View Compilation Error

0_0_20451155_17999.cpp: In function 'int main()':
0_0_20451155_17999.cpp:53:21: error: 'b' was not declared in this scope
         scanf("%s", b);
                     ^
0_0_20451155_17999.cpp:61:42: error: 'c' was not declared in this scope
         for (int i = 0; i < len / 2; i++)c[i]='4';
                                          ^
0_0_20451155_17999.cpp:62:42: error: 'c' was not declared in this scope
         for (int i = len/2; i < len; i++)c[i]='7';
                                          ^
0_0_20451155_17999.cpp:63:9: error: 'c' was not declared in this scope
         c[len] = 0;
         ^