输入“正弦”时，C计算器程序始终显示错误？

Question

Hi I am new to programming and have been working on a calculator for a while now. I am trying to add some trig functions in and I am having trouble with sine. The other functions work (+, -, *, /) but when I put in "sine" it skips to the part of the code where it says it is an incorrect function. Please help out with my code. Thanks!

#include <stdio.h>
#include <math.h>


int main() 
{

    float firstnum, secondnum, angle, answer, pi;
    char function, sine;


    pi = atan(1.0)*4;


    printf("\nHello and welcome to my calculator!\n");

    while(1) 
    {      

        printf("\nPlease input the function you would like to use.  These include +, -, *, /, sine.\n");     
        scanf("%s", &function);   



        switch(function)
        {
            case '+': 
            printf("\nNow please input the two variables.\n");  
            scanf("%f", &firstnum);
            scanf("%f", &secondnum);
            answer = firstnum+secondnum;
            break;

            case '-': 
            printf("\nNow please input the two variables.\n");  
            scanf("%f", &firstnum);
            scanf("%f", &secondnum);
            answer = firstnum-secondnum;
            break;

            case '*': 
            printf("\nNow please input the two variables.\n");  
            scanf("%f", &firstnum);
            scanf("%f", &secondnum);
            answer = firstnum*secondnum;
            break;

            case '/': 
            printf("\nNow please input the two variables.\n");  
            scanf("%f", &firstnum);
            scanf("%f", &secondnum);
            answer = firstnum/secondnum;
            break;

            case 'sine':
            printf("\nPlease enter the angle.\n");
            scanf("%f", &angle);
            answer = sin(angle);
            break;



            default: printf("Sorry, that is an incorrect function.  The only available choices are +, -, *, /, sine.");
            break;
        }   

        printf("Your answer is %f \n", answer);  
        printf("\nWhen you are ready to quit, simply press Ctrl + C or just hit the X button in the top right.\n");
    } 

     return 0;
}

Answer 1

'sine'

That is a multi-character literal. function is a single character. It's integral value is checked in the switch statement. You will likely never be able to consume a single character from the user which matches sine in the way that you are attempting to do so. Read a string (a char* ) instead.

From the standard:

C99 6.4.4.4p10: "The value of an integer character constant containing more than one character (eg, 'ab'), or containing a character or escape sequence that does not map to a single-byte execution character, is implementation-defined."

Answer 2

C does not have a first class string type. This means that you cannot use a switch statement for strings, you will need to use functions such as strlcmp for string comparison.

Depending on your objective (either making a calculator, or learning C) it might be wise to either switch to a different language with higher abstraction levels, or start with lower level exercises from a good C textbook.

Also, please be aware that working with strings and user input correctly in C, that is without security holes, is much more difficult than it would seem at first. If your objective is learning a language perhaps learning C++ is a better bet where you have std::string to handle your comparisons and iostreams to handle your input and output.