Calculator program requires + and - to be entered twice - c

Question

I am writing a simple calculator program to add, subtract, multiply, divide and do exponential equations. The program must repeat until "q0" is entered and it must have two functions called by the main function like I have written. The problem is when I run the program the first number shows up fine and then I can multiply, divide, and do exponential functions fine but if I want to add or subtract I must enter the + or - twice for it to count it. This is the program I have written:
#include <stdio.h>
#include <math.h>
void scan_data(char *oprter, double *oprand);
void do_next_op(char oprter, double oprand, double *accum);
int
main(void)
{
double nmber, /* input/output - operand */
total; /* output - final result */
char sign; /* input/output - operator */
total = 0;
do {
scan_data(&sign, &nmber);
do_next_op(sign, nmber, &total);
} while (sign != 'q');
printf("Final result is %.2f", total);
return (0);
}
/*
* Gathers operator and operand to perform calculation
* Post: results are stored in cells pointed to by oprter, and oprand
*/
void
scan_data(char *oprter, /* input - amount being withdrawn */
double *oprand) /* output - number of tens */
{
double amount;
amount = 0;
scanf("%c", &*oprter);
scanf("%lf", &amount);
*oprand = amount;
}
/*
* Performs calculation and displays results
* Pre: oprter and oprand are defined
* Post: function results are stored in cell pointed to by accum
*/
void
do_next_op(char oprter,
double oprand,
double *accum)
{
double tot_amount;
tot_amount = *accum;
switch(oprter)
{
case '+':
tot_amount = (tot_amount + oprand);
printf("result so far is %.2f\n", tot_amount);
break;
case '-':
tot_amount = (tot_amount - oprand);
printf("result so far is %.2f\n", tot_amount);
break;
case '*':
tot_amount = tot_amount * oprand;
printf("result so far is %.2f\n", tot_amount);
break;
case '/':
tot_amount = tot_amount / oprand;
printf("result so far is %.2f\n", tot_amount);
break;
case '^':
tot_amount = pow(tot_amount, oprand);
printf("result so far is %.2f\n", tot_amount);
break;
}
*accum = tot_amount;
}
An example of what it is doing follows:
+5
result so far is 5.00
+5
-5
++5
result so far is 10.00
--5
result so far is 5.00
*2
result so far is 10.00
/2
result so far is 5.00
^2
result so far is 25.00
q0
Final result is 25.00

Answer 1

The problem is most likely that scanf leaves the newline in the input buffer when you do
scanf("%lf", &amount);
That means that the next call to
scanf("%c", oprter);
it will read that newline.
This can be solved by asking scanf to read and skip all leading whitespace by adding a single space to the scanf format string like
scanf(" %c", oprter);
// ^
// |
// Note space here