Question

I want to get the user input for variable L, but the scanf function is not working, and the program will jump and print the next cost statement and exit if I try to input anything.
I am new to C, and hope can get some help here. Thanks. Code below:

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

 int main()
 {
    float L = 0; // L is litre
    float gallon;
    gallon = 3.785 * L;
    char x[2] = {'u', 'd'}; // u is unleaded and d is diesel
    float cost;
    printf("Hello, welcome to PetrolUpHere!!\n");
    printf("Would u like unleaded or diesel fuel?");
    scanf("%s", &x[2]);
    printf("Enter the litre you want to fuel:");
    scanf("%.2f", &L); //SCANF NOT WORKING
    switch (x[2]) {
        case 'u':
            cost = 1.98 * gallon;
            printf("The cost is :%.2f ", cost);
            break;
        case 'd':
            cost = 1.29*gallon;
            printf("The cost is :%.2f ",cost);
            break;
    }
    getch();
    return 0;
}

Answer 1

There are a number of problems here:

scanf("%s", &x[2]);

I imagine you wanted to read a string into the variable x. Instead, you’re saying “read a string into memory 2 positions past where x points”. In this case that memory will be out of bounds. You should do this, since you only care about one character:

char input;
scanf("%c", &input);

Your switch statement is similarly broken; x[2] is again out of bounds. Use input from the above code instead.

As others have pointed out, using %.2f is not what you want to do when reading in L. Use %f instead. Generally you should only do something like that with format specifiers when printing out variables, rather than reading them in. Eventually you won’t be using scanf anyway, since it’s not a particularly safe way of getting input.

Finally: it seems like your understanding of how C strings work is shaky at best. This is understandable, since this is a fairly confusing topic for anyone who hasn’t worked in C before, and especially for novice programmers. Here’s one explanation; I’m sure you can find many more, probably better ones if you look.