How can I make my current code efficient by avoiding if-else conditions?

Question

Programming language: C
Platform: PIC Microcontroller 8-bit
Number of problems: 2

I'm using a 4-digit 7-segment display for showing numbers. I've a few functions that display some letter/digit on the 7-segment like:

zero() // displays 0 on the 7-segment.
one() // displays 1 on the 7-segment.
two() // displays 2 on the 7-segment.
...

Now I've a number (say 1435) to be shown on the 7-segment display. My current algorithm is as follow:

Extract individual digits from the number 1435 (that's separate the digits as 1, 4, 3, 5). sds
- 1 will be displayed to digit1 of 7-segment.
- 4 will be displayed to digit2 of 7-segment.
- 3 will be displayed to digit3 of 7-segment.
- 5 will be displayed to digit4 of 7-segment.
To display these individual digits, I'm using 'ten' if-else conditions as follow:
- If the digit to be displayed == 0 -> run the function zero(); else
- If the digit to be displayed == 1 -> run the function one(); else
- If the digit to be displayed == 2 -> run the function two();
- ...
- ...
- ...

So this implementation (for the number 1435) to be printed runs several if-else checks.

2 checks for displaying the digit 1
5 checks for displaying the digit 4
4 checks for displaying the digit 3
6 checks for displaying the digit 5
17 total checks that run "periodically and unnecessarily" in the loop() function even if the number is not changed (This is problem number 1).

Problem number 2: This implementation is inefficient as when I need to increment/decrement that number (1435 to 1436 then to 1437 so on..), the number of if-else checks are also changed so the variation in numbers is not smooth. This means that as '0' is the first in if-else checks so it displays quickly. On the other hand '9' is the last in the if-else checks, so it has to undergo ten checks before it gets displayed. This makes implementation much slower as the digit to be displayed grows from 0 towards 9. How can Implement to solve the two problems?

Thanks in advance.

XY-problem. Looks like a very wrong approach. Do you think `putc` calls 256 different functions to print a character? — too honest for this site, Feb 08 '16 at 20:55
`switch` is more performant than if-else, provided you follow some discipline — dlmeetei, Feb 08 '16 at 20:55
Why not get the each digit and run each digit through a `switch` that calls the right function. This should always be the same execution complexity as long as you always have a 4 digit number. — NathanOliver, Feb 08 '16 at 20:56
@NathanOliver: That whole idea to have one function per symbol to output is a bad one. — too honest for this site, Feb 08 '16 at 20:57
@Olaf I get that. I am just assuming the OP has to use those functions. — NathanOliver, Feb 08 '16 at 20:58
Array of function pointers is probably less efficient than a switch — M.M, Feb 08 '16 at 20:59
@NathanOliver: I don't see that requirement. Looks like a typical XY-problem. — too honest for this site, Feb 08 '16 at 20:59
@FarazAhmad Either way is a problem description looking for solution, no code, no research, no effort. Doesn't qualify as a SO question. — 101010, Feb 08 '16 at 20:59
@Hogan, yeah I've already visited the array of function pointers. The example I've copied code from uses three functions (with int arguments) and integer return types. But in my case there is no argument, and the return type is void, so I'm unable to implement it in my case without error. — Faraz Ahmad, Feb 08 '16 at 21:00
@M.M in some cases maybe but here we have to call the function anyway, and there are exactly 10 of them. How can an array of pointers not be faster? — Hogan, Feb 08 '16 at 21:01
@FarazAhmad - just because you can't implement it does not mean it is the wrong way to do it. — Hogan, Feb 08 '16 at 21:02

score 3 · Accepted Answer · answered Feb 08 '16 at 21:01

3

You can use pointers to functions in a table :

typedef void (*func)(); // type for functions
func functions[] = { zero, one, two, three, ... }
functions[3](); // example, will call three()

You have to extract the digit you want and use it as an index in the table....

answered Feb 08 '16 at 21:01

Jean-Baptiste Yunès

34,548
4
48
69

Yup it works, I was already doing this in my code. But the mistake I was making was calling the array as functions[3] instead of functions[3](). Thanks. Can you please confirm it will be efficient than the if-else and switch? Thanks again. – Faraz Ahmad Feb 08 '16 at 21:05
Seriously? You need to optimize for nanoseconds? – Jean-Baptiste Yunès Feb 08 '16 at 21:07
@FarazAhmad - everyone is saying this is the way to do it? What exactly do you want as confirmation? If you want confirmation then test it yourself. – Hogan Feb 08 '16 at 21:07
@Jean-BaptisteYunès: This is a PIC MCU which is not built for such code actually. – too honest for this site Feb 08 '16 at 21:09
@Jean-BaptisteYunès - it could matter, this is a micro-controller. If it is a real time system then every nanosecond counts. – Hogan Feb 08 '16 at 21:09
@Hogan: Likely every >100ns. The PIC normally runs at few MHz. – too honest for this site Feb 08 '16 at 21:16
Just tested the code with this solution (simulation on proteus software). Proteus says, CPU (i.e. microcontroller) is out of memory. When I was using if-else, it ran without this notice but with the problems stated in the question. – Faraz Ahmad Feb 08 '16 at 21:30
@Jean-BaptisteYunès, yes I'm implementing the push button increment/decrement so I need to get as fast as possible. – Faraz Ahmad Feb 08 '16 at 21:30

How can I make my current code efficient by avoiding if-else conditions?

1 Answers1