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Loops & Ranges

Loops are programmatic constructs for repeated evaluation and execution. Blade has three statements that are used to support looping over data. The while, iter and for loops. All three of them evaluate conditions and continue to execute so long as those conditions are true. If the conditions are false as at the start of their iteration, they simply do not execute.

It is sometimes convinent to halt the execution of a loop. The break keyword acts as a mechanism for achieving just that. At other times, one simply wants to skip the current iteration and move on to the next. For the later, the continue keyword exists just for that. The break and continue keywords are both valid in all looping techniques.

While Loops


The while loop is very straightforward in application and self explainable. Simply put, while a condition is true, execute the following.

For example:

%> var age = 13
%> while age < 18 {
..   echo 'Happy ${age}th birthday'
..   age++
.. }
'Happy 13th birthday'
'Happy 14th birthday'
'Happy 15th birthday'
'Happy 16th birthday'
'Happy 17th birthday'

In a while loop, the user has the responsibility to evaluate the condition to false. In our example, we incremented the value of age before we end the loop so that age can finally reach a value at which the loops condition evaluation returns false.

That’s it!

Do…While Loops


The do…while loop behaves very similar to the while loop except that it runs the body of the loop once before evaluating the condition.

For example:

%> var i = 10
%> do {
..   echo 'Iteration ${i}'
..   i--
.. } while i > 0
Iteration 10
Iteration 9
Iteration 8
Iteration 7
Iteration 6
Iteration 5
Iteration 4
Iteration 3
Iteration 2
Iteration 1

In a do…while loop the condition is written after the body of the loop. Like the while loop, the responsiblility to evaluate the condition to false is on the user.

Iter Loops


Ah! We are here… Iter loops (sighs!). Iter loops in Blade are akin to for loops in C or JavaScript. And soon enough, you’ll understand why Blade introduced an new keyword (iter) instead of just calling it a for loop.

Iter loops are simple to construct. An iter loop requires an initializer (variable declaration), a condition and a loop control statement (usually an increment/decrement statement); all three being optional and separated by a semi-colon (;).

For example:

%> iter var i = 0; i < 5; i++ {
..   echo 'Iteration ${i}'
.. }
'Iteration 0'
'Iteration 1'
'Iteration 2'
'Iteration 3'
'Iteration 4'

When the initializer is not given, Blade assumes it has been declared somewhere in the code. If it wasn’t, trust that your code will run into one of many syntax error or exception depending on your code.

For example:

%> iter ; i < 10; i++ {
..   echo i
.. }
Unhandled Exception: 'i' is undefined in this scope
  StackTrace:
    File: <repl>, Line: 1, In: <script>

A good practice would look something like the below:

%> var i = 4
%> iter ; i < 10; i++ {
..   echo i
.. }
4
5
6
7
8
9

When the condition or loop control is not is not given, your code will most likely run infinitely

%> iter var i = 0; ; i++ {
..   echo i
.. }
1
2
3
4
5
6
... # to infinity

Unless you have a break statement somewhere in your code.

%> iter var i = 0; ;i++ {
..   echo i
..   if i == 5 break
.. }
0
1
2
3
4
5

@note: break and continue are statements in Blade not expressions.

A simple way to write an infinite loop that runs until you decide to end it within the block is like this.

%> iter ;; {
..   echo i
.. }

The above iter statement will run infinitely.

Since we’ve seen an example using the break statement to halt the execution of a block, it’s only nice that we show how the continue statement to skip running the rest of an iteration.

For example:

%> iter var i  = 0; i < 10; i++ {
..   if i <= 5 
..     continue
..   echo '${i} > 5'
.. }
'6 > 5'
'7 > 5'
'8 > 5'
'9 > 5'

Ranges


Ranges are simple numeric iterables. i.e. They are structures that can be iterated/looped through. Ranges are in the format start..end. They include a starting number (inclusive) and an ending number (non-inclusive) separated by a range operator (..).

For example:

%> 0..10
<range 0-10>
%> 10..5
<range 10-5>

Ranges are valid in any direction. That is, they can either ascend (end greater than start) or descend (start greater than end). They are also evaluated in order. They can also be constructed from variables or a mixture of constant number and variables as desired.

%> var a = 20
%> 5..a
<range 5-20>
%> a..10
<range 20-10>
%> var b = 16
%> a..b
<range 20-16>

Ranges also have a few methods.

range.lower()
Returns the lower limit of the range. For example:
%> (10..100).lower()
10
range.upper()
Returns the upper limit of the range. For example:
%> (20..30).upper()
30

@note: The parenthesis (()) around the range in our example is important to make sure that the upper limit of the range is not interpreted as a number since the range was not assigned to a variable.

Other examples of iterables are Strings, Lists, Dictionaries, Bytes etc.

For Loops


I promised you’ll soon find out why Blade didn’t name the iter loop for loops. Here is why. The construct of the iter loop is familiar with programmers coming from C-like languages, but not so much for those coming from non-C background. The construct of the iter loop make it monstrous and for this reason, Blade’s for loop is much simplier and easier to write and use.

Rather than iterating arithemetic progressions, for loops iterate over items in an iterable in the order in which they appear in the iterable.

For example (Rhymes…):

%> for i in 1..10 {
..   echo i
.. }
1
2
3
4
5
6
7
8
9

The for keyword MUST be paired with the in keyword. The for loop works for all iterable items. For example, we can iterate the elements of the string Hello as follows:

%> for x in 'hello' {
..   echo 'Letter ${x}'
.. }
'Letter h'
'Letter e'
'Letter l'
'Letter l'
'Letter o'

That’s the basic idea. We’ll show more examples as we go through the different iterables we’ll come across in the tutorial.


Infact, we’ll like to get you exited ahead of time by sharing a simple secret — In Blade, you can convert any class into an iterable simply by implementing two decorators and it’ll work perfectly with the for loop.




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Copyright © 2021 Ore Richard Muyiwa. Distributed under the MIT license.