Casting and parsing

Convert text prices into numbers you can add together. Along the way, you will learn how q distinguishes numbers from text, how casting differs from parsing, and how to check a conversion before trusting its result.

Start PeachQ and enter the examples at its q) prompt. In the examples below, q) marks what you enter; the following lines show the result. Do not type the prompt itself. For the complete conversion syntax, see the cast reference.

Numbers and text are different

Enter these two values:

q)42
42
q)"42"
"42"

The first value is a number. The second is text: the quotation marks tell q to treat the characters 4 and 2 as a string. A type describes what kind of value you have. These values look similar to us, but q needs a number to do numeric arithmetic.

Cast an existing number

Casting converts an existing value to another type. Use $ with the target type on the left and the value on the right:

q)`long$9.4
9
q)`long$9.6
10

A long is a whole-number type. The backtick in `long makes the name a symbol, a name value used here to identify the target type. It is different from the quoted text strings above.

These casts round to the nearest whole number: 9.4 becomes 9, while 9.6 becomes 10. They do not simply discard the decimal part. We use values away from halfway points here so the rounding is easy to see.

Parse a number from text

Parsing interprets text as a value. Use an uppercase type letter in quotes on the left of $. "J" asks q to read a whole number of type long:

q)"J"$"42"
42
q)1+("J"$"42")
43

The parsed result no longer has quotation marks. It is a number you can add to. The parentheses group the conversion, so this example first reads "42" as a number and then adds 1.

For a decimal number, use "F" to parse a float, a numeric type that can hold a fractional part:

q)"F"$"12.50"
12.5

The trailing zero is formatting in the original text, not part of the numeric value. The result is still twelve and a half.

Casting or parsing?

What you have	What you want to do	Example
A number, 9.6	Convert it to a whole-number type	`long$9.6
Text, "42"	Read it as a whole number	"J"$"42"
Text, "12.50"	Read it as a decimal number	"F"$"12.50"

You may also see a lowercase letter used for casting. "j" is the short form of `long:

q)"j"$9.6
10

Case matters: lowercase "j" casts; uppercase "J" parses text. You do not need to memorize the full list of type letters to use these examples.

Convert a list of prices

A list holds several values. Put these three strings inside parentheses, separated by semicolons. The colon assigns the list to a name, priceText, so we can use it again:

q)priceText:("12.50";"3.25";"4.00")
q)priceText
"12.50"
"3.25"
"4.00"

Assignment does not print a result. Entering the name displays its value. Now parse the whole list and save the resulting numbers as prices:

q)prices:"F"$priceText
q)prices
12.5 3.25 4

The same "F"$ operation reads each string. q displays this numeric list on one line, with spaces between its values. sum adds the values in a numeric list:

q)sum prices
19.75

That total uses valid input. Next, check what happens when a price is not a number.

Check invalid input before adding

Suppose the second price is the text "oops":

q)badText:("12.50";"oops";"4.00")
q)parsed:"F"$badText
q)parsed
12.5 0n 4

The float parser returns 0n for this invalid text. This is a null, meaning a missing numeric value. It is not the number zero, and it is not a valid price.

Use null to check each value, then any to ask whether at least one value is missing:

q)null parsed
010b
q)any null parsed
1b

These are boolean results: 1 means true and 0 means false. The suffix b marks booleans. 010b is a list of three answers: only the second value is null. 1b says there is at least one null. In any null parsed, q first checks null parsed, then applies any to those answers.

Do not accept a total while that check returns 1b. sum skips null values, which could make an incomplete total look successful. Correct the source price and parse again. Here we restore 3.25, check all three prices, then add them:

q)priceText:("12.50";"3.25";"4.00")
q)prices:"F"$priceText
q)prices
12.5 3.25 4
q)any null prices
0b
q)sum prices
19.75

0b means none of these parsed prices is missing. The checked total is 19.75. This check detects missing parsed values; it does not decide whether a valid number is a sensible price for your application.

Try it yourself

Edit and run these examples to test your understanding. Each run starts fresh; Reset restores the original code.

1. Cast a number

Convert 7.8 to a long. What result do you expect?

q)`long$7.8
8

Open in REPL

The cast rounds to the nearest whole number, which is 8 here.

2. Parse a price

Read the text "6.75" as a float.

q)"F"$"6.75"
6.75

Open in REPL

Uppercase "F" parses the text as a numeric value.

3. Find invalid text

Parse ("2.50";"unknown";"1.25") as floats and check which value is missing.

q)exercisePrices:"F"$("2.50";"unknown";"1.25")
q)null exercisePrices
010b

Open in REPL

The second string could not be read as a float. Fix that input before accepting a total.