Language: C

float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}

Compiler output:
main.c:1:15: warning: data definition has no type or storage class
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |               ^
main.c:1:15: error: type defaults to 'int' in declaration of 'i' [-Wimplicit-int]
main.c:1:17: error: return type defaults to 'int' [-Wimplicit-int]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                 ^~~~
main.c: In function 'main':
main.c:1:24: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                        ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
main.c:1:24: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                        ^~~~~
main.c:1:24: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:57: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                                                         ^~~~~~
main.c:1:57: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:57: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:57: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:71: error: implicit declaration of function 'hypot' [-Wimplicit-function-declaration]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                                                                       ^~~~~
main.c:1:1: note: include '<math.h>' or provide a declaration of 'hypot'
  +++ |+#include <math.h>
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
main.c:1:71: warning: incompatible implicit declaration of built-in function 'hypot' [-Wbuiltin-declaration-mismatch]
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                                                                       ^~~~~
main.c:1:71: note: include '<math.h>' or provide a declaration of 'hypot'
main.c:1:105: error: expected ')' before ';' token
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                                                               ~                                         ^
      |                                                                                                         )
main.c:1:106: error: expected ';' before '}' token
    1 | float a,b,c,d;i;main(){scanf("%f%f%f%f",&a,&b,&c,&d);i=!printf("%f\n",hypot((double)(a-c),(double)(b-d));}
      |                                                                                                          ^
      |                                                                                                          ;