c - Assigning memory dynamically or on the stack based on a condition (if statement) -

i have c program in array of floats has elements accessed quite duration of program. size of array depends on argument user input , therefore vary. generally, size small enough (~ 125 elements) memory of array can placed on stack , allocation , accessing faster. in rare cases, array may large enough such requires dynamic allocation. initial solution following:

if(size < threshold){      float average[size]; } else{     float *average;     average = (float*)malloc(sizeof(float) * size );  }  // stuff average 

this gives error @ compile time. how 1 address such problem?

the declaration of average has limited lifetime in code; lives end of block. in other words, way declare average makes available inside if/else block.

i suggest splitting in 2 functions: 1 handles allocation; other work. this:

void do_average(int size, int threshold) {     if (size < threshold) {          float avg[size];          average(size, avg);     } else {          float *avg = malloc(sizeof(*avg)*size);          assert(avg != null);          average(size, avg);          free(avg);     } }   void average(int avg_size, float avg[static avg_size]) {     /* stuff avg */ } 

you may want think how handle (unlikely) event of malloc() returning null. assert() might not better choice. put there make sure don't forget check error.

note: mentioned in comments, opportunity use vla parameter declaration.