[Solved] Using printf and scanf functions for doubles in C

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You must pass a pointer to a variable of the specified type when using scanf.

double itemCost;
double paidMoney;
int changeDue;

printf("How much does the item cost: ");
scanf("%lf", &itemCost);
// ----------^

printf("How much did the coustomer pay: ");
scanf("%lf", &paidMoney);
// ----------^

Also, you’re neglecting to check the return value of scanf. This is not optional! scanf returns the number of items successfully assigned. If it returns N, but you specified M variables to be assigned, then the last (N-M) variables are left unassigned (and in your case uninitialized).

Try something like this:

for (;;) {
    printf("How much did the coustomer pay: ");
    if (scanf("%lf", &paidMoney) == 1)
        break;   // success
    printf("Invalid input!\n");
}

solved Using printf and scanf functions for doubles in C