Switch statement using string on an array

Question

#include<stdio.h>

int main(){

    char name[20];

    printf("enter a name ");
    scanf("%s",name);
    switch(name[20]){
        case "kevin" : 
        printf("hello");
        break;
    }
    printf("%s",name);
    getch();
}

It seems it will not work. Is this possible? I mean is there any way we can make a switch statement of a string. How to solve the problem, actually?

Solution 1

Switch statements in C aren't smart like one's found in other languages (such as Java 7 or Go) you cannot switch on a string (Nor can you compare strings with ==). Switch can only operate on integral types (int, char, etc).

In your code you call switch with: switch(name[20]). That means switch(*(name + 20)). In other words switch on the 21st char in name (because name[0] is the first). As name only has 20 chars you are accessing whatever memory is after name. (which could do unpredictable things)

Also the string "kevin" is compiled to a char[N] (where N is strlen("kevin") + 1) which contains the string. When you do case "kevin". It will only work if name is in the exact same piece of memory storing the string. So even if I copied kevin into name. It still would not match as it is stored in a different piece of memory.

To do what you seem to be trying you would do this:

#include <string.h>
...
    if (strcmp(name, "kevin") == 0) {
        ...
    }

String compare (strcmp) returns different values based on the difference in the strings. Eg:

int ord = strcmp(str1, str2);
if (ord < 0)  
    printf("str1 is before str2 alphabetically\n");
else if (ord == 0) 
    printf("str1 is the same as str2\n");
else if (ord > 0)  
    printf("str1 is after str2 alphabetically\n");

Side note: Dont use scanf("%s", name) in that form. It creates a common security problem use fgets like this: (there is a safe way to use scanf too)

#define MAX_LEN 20
int main() { 
    char name[MAX_LEN]; 
    fgets(name, MAX_LEN, stdin);
    ...

Solution 2

Switch statements work on int values (or enum), but not on char arrays.

You could do

if (strcmp(name, "kevin")==0) {
    printf("hello");
}
else if (strcmp(name, "Laura")==0) {
    printf("Allo");
}
else if (strcmp(name, "Mike")==0) {
    printf("Good day");
}
else  {
    printf("Help!");
}

Solution 3

There are plenty of ways to go about this! For example, use a...

3-letter hash

#include <stdio.h>

int main(){

    char name[20];

    printf("enter a name ");
    scanf("%s",name);
    switch((int)*name * (int)*(name+1) * (int)*(name+2)){
          case (1275226) : // "kevin"
            printf("hello %s.\n", name);
            break;
          case (1293980) : // "astro"
            printf("welcome %s.\n", name);
            break;
    }
    printf("%d",(int)*name * (int)*(name+1) * (int)*(name+2));
}

Comments

[autistic]

Which book are you reading? I ask this because those who read books usually don't have this question; The book answers it. If you're not using a book as a guide, then you're probably a liability; Stop learning C, and learn a programming language that doesn't allow you to accidentally shoot yourself in the feet. Alternatively, get yourself a book... (duh, hintedy hint)

[PNC]

im not reading any book :) uh.. c language is our lesson and we have a project about it. yeah i want to stop learning this we are just forced to do so :))

[Jiminion]

Isn't name[20] undefined?

[This isn't my real name]

Please note that string literals do not compile to const char *. String literals are arrays of char, and have type char[n] where n is one more than the number of bytes in the string ("ABCD" has four bytes, so it would be of type char[5]), and when used in a pointer context, they evaluate to pointers which are of type char *. It is true that you aren't allowed to modify their content, but for historical reasons they do not have const type.

[Triss Healy]

Cheers, changed the text to fix that. I wasn't sure if C recognized string literals as const as C++ seems to, C won't complain about loosing const-ness.

[This isn't my real name]

Incidentally, here's a subtlety for you. While it is true that the string literal "kevin" is an array N of char where N is equal to strlen("kevin") + 1, this does not hold true for all string constants. Consider the perfectly valid string constant "degenerate\0string\0constant", which (if I've counted correctly) is is an array 27 of char, not an array 11 of char.

[jjxtra]

For strings of length 8 or less you should be able to switch on a unsigned long long by bit shifting each char.