Big if else statement

Question

If have a big ( about 100 plus) if else statement like below and the if else condition might be irregular(for example some depends on 3 variables, some on 4), is there any way of making it simpler?

Basically I have a table of around 100 plus rows, with the a,b,c and d as the column. Based on a,b,c and d, I need to perform 3 different type of function.

The table describes a set a business rules.

 uint8 a;
 uint8 b;
 uint16 c;
 uint8  d;

 if      ( a == 1 &&   b == 1           && c == 0)           { functionA();}
 else if ( a == 5 &&   b == 5           && c == 2 && d == 2) { functionB();}
 else if ( a == 1 && (b ==36 || b == 7) && c == 0)           { functionC();}  
 else if ( a == 3 &&   b == 3                     && d == 50) {functionA();}
    :
    :

Answer 1

There are lots of ways to make it simpler, for example:

• you can populate a map from a struct holding a, b, c, and d values to check for to the function to call (the code to populate the map may still be a mess, but it'll be faster and cleaner executing; can add two keys for cases ala b == x || b == y)
• you can manually factor the conditions: given your example, if (a == 1) if (b == 1 && c == 0) functionA(); else if ((b == 36 || b == 7) && c == 0) functionC();. switch statements may make this cleaner. In such a factoring, you can also use <, <=, >, and/or >= to divide larger search spaces, improving performance to from O(N) to O(log2N).
• for the common simple case of testing a, b, c, and d once, use a macro ala #define ELIF_ABCD(A, B, C, D, F) else if (a == (A) && b == (B) && c == (C) && d == (D)) F();. Add macros as necessary for other combinations of tests e.g. ABD, ABC, AD.
• (may make the code more cryptic), but could explore bit-shifting and ORing together the values into a large enough type (int64_t) then e.g. binary search an array of function pointers

Something to look out for though is that the if/else chain may contain things like:

if (a == 1 && c == 3 && d == 2) functionY();
else if (a == 1 && b == 2 && c == 3) function X();

Here, the order is significant as an input can match both. This aspect can easily get lost or altered if the searches are factored differently or some manner of indexing to function pointer is used, which is one argument in favour of the macro approach above.