver block
and the block passed as a parameter.

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Depending on whether the evaluation
returns the Boolean true or false,

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you must execute once, several times,
or even not at all

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the expression self doSomething:
it must be put in a block.

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Let's do a small exercise.

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If I give you
the two following expressions,

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repeat from 1 to: n self doSomething,

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should I use parentheses or brackets?

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If I write x ifEmpty: self doSomething,

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should I use parentheses or brackets?

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I'll give you the answers.

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In both cases, you should use brackets.

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In the first case,
if you want to repeat self doSomething,

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you have to send the message value
to this block several times.

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In the second case, x ifEmpty:,

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if x isn't empty,
you shouldn't execute self doSomething.

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If x is empty, you have to.

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To sum up, we saw in this course

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that you shouldn't mix up
parentheses and brackets.

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Parentheses are about changing the order
of computation in an expression.

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Brackets are about freezing

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expressions

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and to control their execution
or even to execute them several times.