FOR Statements in Python

A for statement in PyJ has the syntax:

<for statement> ::= “for” <identifier> “=” <expression> “:” <expression> <statement>+ “end”

Once again, we know that a for statement is coming because the current symbol is forSy, which is to say that we say the identifier “for.” Recall that each non-terminal is implemented by a function in the parser, so in this case we re­quire the functions identifier, expression, and statement. The basic code is as follows:

Error detection in this parser is lacking, because that takes a large amount of error unrelated to the basic task. If the for is not followed by an identifier, then an error will be indicated. There are many others that could have been tested for.

This code is only a parser, though. In the real language, there are code gen­eration and other issues. This is a pretty simple for statement as languages go. There are semantic (meaning) issues that should be addressed, but that are not related to parsing. Here is the final code for the for loop with the code generation:

Error detection in this parser is lacking, because that takes a large amount of error unrelated to the basic task. If the for is not followed by an identifier, then an error will be indicated. There are many others that could have been tested for.

This code is only a parser, though. In the real language, there are code gen-eration and other issues. This is a pretty simple for statement as languages go.

There are semantic (meaning) issues that should be addressed, but that are not related to parsing. Here is the final code for the for loop with the code generation:

if sy == forSy:                         # FOR statement

sy = nextSy()

if sy == identSy:                  # Loop control variable

defineIdent(ident, FLOAT)     # define it – symbol

                              # table stuff

lcvr = ident                  # remember it

sy = nextSy()                 # skip it

if sy == assignSy:            # Equals, skip

sy = nextSy()

lcv = convertIdent(ident)     # symbol table stuff

gen1n(“for (“+lcv+”=”)

expression()                  # Start value

gen1n(“; “+lcv+”<=”)          # Terminal condition

if sy == colonSy:             # : skip it

sy = nextSy()

expression()

gen1(“; “+lcv+”=”+lcv+”+1) {“) # Increment

while sy != endSy:             # Statements until

# the END

statement()

sy = nextSy()

gen1(“}”)

undefine(lcvr)                 # symbol table stuff

else:

println(“Syntax error in FOR”)

 

The PyJ for loop vs. the C for loop is as follows:

Here’s how that translates:

There are places in the commentary where it references the “symbol table.” That is where we look up user defined symbols to see it they are defined and what they are. In PyJ, a symbol can be defined or not. If defined, it can be a floating point number or a function. A for loop control variable is defined in the for state­ment and is undefined at the end of the loop, so that it cannot be used outside of the loop.

Source: Parker James R. (2021), Python: An Introduction to Programming, Mercury Learning and Information; Second edition.