My switch statement is always returning the first case?

Question

I'm having some trouble with this code. I'm pretty new to a switch statements and enumerated types so may be overextending a bit. I managed to work this to enter the switch statement, but it keeps returning the first case. Any ideas why?

#include <stdio.h>
#include <string.h>

enum express {ADD, SUB, AND, OR, XOR, SHL, SHR};
express m_express;

express switchint(char *str);

int main(){
    unsigned int n1=0x00;
    unsigned int n2=0x00;
    char action[5];
    printf("Enter an expression: ");
    scanf("%x, %s, %x", &n1, action, &n2);
    m_express=switchint(action);
    unsigned int result;
    switch(m_express){

        case ADD:
            printf("add works");
            break;
        case SUB:
            printf("SUB works");
            break;
        default:
            printf("Default");
            break;
     }
}

express switchint(char *str){
    if( strcmp(str, "add")){
        return ADD;
    }
    else if ( strcmp(str, "sub")){
       return SUB;
    }
    else if ( strcmp(str, "and")){
        return AND;
    }
    else if ( strcmp(str, "or")){
        return OR;
    }
    else if ( strcmp(str, "xor")){
        return XOR;
    }
    else if ( strcmp(str, "shl")){
        return SHL;
    }
    else {
        return SHR;
    }
}

I haven't written the rest of the switch cases I need yet. Any help solving this issue is greatly appreciated!

Do you know what is the meaning of `strcmp` and its return value? Have you used a debugger to step through the code? — StoryTeller - Unslander Monica, Jul 30 '17 at 22:53
hint: what happens if `switchint` function falls off the end? Also, `strcmp` doesn't work like you think it does. — o11c, Jul 30 '17 at 22:54
You need to read [How to debug small programs (by Eric Lippert)](https://ericlippert.com/2014/03/05/how-to-debug-small-programs/) — StoryTeller - Unslander Monica, Jul 30 '17 at 22:54
@o11c - In this particular case it cannot in any way fall off the end. Although I'm not sure how pedantic are compilers at checking that. — StoryTeller - Unslander Monica, Jul 30 '17 at 22:55
Hm, actually I think the *first* bug is with the call to `scanf` ... the commas in the format string, and then the return value is not checked for failure (not to mention the buffer overflow due to not using `%4s`) — o11c, Jul 30 '17 at 22:58
Check the return value of `scanf`, it may be you are switching based on a bogus value — M.M, Jul 30 '17 at 23:23

score 4 · Accepted Answer · edited Jul 30 '17 at 22:55

4

strcmp returns 0 if both strings are equal. You should rewrite your checks:

if( !strcmp(str, "add")) 
{
}
else if ( !strcmp(str, "sub")){
       return SUB;
}

edited Jul 30 '17 at 22:55

Petr Skocik

58,047
6
95
142

answered Jul 30 '17 at 22:54

ggwellplayed

56
2

This turned out to be half the issue. My scanf statement foolishly used commas also. Does this mean !strcmp(...) and strcmp(...)==0 work the same way? – Benjamin M Jul 30 '17 at 23:03

score 2 · Answer 2 · answered Jul 30 '17 at 22:55

2

strcmp returns 0 when both strings are equal. So your comparison in your switchint function should be:

if(!strcmp(str, "add")) { ... }

Same goes for the other comparisons.

answered Jul 30 '17 at 22:55

CRoemheld

889
7
26

score 0 · Answer 3 · answered Jul 30 '17 at 23:03

According to the description of the function in the C Standard (7.23.4.2 The strcmp function)

3 The strcmp function returns an integer greater than, equal to, or less than zero, accordingly as the string pointed to by s1 is greater than, equal to, or less than the string pointed to by s2.

So you should write at least like

express switchint( const char *str){
                   ^^^^^
    if( strcmp(str, "add") == 0 ){
        return ADD;
    }
    else if ( strcmp(str, "sub") == 0 ){
       return SUB;
    }
    // and so on

My switch statement is always returning the first case?

3 Answers3