The order of operator precedence specifies the standard order to be used when calculating any mathematical expression. That is to say, when using any of the mathematical operators we've described previously, there's a universally agreed-upon order in which to evaluate them. You may recall learning this in basic arithmetic classes, however let's refresh our memories.

In simple terms, the precedence of an operator specifies how "tightly" two expressions are bound together. For example, when calculating the expression 1 + 5 * 3, rather than working out the math from left to right, we obey the order of precedence. This states that the multiplication operator ranks higher than the addition operator. Therefore we first multiply five by three and then add one, this equals 16. If however we calculated the expression from left to right we would get the answer 18, which is obviously incorrect. We need to keep this in mind while creating our own MetaStock formulas, since whenever MetaStock Professional calculates a piece of coding it refers to the order of operator precedence.

H-(H-L)*2

MetaStock would evaluate this first by calculating the inner most parenthesis `(H-L)'. The multiplication would then be calculated since it has a higher precedence than subtraction `(H-L)*2'. Finally, the subtraction from the high would be calculated `H-(H-L)*2'.

Conversely, presume we asked someone who didn't understand the use of parentheses, to code the same thing. They would write the code as `H-H-L*2'. Now when MetaStock Pro calculates this, it would initially multiply the low by two `L*2', since multiplication has higher precedence than subtraction. Then, the high would be subtracted from the high `H-H.' Finally, the result of `L*2' would then be taken from this.

As you can see parentheses dramatically affects the outcome.

Periodicity

Earlier we discussed price array identifiers saying that they reflect the open, high, low, close, volume and open interest. The astute reader may have noticed the wording of each definition made no reference to a particular period, i.e. daily, weekly or monthly. For example the definition for the open was as follows: "This is the price of the first trade for the period." Since there was no reference to a particluar period, we must identify which opening price we are referring to.

To do this we need to pull up the periodicity tab which is located in the `X-Axis Properties' dialog. Simply, select X-Axis from the `Format' menu, or right-click on a chart's X-Axis and choose `X-Axis Properties' from the shortcut menu.

Within the `X-Axis Properties' dialog, or using the periodicity button, we can indicate the period we wish the chart to be displayed in (ie, daily, weekly, monthly, quarterly or yearly periods). We can also select the "other" option which allows us to define the number of periods over which to compress the data, thereby selecting whatever period we like.

Now that we know where it's located, remember to keep an eye on the periodicity, as it can greatly affect our price array identifiers and MetaStock data. For example, assume we were using the ref(c,-1) function (we'll discuss this function later) which returns the close from the previous period. If the periodicity was set to weekly, the previous close would be from 1 week ago. Conversely, if it was set to daily, the previous close would be from one day ago.

The effects of periodicity are also seen in two other data array identifiers, the volume and open interest. MetaStock calculates volume and open interest by merely summing the values over the selected period. For example, a chart showing weekly periodicity would sum the entire week's volume in one period. As such, you need to monitor this closely when using indicators such as the average true range when setting your stop loss or when backtesting systems with MetaStock and TradeSim.

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