Here's a simple C program showing an array of structs:
======================================================
struct prec {
   char op;
   int preced;
   char assoc;
};

int main() {
   struct prec p[] =
      {{'^', 4, 'R'},
       {'*', 3, 'L'},
       {'/', 3, 'L'},
       {'+', 2, 'L'},
       {'-', 2, 'L'},
       {'(', 1, 'R'},
       {')', 1, 'R'},
       {'?', 0, '?'}};
   int i = 0;
   for (i = 0; p[i].op != '?'; i++)
      printf("i=%i: op=%c, preced=%i, assoc=%c\n",
         i, p[i].op, p[i].preced, p[i].assoc);
}
Output:
% cc -o prec1 prec1.c
% prec1
i=0: op=^, preced=4, assoc=R
i=1: op=*, preced=3, assoc=L
i=2: op=/, preced=3, assoc=L
i=3: op=+, preced=2, assoc=L
i=4: op=-, preced=2, assoc=L
i=5: op=(, preced=1, assoc=R
i=6: op=), preced=1, assoc=R
===================================================
This can be rewritten in the following awkward form:
===================================================
struct prec {
   char op;
   int preced;
   char assoc;
};

int main() {
   int i = 0;
   struct prec p0 = {'^', 4, 'R'};
   struct prec p1 = {'*', 3, 'L'};
   struct prec p2 = {'/', 3, 'L'};
   struct prec p3 = {'+', 2, 'L'};
   struct prec p4 = {'-', 2, 'L'};
   struct prec p5 = {'(', 1, 'R'};
   struct prec p6 = {')', 1, 'R'};
   struct prec p7 = {'?', 0, '?'};

   struct prec p[8];
   p[0] = p0;  /* 0 */
   p[1] = p1;  /* 1 */
   p[2] = p2;  /* 2 */
   p[3] = p3;  /* 3 */
   p[4] = p4;  /* 4 */
   p[5] = p5;  /* 5 */
   p[6] = p6;  /* 6 */
   p[7] = p7;  /* 7 */
   
   for (i = 0; p[i].op != '?'; i++)
      printf("i=%i: op=%c, preced=%i, assoc=%c\n",
         i, p[i].op, p[i].preced, p[i].assoc);
}
Same output ...
======================================================
But the following form produces syntax errors:
======================================================
struct prec {
   char op;
   int preced;
   char assoc;
};

int main() {
   int i = 0;
   struct prec p[8];
   p[0] = {'^', 4, 'R'};  /* 0 */
   p[1] = {'*', 3, 'L'};  /* 1 */
   p[2] = {'/', 3, 'L'};  /* 2 */
   p[3] = {'+', 2, 'L'};  /* 3 */
   p[4] = {'-', 2, 'L'};  /* 4 */
   p[5] = {'(', 1, 'R'};  /* 5 */
   p[6] = {')', 1, 'R'};  /* 6 */
   p[7] = {'?', 0, '?'};  /* 7 */
   
   for (i = 0; p[i].op != '?'; i++)
      printf("i=%i: op=%c, preced=%i, assoc=%c\n",
         i, p[i].op, p[i].preced, p[i].assoc);
}
Output:
% cc -o prec3 prec3.c
"prec3.c", line 10: syntax error before or at: {
"prec3.c", line 11: syntax error before or at: {
"prec3.c", line 12: syntax error before or at: {
"prec3.c", line 13: syntax error before or at: {
"prec3.c", line 14: syntax error before or at: {
"prec3.c", line 15: syntax error before or at: {
"prec3.c", line 16: syntax error before or at: {
"prec3.c", line 17: syntax error before or at: {
"prec3.c", line 22: cannot recover from previous errors
==========================================================
The syntax errors are there because the curley
bracket notation can only be used as part of
a declaration in C.