Language: C

j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;

Compiler output:
main.c:1:1: warning: data definition has no type or storage class
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      | ^
main.c:1:1: error: type defaults to 'int' in declaration of 'j' [-Wimplicit-int]
main.c:1:3: error: type defaults to 'int' in declaration of 'p' [-Wimplicit-int]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |   ^
main.c:1:12: error: type defaults to 'int' in declaration of 'n' [-Wimplicit-int]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |            ^
main.c:1:14: error: return type defaults to 'int' [-Wimplicit-int]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |              ^~~~
main.c: In function 'main':
main.c:1:14: error: type of 'i' defaults to 'int' [-Wimplicit-int]
main.c:1:80: error: implicit declaration of function 'scanf' [-Wimplicit-function-declaration]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |                                                                                ^~~~~
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'scanf'
  +++ |+#include <stdio.h>
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
main.c:1:80: warning: incompatible implicit declaration of built-in function 'scanf' [-Wbuiltin-declaration-mismatch]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |                                                                                ^~~~~
main.c:1:80: note: include '<stdio.h>' or provide a declaration of 'scanf'
main.c:1:101: error: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      |                                                                                                     ^~~~~~
main.c:1:101: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:101: warning: incompatible implicit declaration of built-in function 'printf' [-Wbuiltin-declaration-mismatch]
main.c:1:101: note: include '<stdio.h>' or provide a declaration of 'printf'
main.c:1:1: error: expected declaration or statement at end of input
    1 | j,p[1<<20],n;main(i){for(;++i<999;)if(!p[i])for(j=i+i;j<1<<20;j+=i)p[j]=1;for(;scanf("%d",&n),j=0,n;printf("%d\n",j))for(i=2;i<=n/2;i++)if(!p[i]&!p[n-i])j++;
      | ^