View Compilation Error

0_0_20994965_15880.cpp: In function 'int main()':
0_0_20994965_15880.cpp:27:29: error: 'memset' was not declared in this scope
         memset(f,0,sizeof(f));
                             ^
0_0_20994965_15880.cpp:41:25: error: 'a' was not declared in this scope
             for(int j = a[i].x; j>0; j--)
                         ^
0_0_20994965_15880.cpp:49:16: error: 'j' was not declared in this scope
             if(j==0)
                ^
0_0_20994965_15880.cpp:51:24: error: 'a' was not declared in this scope
                 sum += a[i].y;
                        ^