Switch Case doubt

[Esmerelda]

#include<stdio.h>
main()
{
    int a=1;
    switch(a)
    {
        case isdigit('1'):printf("Hello");
    }
}

This code works fine Hello is printed

#include<stdio.h>
main()
{
    int a=1;
    switch(a)
    {
        case isalpha('a'):printf("Hello");
    }
}

But this code gives an error
It says case label does not reduce to an integer

[DJSAN10]

It says case label does not reduce to an integer

Maybe printing isalpha('a') might help. Just because isalpha on true returns non-zero integer , not necessarily 1. So can be a problem. Just try that out

[L7Sqr]

Neither case will work for me. The tags for case statements must be constants, they can not be the result of a function call.

It might be the case that, on your platform, the isdigit is evaluated at pre-processor time to be 1 allowing for you to compile. Can you show the result of the pre-processor on your first example?

[Moschops]

What exactly are you trying to do?

isdigit('1') will always have the same value. isalpha('a') will always have the same value. Why bother using a function to generate values that never change and are always the same?

What are you trying to do here?

[Esmerelda]

@DSJAN10
yea u r right isalpha('a') returned 1024 when i printed it.

@L7sqr
Well they can be the result of an expression right?? case : 'B'>'A' is right. As long as it evaluates to an integer or character constant (with no variables in the expression) it works fine. Anyway wat did u pre processor time?
@Moschops
Absolutely nothing..one of my students had this doubt...none of the functions work with case except isdigit.

[deceptikon]

#include<stdio.h>
main()
{
    int a=1;
    switch(a)
    {
        case isdigit('1'):printf("Hello");
    }
}

This code works fine Hello is printed

#include<stdio.h>
main()
{
    int a=1;
    switch(a)
    {
        case isalpha('a'):printf("Hello");
    }
}

But this code gives an error
It says case label does not reduce to an integer

It doesn't reduce to a *constant integer expression*. So no, that code shouldn't compile because it has a constraint violation.

[L7Sqr]

@L7sqr
Well they can be the result of an expression right?? case : 'B'>'A' is right. As long as it evaluates to an integer or character constant (with no variables in the expression)

A function call (with a variable input) is not a constant expression. It might be the case that a compiler is tuned to evaluate a lookup-table macro expansion and replace it with a constant expression given a constant input but I wouldn't count on such behavior.

[sonal_salkade]

Case expression should be a constant or an expression that reduces to a constant.

[WaltP]

Case expression should be a constant or an expression that reduces to a constant.

It surprises me that no one before you said anything even remotely like that! Oh wait...! Look at what L7QSR and deceptikonsaid... :icon_rolleyes: