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Multiple exercises from one exercise with selectors
Multiple exercises from one exercise with selectors

Learn how to use multiple exercises from one exercise with selectors

Updated over a week ago

We often find ourselves in a situation where we have one kind of exercise, but we want different variations on it. For example, say you have the following exercise which asks you to add two integers:

`\$a + \$b`

We may specify `\$a = rand(1,10)` and `\$b= rand(1,10)`, for the first exercise in this package. And `\$a = rand(11,100)` and `\$b= rand(11,100)`, for the second exercise in this package. And `\$a = rand(101,1000)` and `\$b= rand(101,1000)` for the third question.

For question 4,5 and 6, we do the same with the operator changed from plus to minus, and so on, and so forth. Because those 6 exercises are so similar in nature, they probably have similar feedback rules, a similar explanation in the solution, a similar solution field, similar hints, etc.

We often use the following strategy in those kind of exercises.

## Step 1

Call the first variable `\$selector`, and give it as value an array with the keys being a description of the instance. We usually use integers as values.

We use the PHP 5.4+ array notation. For more info, see http://php.net/manual/en/language.types.array.php.

In the platform, variables which are arrays are interpreted as taking a random item from the array.

## Step 2

The following variable is called json. Here, all the properties which are unique to the selector can be specified. You could also use conditionals (`\$selector` == 0 ? rand(1,10) :`\$selector`r == 1 ? ... etc.etc.). The advantage of this method is that all the data specific to a given selector is given in one place.

The function json_encode is used to make a string from an array. At this moment, the SOWISO platform only understand variables which are strings. If you want to use an array you have to encode it as a string first.

## Step 3

We can now extract the variables in the following way:

The function json_decode is used to make an array from a string, (if the second parameter is set to true). In this way you can access a value from the array which is encoded as a string.