View Compilation Error

0_0_15517068_23322.cpp: In function 'int main()':
0_0_15517068_23322.cpp:20:84: error: lvalue required as left operand of assignment
                 if(!(a[i]='_'||(a[i]>='a'&&a[i]<='z')||(a[i]>='A'&&a[i]<='Z')||a[i]=' '))
                                                                                    ^