Something interesting I found while parsing some strings. Behavior of calculations inside of FOR loops.

While doing some optimizations I realized one part of my string parser was running super slow over a 82 char string. After some debugging I found that the LEN() function(returns the length of the string), if called in the FOR loop declaration it counts the characters every loop. Calculations in lower/upper bounds of FOR loops should be avoided if possible.

Results

108 chars Time took (ms)
Consts, 0 to 108 164
0 to LEN(str) 19660
0 to LEN(str), calculated before loop 184

So one line of code can make your code run 111x faster.

MEASUREMENT CODE

//------------KNOWING LENGTH IN ADVANCE -------------------
vTest1.OldTime := LTIME();
FOR i:= 0 TO cRepeatCalcTimes DO
    FOR j:= 0 TO 108 DO
        vResult := vLongString[j];
    END_FOR
END_FOR
vTest1.ExecTime := LTIME() - vTest1.OldTime;

//------------CALCULATING LENGHT IN THE DECLARATION--------
vTest2.OldTime := LTIME();
FOR i:= 0 TO cRepeatCalcTimes DO
    FOR j:= 0 TO LEN(vLongString) DO
        vResult := vLongString[j];
    END_FOR
END_FOR
vTest2.ExecTime := LTIME() - vTest2.OldTime;

//------------CALCULATING LENGHT BEFORE LOOPING -------------------
vTest3.OldTime := LTIME();
FOR i:= 0 TO cRepeatCalcTimes DO
    vLenOfString := LEN(vLongString);
    FOR j:= 0 TO vLenOfString DO
        vResult := vLongString[j];
    END_FOR
END_FOR
vTest3.ExecTime := LTIME() - vTest3.OldTime;

In real life, array lengths are not always known so lets write some PROs and CONs of each type of looping code presented.

KNOWING LENGTH IN ADVANCE

+ FAST
- IF \0 STRING CHANGES DURING LOOPING, 
  ITERATING FOR NO REASON (ADD EXIT CLAUSE IN THIS CASE)
- ITERATES TO 108 char REGARDLESS WHERE THE STRING ENDS
- CAN READ OUT OF BOUNDS (IF LENGTH OF VARIABLE CHANGES)
- LENGTH NOT ALWAYS KNOWN

CALCULATING LENGTH IN THE DECLARATION

+ SAFE LENGTH CALCULATION
+ IF \0 STRING CHANGES DURING LOOPING, STILL SAFE
- SUPER SLOW AS LOOP EVALUATES LEN() EVERY LOOP

CALCULATING LENGTH BEFORE LOOPING

+ ALMOST AS FAST KNOWING THE LENGTH
- IF \0 STRING DECREASES DURING LOOPING, 
  ITERATING FOR NO REASON (ADD EXIT CLAUSE IN THIS CASE)
- REQUIRES A HELPER VARIABLE (MEMORY)

Wise man said once to use the appropriate tool for the job, it seems the advice still holds.

As always, the complete source code is on my github repository dedicated to speed testing.

Speedtest: Converting BOOL[1..32] to BYTE[1..4]

Speedtest: Calculations inside loop declarations

Speedtest: ST vs LADDER vs FBD vs CFC

Speedtest: IF-ELSIF vs CASE

Speedtest: Integer vs Floating point math