View Compilation Error

0_0_20723997_3953.cpp: In function 'int main()':
0_0_20723997_3953.cpp:25:22: error: 'strlen' was not declared in this scope
    for(i=0;i<strlen(a);i++)
                      ^
0_0_20723997_3953.cpp:30:22: error: 'strlen' was not declared in this scope
    for(i=0;i<strlen(b);i++)
                      ^
0_0_20723997_3953.cpp:35:22: error: 'strlen' was not declared in this scope
    for(i=0;i<strlen(a);i++)
                      ^