foo.c: In function 'main':
foo.c:8:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for(int i=1;i<=n;i++) scanf("%d",&a[i]);
  ^
foo.c:8:2: note: use option -std=c99, -std=gnu99, -std=c11 or -std=gnu11 to compile your code
foo.c:9:10: error: redefinition of 'i'
  for(int i=1;i<=n;i++) all+=a[i];
          ^
foo.c:8:10: note: previous definition of 'i' was here
  for(int i=1;i<=n;i++) scanf("%d",&a[i]);
          ^
foo.c:9:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for(int i=1;i<=n;i++) all+=a[i];
  ^
foo.c:11:10: error: redefinition of 'i'
  for(int i=1;i<=n;i++){
          ^
foo.c:9:10: note: previous definition of 'i' was here
  for(int i=1;i<=n;i++) all+=a[i];
          ^
foo.c:11:2: error: 'for' loop initial declarations are only allowed in C99 or C11 mode
  for(int i=1;i<=n;i++){
  ^