/in/foo.c: In function 'main':
/in/foo.c:26:3: warning: implicit declaration of function 'memset' [-Wimplicit-function-declaration]
   memset(speed, 0, sizeof(speed));
   ^~~~~~
/in/foo.c:26:3: warning: incompatible implicit declaration of built-in function 'memset'
/in/foo.c:26:3: note: include '<string.h>' or provide a declaration of 'memset'
/in/foo.c:34:8: warning: implicit declaration of function 'abs' [-Wimplicit-function-declaration]
     if(abs(speed[i]-speed[j]) > k){
        ^~~