Subroutine Subprograms

1
Subroutine Subprograms

• Program units designed to perform a particular
  task. They differ from functions by the
  following
• Functions are designed to return a single value
  Subroutines often return more than one value or
  none at all
• Functions return values via function names that
  must be declared Subroutines return values via
  arguments
• Function is reference by using its name in an
  expression subroutine is referenced by a CALL
  statement

2
Subroutine Syntax

• subroutine ltname of subprogramgt (ltargument
  listgt)
• ltdeclare all variables usedgt
• ltstatements of executiongt
• return
• end
• Example
• subroutine promptnum(tempC)
• real tempC
• print , Enter a temperature in Celsius
• read , tempC
• return
• end

3
How to access a subroutine

• call ltnamegt(ltargument listgt)
• Example
• call promptnum (tempC)
• call printtable()
• call doy2monday(doy,month,day,year)

4
Rules for the argument list

• Variables being passed back and forth must be
  declared in both areas
• Order of argument list must be same in the CALL
  line and the SUBROUTINE line
• Argument list does not have to use the same
  variable names, but must reference same memory
  locations

5
Character data types

• All variables of data type character are 1
  character in length unless otherwise specified
• character gender
• m or n if enter male, only m will be
  stored in the gender variable
• To have variables be a string of characters
  (i.e., words), must specify length in declaration
• character gender6, cyear4, fname32
• character10 cmonth, cyear, cday

6
More about character strings

• If character variable is allocated more units
  than string, string is left justified
• character gender10
• gender male
• print , gender
• Output male______

7
Combining and extracting strings

• Concatenation in F77 //
• character3 word1, word2
• character words1and26, phrase20
• word1 mon
• word2 day
• words1and2 word1 // word2
• phrase Today is // word1 // word2
• Extraction in F77
• phrase Beth Hall is cool
• word1 phrase(3) or word1 phrase(13)
• word2 phrase(79)

8
We Try It!

• Write a program that converts decimal degrees
  into degrees, minutes and seconds

9
We Try Another!

• Write a program to convert polar coordinates (R,
  THETA) to rectangular coordinates (X, Y).

10
You Try It!

• Write a program that has a starting unit table
  and a resulting unit table. In the starting unit
  table, provide an option to quit program. Based
  on the selection, either quit or display a table
  asking them to choose the conversion unit result
  (e.g., feet, kilometers, inches, meters, etc.).
  Based upon their choices, compute the result.
  Use subroutines, functions, IF statements, do
  whiles.