Problem with calculator program

Question

Good morning, I would like to know, why is my calculator program not printing the value of operations?? Below is the code:

#include <stdio.h>

int main (void)

{

    float A, B;
    char op;

    scanf("%f", &A);
    scanf("%c", &op);
    scanf("%f", &B);

    switch('op')
            {

            case 43 : printf("%f", A + B);
            break;

            case 45 : printf("%f", A - B);
            break; 

            case 42 : printf("%f", A * B);
            break;

            case 47 : printf("%f", A / B);
            break;

            }

return 0;
}

Answer 1

Beyond the problem of switch('op'), that should be without quotation marks as already mentioned in the comments, your scanf is not consuming the input the way you imagine. Let’s test for example the sum of 1.0 and 2.0. You type:

1.0ENTER
+ENTER

And the program ends, without asking for the third number! That’s because the ENTER after the first float is considered the character you were expecting in the second scanf. If you type the whole expression in one line, the program works normally:

1.0+2.0ENTER

A simple solution (but not very pretty) to read line by line is to put a getchar after reading the first float:

scanf("%f", &A);
getchar();
scanf("%c", &op);
scanf("%f", &B);

^{http://ideone.com/Guc1Kj}

Or, as @Jjoao suggested, simply add a space in what is expected by the second scanf:

scanf("%f", &A);
scanf(" %c", &op);
scanf("%f", &B);

^{http://ideone.com/2n5uMO}

Answer 2

Sorry to answer again but now in a way more according to what the OP intention.

1. case 43 --> case '+' makes things more readable

2. scanf returns a value (number of elements read) -- can be used for validation

3. in format scanf spaces mean zero or whites (spaces and " n") which is very important for operator reading.

4. Vertical symmetries in indentation make things more readable

Code:

#include <stdio.h>

int main (){
   float A, B;
   char op;

   if (scanf("%f %c %f", &A, &op , &B) == 3)
       switch(op) {
          case '+' : printf("%f", A + B); break;
          case '-' : printf("%f", A - B); break; 
          case '*' : printf("%f", A * B); break;
          case '/' : printf("%f", A / B); break;

          default : printf("???\n"); 
      }

   return 0;
}

Answer 3

If I’m not mistaken the switch of C is only of integer primitive values (this may have changed in the latest versions) another detail in char op you are assigning two characters soon the last will not be caught so even if it were possible to catch char on switch will never match one case so that it can pick up two or more characters you should declare a character array

char op[3]; // the first two positions for the 4 and 3 and the last position for the /0

That’s how it works:

#include <stdio.h>

int main (void)

{

    float A = 3;
    float B = 9;
    int op = 43;

    //scanf("%f", &A);
   // scanf("%c", &op);
   // scanf("%f", &B);

    switch(op)
            {

            case 43 : printf("%f", A + B);
            break;

            case 45 : printf("%f", A - B);
            break; 

            case 42 : printf("%f", A * B);
            break;

            case 47 : printf("%f", A / B);
            break;

            }

return 0;
}

Answer 4

One of the easiest ways to write C is to generate it. In this case with flex.

The typical structure of a program flexis:

%%
regExp    {ação C}
...
%%

In our case the file calc.fl is:

F   [ ]*[0-9]+(\.[0-9]+)?[ ]*
%%
  float A, B;

{F}\+{F}    { sscanf(yytext,"%f + %f",&A, &B);   printf("%f\n", A+B); }
{F}\*{F}    { sscanf(yytext,"%f * %f",&A, &B);   printf("%f\n", A*B); }
{F}\/{F}    { sscanf(yytext,"%f / %f",&A, &B);   printf("%f\n", A/B); }
{F}\-{F}    { sscanf(yytext,"%f - %f",&A, &B);   printf("%f\n", A-B); }
\n          {}

%%

To compile:

flex -o calc.c calc.fl
cc  -o calc cal.c -lfl 

This solution accepts questions repeatedly until CTR-D; the user can insert spaces and new-Lines freely (spaces in format sscanf).

Answer 5

I may be wrong but I did not find any value being passed to enter the switch. I did it that way and it worked:

int main (void)

{

float A, B;
int op;

printf("Entre com a opcao: ");
scanf("%d", &op);
printf("Entre com o valor de A: ");
scanf("%f", &A);
printf("Entre com o Valor de B: ");
scanf("%f", &B);



switch(op)
        {

        case 43 : printf("%f", A + B);
        break;

        case 45 : printf("%f", A - B);
        break; 

        case 42 : printf("%f", A * B);
        break;

        case 47 : printf("%f", A / B);
        break;

        }

        return 0;
        }