Loops and how to avoid them

Loops are bad (but are they ``the embodiment of pure evil''?). They slow things down (but, they are not that bad in some cases; for simple applications speed is not an issue and they can be clear to read). How to avoid them can be complex (see links above) but there are a few simple rules.

I think here it's just best to go through,

http://www.dfanning.com/code\_tips/slowloops.html

no point me cutting and pasting it all in here!

Use ## (matrix multiply i.e. multiplies the rows of the first array by the columns of the second array) OR use # (multiplies the columns of the first array by the rows of the second array, i.e. reverse of a standard matrix multiply) to avoid loops (see example in Other tips and tricks: colorbars and contours on images).

Write functions to always deal with scalars or vectors e.g.

FUNCTION temp_to_theta,temp,pressure,ref_press=ref_press

;+
;FUNCTION temp_to_theta,temp,pressure,ref_press=ref_press
;JOHN MARSHAM 18/3/2005 
;Takes temperature (K or C),  and pressure (any, mbar if ref_press not set) '
;returns potential temperature (K or C) adjusted to ref_press'
;If ref_press not set uses 1000 mbar
;-

if n_params() ne 2 then begin
print,'temp_to_theta,temp,pressure,ref_press=ref_press'
print,'Takes temperature (K (or C)),  and pressure (hPa) '
print,'returns potential temperature (K (or C)) adjusted to ref_press'
print,'If ref_press not set uses 1000 mbar'
return,-1
endif

if not keyword_set(ref_press) then ref_press = 1000.

; this auto-conversion is slightly dodgy but I don't tend to look at
; temperatures below -123 K !
if max(temp) lt 150 then begin
 temp=temp+273.15
 degC=1
 print,'converting deg C to K - will convert back'
endif else begin
 temp=temp  
 degC=0
 print,'Not converting deg C to K - already K'
endelse

theta=temp*(ref_press/pressure)^(0.286)

if degC eq 1 then theta=theta-273.15

return,theta

end

will convert temp to potential temp where temp is a scalar (eg temp=10.) or a vector (temp=[5.,10.,12.]) eg

 
IDL> print,temp_to_theta(10.,700.)
converting deg C to K
      40.4085
IDL> print,temp_to_theta([10.,20.],700.)
converting deg C to K
      40.4085      51.4824
IDL> print,temp_to_theta([10.,20.],[700.,700.])
converting deg C to K
      40.4085      51.4824
IDL> print,temp_to_theta([10.,20.],[700.,600.])
converting deg C to K
      40.4085      66.1147

For further discussion (`` are they the embodiment of pure evil?'') see http://www.dfanning.com/tips/forloops.html