View Compilation Error

0_0_24501818_21342.cpp: In function 'int main()':
0_0_24501818_21342.cpp:21:16: error: 'strlen' was not declared in this scope
    if (strlen(a) > 16 && strlen(a) < 8) {
                ^
0_0_24501818_21342.cpp:25:22: error: 'strlen' was not declared in this scope
    int len = strlen(a), i;
                      ^
0_0_24501818_21342.cpp:26:9: error: 'i' was not declared in this scope
    for (i = 0; i < len; i++) {
         ^
0_0_24501818_21342.cpp:33:8: error: 'i' was not declared in this scope
    if (i < len || (b + c + d + e) < 3) cout << "NO" << endl;
        ^