What is meant by the phrase one block of code can be nested in another?

Answer 1

It simply means one code block contains another code block. How you achieve this depends on the language, but in many languages we use opening and closing braces to define a code block.

function foo ()

{ // start of function block

{ // start of nested block

// ...

} // end of nested block

} // end of function block

Nested code blocks are typically used to define a new scope within an existing scope. Any variables declared within the nested block are local to the scope of that block and will fall from scope when we exit the nested block. This can be useful when we want to instantiate new variables within a larger scope but want to localise them to the code block that actually uses them. In some languages (such as C++) this also allows us to redefine names that were initially defined by the enclosing scope, temporarily hiding the original names from within the nested scope (only the local names are visible). When we exit the nested block, the original names become visible again. However, referring to two separate variables by the same name within two scopes of the same function can make code difficult to read, so this is best avoided.