ITEC 109 2014spring ibarland

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lab13-syntax
syntax so far
function-call, variable-assignment

“Syntax” is the grammatical rules that define what is a legal program. We've been writing function-calls and variable-assignments for a while, but it's also useful to see/state the general rule.

• Syntax of function call¹:
  name_function( expression)

  In words: “To call a function: functionName-openParen-arguments.”

  Examples:

  • sqrt(25.0)
  • pow(3,4)
  • len( "hello" )
  • substring( "hello", 2, 4 )
  • sqrt( 21.0 + 4.0 )
  • sqrt( len("abcdefghijklmnopqrstuvwxy") )
  • show( makePicture( pickAFile() ) )
  We've seen examples that return answers: sqrt( 12+4 )
  as well as examples which change some pixels on the screen, but don't return answers: show( … );
  We can use the answer from one function as the argument to another (!): sqrt( len( … ) )
  Note that it does not make sense to write sqrt( show( …) ): show does not return an answer, so we can't use it as the input to sqrt.

• Syntax of assigning to a variable:
  name_var = expression

  Examples:

  • n = 8
  • poem = "Candy is dandy \ But liquor is quicker"
  • quizPct = float(quizScore) / float(quizPossible) * 100.0
  • theChosenFile = pickAFile()
  We see that this rule requires the variable-name on the left-hand-side and not vice-versa; 8 = n is not valid!

  Note that assigning to a variable does not return a value; this means you can't use it as part of a bigger expression: sqrt( 21.0 + x = 4).

  I suggest pronouncing = as “gets”, since represents an action (put a value into a particular variable (box)), and is not really about equality.

• Some terms:
  • value: The pieces of information that python uses. Examples: 25, "hello".
  • type: a set of values, recognized by python. Examples: int, float, char, string.
  • variable — a named box that can some particular value.
  • assigning to a variable — putting a value inside a variable (box).
  • “Calling a function” corresponds to asking a question (e.g. “What's the square root of 25” or “What's the length of the text "abracadabra"?”).
  • argument: the value passed (given) to a function. For example, sqrt(25.0) is calling sqrt with the argument 25.0. (The same is true of sqrt(21.0+4.0) — the argument is 25.0; it is the result of evaluating 21.0+4.0.)
  • We will see:
    “writing a function” is writing down the instructions to answer that question, whenever it gets asked in the future. Its syntax will start with the keyword “def”.

    Note that calling a function is very different than writing a function!

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©2014, Ian Barland, Radford University Last modified 2014.Feb.17 (Mon)

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