why the following code gives the first mobile no. irrespective of name

Question

Please explain why the following code gives the first mobile no. irrespective of entered name:

#include<stdio.h>
#include<conio.h>

int mobileno(char[]);

int main(){

 char input[20];
 gets(input); 
 printf("%s",input);          
 int x;
 x = mobileno(input);  
printf("%d",x);
switch(x){
          case 0:printf("7255222319");break;
          case 1:printf("9404521113");break;
          case 2:printf("9326513553");break;
          case 3:printf("9845621611");break;
          case 4:printf("4556312390");break;
          case 5:printf("9245178190");break;
          case 6:printf("9214188214");break;
          case 7:printf("7014773244");break;
          case 8:printf("7898888044");break; 

          };    

    getch(); 
    return 0;
}

int mobileno(char s[20]){


    if( s="katrina" )
    {return 1;}else{
    if( s="shahid" ){return 2;}else{
    if( s="ranbir" ){return 3;}else{
    if( s="sharukh" ){return 4;}else{
    if( s="hema" ){return 5;}else{
    if( s="amitabh" ){return 6;}else{
    if( s="shashnk" ){return 7;}else{
    if( s="raj" ){return 8;}else{
    if( s="aishwarya" ){return 0;}else{

        return 9;}}}}}}}}};

    };

Answer 1

You cannot compare strings using = (or even by ==, for that matter) operator. You need to use strcmp() for that.

In your code, inside mobileno() function,

if( s="katrina" )

is essentially trying to assign the base address of the string literal "katrina" to s. It is nowhere near a comparison.

That said,

1. Never use gets(), it suffer from buffer overflow problem. Use the safer alternative, fgets()
2. Recommended signature of main() is int main(void)

Answer 2

this code

if( s="katrina" )

assigns "katrina" to s and the result is not 0, i.e. "true". To compare, use strcmp instead. While you can compare values with "==", don't use it for strings as it would only compare the address, and two identical strings could still have different addresses.

Answer 3

Two problems in your if conditions:

if( s="katrina" )

Assignment instead of equality :

a=b means assigning the value of b to a.

a==b' means checking if both a and b have same values.

Char array comparison: Use strcmp() instead:

 if( strcmp(s,"katrina")==0 )

Answer 4

In this function

int mobileno(char s[20]){


    if( s="katrina" )
    {return 1;}else{
    if( s="shahid" ){return 2;}else{
    if( s="ranbir" ){return 3;}else{
    if( s="sharukh" ){return 4;}else{
    if( s="hema" ){return 5;}else{
    if( s="amitabh" ){return 6;}else{
    if( s="shashnk" ){return 7;}else{
    if( s="raj" ){return 8;}else{
    if( s="aishwarya" ){return 0;}else{

        return 9;}}}}}}}}};

    };

there is used the assignment operator = that assigns the address of the first characters of string literals in if conditions. For example in thsi statement

    if( s="katrina" )

pointer s is assigned to the address of the first character of string literal "katrina" that is evidently is not equal to NULL. So the condition is alway true.:)

If you want to compare to strings you should use standard C function strcmp declared in header <string.h>

Rewrite the function the following way

#include <string.h>

//...

int mobileno( const char *s )
{
    const char *name[] = 
    {
        "aishwarya", "katrina", "shahid", "ranbir", "sharukh",
        "hema", "amitabh", "shashnk", "raj"
    };
    const size_t N = sizeof( name ) / sizeof( *name );

    size_t i = 0;

    while ( i < N && strcmp( s, name[i] ) != 0 ) i++;

    return i;
}

Take into account that function gets is not a C Standard function any more because it is unsafe. Use instead function fgets.