Why this is showing error ? even though it seems all syntax is correct?

c++11

Put single ticks around the items being compared: '[' instead of "[". That way you are comparing char to char instead of char to a string

Related

Getting "illegal parameter" when trying to call the alipay Payment API in the Sandbox
text automatically resizes without media queries (responsive design)
How can I display the number 10525.25 as 10.525,25 using regexp with Dart?
Healthchecks for windows services from MVC
Implicit Bearer flow for swagger asp..net core 3.1
Evaluation of $\int\frac{5x^3+3x-1}{(x^3+3x+1)^3}\,dx$
Can we recover a compact smooth manifold from its ring of smooth functions?
Must imaginary roots come in conjugate pairs?
If $\int_{\mathbb R^2} \frac{\vert f(x)-f(y)\vert}{\vert x-y\vert^2}dxdy<+\infty$ then $f$ is a.e. constant
How are norms different from absolute values?
Are there parabolic and elliptical functions analogous to the circular and hyperbolic functions $\sinh$, $\cosh$, and $\tanh$?
$f\colon\mathbb{R}\to\mathbb{R}$ such that $f(x)+f(f(x))=x^2$ for all $x$?