c beginners calculator

Question

I wrote this simple calculator and I want to know if there is any problems in it or if there is any way to enhance the code and make it simpler and easier and I also want to know if I'm starting to get into the intermediate level instead of beginners level and thank you

#include<stdio.h>

void main()

{
int mode, n1, n2;
float num1, num2, rslt;
char op;

puts("a simple calculator");
printf("enter the first number : ");
scanf("%f", &num1);
printf("which operation you want '+ - / * or mode' : ");
scanf(" %c", &op);
printf("enter the second number : ");
scanf("%f", &num2);

switch(op)
    {
    case '+' :
        rslt = num1 + num2;
        printf("\n %f + %f = %.3f\n\n", num1, num2, rslt);
        break;
    case '-' :
        rslt = num1 - num2;
        printf("\n %f - %f = %.3f\n\n", num1, num2, rslt);
        break;
    case '*' :
        rslt = num1 * num2;
        printf("\n%f * %f = %.3f\n\n", num1, num2, rslt);
        break;
    case '/' :
        rslt = num1 / num2;
        printf("\n%f / %f = %.3f\n\n", num1, num2, rslt);
        break;
    case '%' :
        n1 = num1;
        n2 = num2;
     
        switch(n1%n2)
            {
            case 0 :
                printf("\n%d can be devided by %d\n\n", n1, n2);
                break;
            case 1 :
                printf("\n%d can not be devided by %d\n\n", n1, n2);
                break;
            }
        break;  
    default :
        printf("\ninvalid input\n\n");
    }
}   

Answer 1

Your code is OK, for a beginner, so please don't worry too much about these comments. I bet you program is fine until you enter the data "break me". The reason I can list these points so easily, is because I've done them more times than I care to think of, and I've probably done some of them today. Code Review is always a case of do what I say, not what I do. I hope that helps, if there is anything that isn't clear please say and I will try and make it clearer.

1. Your code lacks comments. There are all sorts of wise people who say you should have x% comments in your code, but all you need to do is to explain what your code does. Sound stupid, but when you look at this in a years time, you will know what I mean.
2. Variable names should be descriptive, it means you don't need to write so many comments. n1, n2, num1, num2 and op are all examples of variable names I think could be more descriptive/precise. You shouldn't have to think when you read code, firstNumber and secondNumber are easier to understand than n1 and n2.
3. Initialise variables unless you are 100% certain you don't need to. rslt is not initialised when op is % or default. This could cause a crash or at least garbage data. Also check what happens if scanf fails (can it fail?), is the output parameter set to a certain safe value?
4. Only declare a single variable per line. It makes it easier to check what has been initialised and what hasn't and to change types.
5. why did you use puts() and the printf()? Why not just printf() to make the code more uniform?
6. The printf statements inside the switch statements could be moved to the end to 'refactor' you code slightly.
7. Putting a switch inside a switch increases the complexity of the code massively. In this case it might be better to replace the whole block with:

.

{
  int n1 = (static_cast<int>(num1); 
  int n2 = (static_cast<int>(num2);   
  printf("\n%d can %sbe divided by %d\n\n", n1, ((n1%n2)?"":"not"), n2); 
}

EDIT To move the printf outside of the switch, you just need to place the opcode in the general string using the variable you already have op:

printf("\n%f / %f %c %.3f\n\n", num1, num2, op, rslt);

The problem is with the % and default branches of the switch, but you can just use return after their printfs to skip the general one.

Answer 2

regarding:

void main()

results in: untitled.c:3:6: warning: return type of ‘main’ is not ‘int’ [-Wmain]

There are also 3 other compiler warnings:

gcc -ggdb3 -Wall -Wextra -Wconversion -pedantic -std=gnu11 -c "untitled.c" -o "untitled.o"

results in:

untitled.c:3:6: warning: return type of ‘main’ is not ‘int’   [-Wmain]
3 | void main()
  |      ^~~~

untitled.c: In function ‘main’:

untitled.c:37:14: warning: conversion from ‘float’ to ‘int’ may change value [-Wfloat-conversion]

37 | n1 = num1; | ^~~~

untitled.c:38:14: warning: conversion from ‘float’ to ‘int’ may change value [-Wfloat-conversion]

38 | n2 = num2; | ^~~~

untitled.c:6:5: warning: unused variable ‘mode’ [-Wunused-variable]
6 | int mode, n1, n2;
  |     ^~~~

Answer 3

Use floating point

Rather than convert to int and risk problems when the float value is outside the int range, use fmodf() via <math.h>.

case '%' :
    rslt = fmodf(num1, num2);  
    printf("\n%f % %f = %.3f\n\n", num1, num2, rslt);  
    break;