Comparing input character to an array of ints in C

Nobody yet has explained why what you are doing is incorrect, apart from saying that it is "wrong".

A string in C is just a bunch of consecutive characters in memory, where the last character in the string has a value of zero. You can store a string in a char array, or you can point to somewhere in memory by using a char pointer (char*).

When you input a decimal number, you are reading individual characters that happen to be in the range '0' through to '9', maybe prefixed by an optional '-'. You read these as a string, and if you want to treat them as integers you need to convert them to an integer data type (instead of a series of char values).

That's where something like atoi helps, although it is no longer fashionable. You should use strtol. [There is a whole family of these functions for dealing with unsigned, long long, or combinations thereof, as well as double and float types].

That tackles roughly half of your question. Now, you are using strcmp and expecting it to work. There are a couple of things wrong with what you are doing. The major error is that you can't treat an integer as a string. If you really want to use string comparison, you have to convert the integer to a string. That means you do the reverse of what strtol does.

That's a bigger discussion, and in your case it is not the correct way so I won't go into it. But I'd like to point out that, all things being equal, you are sending the wrong types to strcmp. It expects two char* pointers (const char *, really). What you have done is dereferenced your input pointer to a char for the first element, and then pass an int for the second.

strcmp(*input,i[0])

A pointer is basically just a number. It gives the memory address of some data. In this case, the data is expected to be char type (single bytes). The strcmp function is expecting valid memory addresses (stuff that's actually in your stack or heap). But what you give it is *input (the value of the first character in your input string) and i[0] (the number 1).

Those compiler warnings were telling you something quite important, you know! =)

So, just for completeness (although others have answered this already), you should forget the string comparisons (and make a mental note to learn more about strings in C), and do this:

int value = strtol( input, NULL, 10 );

if( value == i[0] )
   printf("1 chosen");
if( value == i[1] )
   printf("2 chosen");

There are other ways to go about this. I could talk about how to convert single-digit numbers from a string, but I think I have already ranted for long enough. Hope this is of some help.

Apart from the various methods listed as answers to your qn.

Why don't you take the user input as int??

#include<stdio.h>

int main()
{

int i[2] = {1,2};
int input;
    printf("Select option:");
    scanf("%d",&input);

    if(input==i[0])
       printf("1 chosen");
    if(input==i[1])
       printf("2 chosen");

return 0;

}

You must use scanf instead of fgets:

int input;
printf("Select option:");
scanf("%d", &input);
if (input == i[0])

etcetera.

See http://en.wikibooks.org/wiki/C_Programming/Simple_input_and_output#Input_using_scanf.28.29

You need to compare the input string vs. some known strings. But you're comparing the first character vs. some ints. strcmp() will do what you need if you pass it the right arguments: two strings.

here you are trying to compare a string to an integer using strcmp:

if(strcmp(*input,i[0]) == 0)

that won't work.

you could do that way:

const char *numbers[2] = {"1", "2"};
char input[3];
printf("Select option:");
fgets(input,3,stdin);

if(strcmp(*input, numbers[0]) == 0)
    printf("1 chosen");
if(strcmp(*input, numbers[1]) == 0)
    printf("2 chosen");

where you compare two strings, instead of comparing a number to a string

or you could convert the input string to an int using sscanf or atoi

int i[2] = {1,2};
char input[3];
int num;

printf("Select option:");
fgets(input,3,stdin);

sscanf(input, "%d", &num);

if(num == i[0])
   printf("1 chosen");
else if(num == i[1]) 
   printf("2 chosen");

i didn't compile them though, maybe there's sth which i am missing but that's the general idea

Use atoi. it will convert your string to integer

 int i[2] = {1,2};
    char input[3];
    printf("Select option:");
    fgets(input,3,stdin);

    if(atoi(input)==i[0]))
       printf("1 chosen");
    if(atoi(input)==i[1]))
       printf("2 chosen");