gnuplot - convert a string variable to lowercase

Question

How do you convert a string to lowercase in gnuplot?
This is a gnuplot string handling question.

Example:- I wish to check a user typed parameter in a gnuplot script....

if (tolower(ARG2) == "ohms") {.....

so by accepting "ohms", "Ohms", or "OHMS".

The preference is to not need to use an external "system" command so that the script is more portable. My current best solution is

  arg2 = system("awk 'BEGIN { print toupper(\"".ARG2."\") }'")

and then test the new string variable "arg2", but awk (or other program) may not be generally available on a non unix system, making the gnuplot script less portable.

I cannot see any enhanced gprintf % format specifiers that modifies string presentation - it seems gprintf is only for converting values.

Answer 1

The full function Macro solution (Thanks theozh) has me thinking again of how to implement this as a function. The idea of using a lookup table to convert characters by equating an ordinal number was a great idea. Encapsulating single character case conversion into a function was the start, and then along with recursion, it has made it possible to handle full strings as I had first looked for. I hope this is now a tidy solution for all. Share and enjoy.

# GNUPLOT string case conversion
# string_case.gnu   M J Pot, 14/1/2019

# Index lookup table strings
UCases="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
LCases="abcdefghijklmnopqrstuvwxyz"

# Convert a single character
# Char to convert is added to string so it is always found to default other chars
toupperchr(c)=substr( UCases.c, strstrt(LCases.c, c), strstrt(LCases.c, c) )
tolowerchr(c)=substr( LCases.c, strstrt(UCases.c, c), strstrt(UCases.c, c) )

# Convert whole strings
# Conversion first char (or return null), and recurse for the remaining
toupper(s) = s eq ""  ?  ""  :  toupperchr(s[1:1]).toupper(s[2:*])
tolower(s) = s eq ""  ?  ""  :  tolowerchr(s[1:1]).tolower(s[2:*])

Addition: Improved solution

This is a re-work of the recursive case conversion as self contained functions. A little more effort has resolved the excessive stack usage of the first solution. I had only been considering strings of single words when I had the problem. Note:- the single character conversion has been made more robust.

# GNUPLOT string case conversion
# string_case.gnu   M J Pot, 29/1/2019
# toupper(), tolower() functions

# Index lookup table strings
UCases="ABCDEFGHIJKLMNOPQRSTUVWXYZ"
LCases="abcdefghijklmnopqrstuvwxyz"

# Convert a single character
# Char to convert is added to string so it is always found to default other chars
# Null strings are returned null
toupperchr(c)= c eq ""  ?  ""  :  substr( UCases.c, strstrt(LCases.c, c), strstrt(LCases.c, c) )
tolowerchr(c)= c eq ""  ?  ""  :  substr( LCases.c, strstrt(UCases.c, c), strstrt(UCases.c, c) )

# Divide & conquer
# A simple linear recursive call uses too much stack for longer strings.
# This undertakes a binary tree division to make the stack growth order log_2(length)
toupper(s) = strlen(s) <= 1 ? toupperchr(s) : toupper( substr(s,1,strlen(s)/2) ) . toupper( substr(s,(strlen(s)/2)+1,strlen(s)) ) 
tolower(s) = strlen(s) <= 1 ? tolowerchr(s) : tolower( substr(s,1,strlen(s)/2) ) . tolower( substr(s,(strlen(s)/2)+1,strlen(s)) ) 

Answer 2

When wishing to compare a string variable independent of case to a known string constant, only a subset of characters need to be converted. This is not an answer to the general situation of case conversion in gnuplot, but may work for many situations.

arg2 = ARG2
# As we want limited comparisons strstrt(”string”, ”key”) can be used....
# Substitute the first occurrence of a character [pat], with another character [repl]
# Can be used to convert an expected word's case, one character at a time
subchr(src, pat, repl) = strstrt(src, pat)?src[*:strstrt(src, pat)-1].repl.src[strstrt(src, pat)+1:*]:src
arg2 = subchr(arg2, "o", "O")
arg2 = subchr(arg2, "h", "H")
arg2 = subchr(arg2, "m", "M")
arg2 = subchr(arg2, "s", "S")
arg2 = subchr(arg2, "d", "D")
arg2 = subchr(arg2, "b", "B")
if ( arg2[1:2] eq "DB" ) {
  # In terms of dB
  .....
  }
else {
  if ( arg2 eq "OHMS" ) {
    .....
    }
  }

The solution is to write a gnuplot string function subchr() which replaces a single matching character, only if found (the ternary ?), and to call it for each of the characters to be converted. Fortunately, gnuplot string range specifiers appear well behaved when indexing before (0) and beyond (stringlength+1), returning null for that region. This allows us to return the string with each character upgraded.

This avoids the need for a system() call to a program like awk, etc.

Answer 3

I am not aware that gnuplot offers uppercase or lowercase functions.

Edit: to my opinion, my original macro solution and the second approach don't have any advantage compared to the latest version. In order to keep it short and not confusing people, I removed the old solutions.

The attempt below is a function which converts the argument into uppercase or lowercase without external tools as desired from the OP. Characters which are not found in the list are kept unchanged.

Script:

### uppercase/lowercase function for gnuplot
reset session

UpperCases= "ABCDEFGHIJKLMNOPQRSTUVWXYZÄÁÂÀËÉÊÈÏÍÎÌÖÓÔÒÜÚÛÙŸÝ"
LowerCases= "abcdefghijklmnopqrstuvwxyzäáâàëéêèïíîìöóôòüúûùÿý"

# upper/lowercase for characters
ucc(c) = ((ucc_i=strstrt(LowerCases, c)) ? UpperCases[ucc_i:ucc_i] : c)
lcc(c) = ((lcc_i=strstrt(UpperCases, c)) ? LowerCases[lcc_i:lcc_i] : c)

# upper/lowercase for strings
uc(s) = (uc_s='', sum[uc_i=1:strlen(s)] (uc_s=uc_s.ucc(s[uc_i:uc_i]),1),uc_s)
lc(s) = (lc_s='', sum[lc_i=1:strlen(s)] (lc_s=lc_s.lcc(s[lc_i:lc_i]),1),lc_s)

s = "...thE qUick brOWn foX jUmPs oVeR The LazY Dog!"
print "Input:     ", s
print "Uppercase: ", uc(s)
print "Lowercase: ", lc(s)
### end of script

Result:

Input:     ...thE qUick brOWn foX jUmPs oVeR The LazY Dog!
Uppercase: ...THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG!
Lowercase: ...the quick brown fox jumps over the lazy dog!