Percentage “%” in format string

Question

(format "Hard-coded value is 100\% and the variable is %s" "50\\%")

Returns

Debugger entered--Lisp error: (error "Invalid format operation %a")
  format("Hard-coded value is 100% and the variable is %s" "50%")
  eval((format "Hard-coded value is 100% and the variable is %s" "50%") nil)
  eval-last-sexp-1(nil)
  eval-last-sexp(nil)
  call-interactively(eval-last-sexp nil nil)
  command-execute(eval-last-sexp)

(Note how in the debugger output, the command has the "%" escaped)

How in heaven can I use a string with a percentage in it?

Answer 1

It is indeed %%. If you type C-h f format <RET> you'll pull up the documentation for the format function, which states:

Use %% to put a single % into the output.

Thus:

(format "Hard-coded value is 100%% and the variable is %s" "50%")

(the % signs never need to be escaped with backslashes).