What is switch and jumping in pointer programming C plus plus?

Answer 1

There's no such thing as "jumping" in C++ (the equivalent of the JMP assembly instruction). A goto is quite similar to a jump, but the jump is localised to the current function. If you need to jump outside the function you need to make a function call instead. However a function call is not the same as a jump because functions return to the caller but a jump does not return.

A goto does not use a pointer, it simply "jumps" to the labelled section of code, where the label must reside within the current function. For instance, the following example demonstrates a procedural loop using goto:

unsigned loop=0;

:again

std::cout << ++loop << std::endl;

if (loop != 100) goto again;

Note that a label begins with a colon.

A switch is similar to a goto where an expression is evaluated and control passes to the label that matches the result of that evaluation. For instance, suppose you have the following conditional goto:

unsigned r = rand() %10 + 1; // a random number in the range [1:10]

if (r<3) goto less_than_three;

else if (r<5) goto less_than_five;

else if (r<7) goto less_than_seven;

else if (r<9) goto less_than_nine;

else goto less_than_eleven;

:less_than_three

// ...

:less_than_five

// ...

:less_than_seven

// ...

:less_than_nine

// ...

:less_than_eleven

// ...

Note that if r is less than 3 then it is also less than 5, 7, 9 and 11, so execution passes through all the labels in succession (unless we were to place another goto, a break or a return before the next label). But if we suppose this is exactly what we want, consider that if the value is 9 or 10, then we have to evaluate all the expressions, r<3, r<5, r<7, r<9 and r<11, before we reach the point where we can make the jump. With a switch, we can achieve the same thing more efficiently:

switch (r)

{

case (1):

case (2):

// ...

case (3):

case (4):

// ...

case (5):

case (6):

// ...

case (7):

case (8):

// ...

case (9):

case (10):

// ...

}

Note that switch labels begin with the case keyword and end with a colon (the parenthesis are optional).

If we do not wish our code to flow through to the next case label we must place a goto, break or return statement before the next case label.

We can also choose a default case. For instance, since the last case has to be 9 or 10, we can combine these into a default case:

switch (r)

{

case (1):

case (2):

// ...

case (3):

case (4):

// ...

case (5):

case (6):

// ...

case (7):

case (8):

// ...

default:

// ...

}

The default does not have to be last option, it simply caters for any result not handled by a specific case label. Execution continues from that point until the end of the switch statement, unless a break, return or goto is encountered.