Why can't I assign a value to a sequential list structure in a loop

Question

#include <iostream>
using namespace std;
#define MAXSIZE 100

typedef struct {
  int elem[MAXSIZE];
  int last;
} seqlist;
int main() {
  seqlist L;
  L.last = 0;
  int i;
  for (i = 0; i < 20; i++)
    ;
  {
    L.elem[i] = i;
    L.last++;
  }
  cout << "The value of L:" << ' ';
  for (i = 0; i < 20; i++) {
    cout << L.elem[i] << ' ';
  }
  cout << "\n"
       << "The last of L:" << ' ';
  cout << L.last << endl;
  return 0;
}

This is the result

The value of L: 1876269952 0 4206312 0 4206304 0 661232 0 6421936 0 16 0 8 0 0 0 0 0 1876144503 0

The last of L: 1

I don't know why this happens. Please help me.

Answer 1

It's hard to notice but you've got a semicolon right after your for loop:

for (i = 0; i < 20; i++); <-----

This means the loop runs 20 times and executes an empty statement. At the end of the loop i is 20 , so the following statements only work on L.elem[20] . Then when you print the values you're just printing the initial garbage values instead of what you expected.

Remove the semicolon and your code works.