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Simple calculator program not letting me use the same operator (+.-.*./) twice

This is a discussion on Simple calculator program not letting me use the same operator (+.-.*./) twice within the C Programming forums, part of the General Programming Boards category; I am trying to make a simply calculator that allows the user to continually modify a result using +,-,*, or ...

  1. #1
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    Simple calculator program not letting me use the same operator (+.-.*./) twice

    I am trying to make a simply calculator that allows the user to continually modify a result using +,-,*, or / , however for some reason my code is only letting me use each operator (+,-,*, or /) one time, before giving me the error message that is supposed to only appear when the user enters a character that is not one of the four basic operators (+,-,*, or /). Actually no matter what I enter the second time it does this. Here is the code. Please help:


    Code:
    //
    //  main.c
    //  calc
    //
    //
    //
    
    
    #include <stdio.h>
    #include <stdlib.h>
    
    
    
    
    #define QUIT 0
    #define CONTINUE 1
    
    
    //---------------math prototype--------------------------
    int math (double first, double second, char operator);
    
    
    int main (void)
    {
        
        int equation=CONTINUE;
        char operation='+';
        double result=10.0, modifier;
    
    
        
        printf("calculator is on.\n");
        
        //printf("%d\n",equation);
        
        while (equation==CONTINUE) {
            
            
    //reading user input of operation and a number:
        scanf("%c%lf", &operation, &modifier);    
    
    
            
    //sending the two values of "result" and "modifier" and the operator character to subfunction called "math"
    // and putting the result of "result"(+ or - or * or /)"modifier" in to the variable "result":
        result = math(result, modifier, operation);
    
    
        
            printf("result=%f\n", result);
        
        }
            
        return 0;
        
    
    
    }
    
    
    
    
    //----------math-----------------------------------------------------
    
    
    int math (double first, double second, char operator)
              {
                  
                  if (operator=='+')
                      return (first+second);
                  
                  else if (operator=='-')
                      return (first-second);
                  
                  else if (operator=='*')
                      return (first*second);
                  
                  else if (operator=='/')
                      return (first/second);
                  
                  else 
                    printf("%c is an unknown operation. Reenter your last line:\n", operator);
                      return (first);
                  
              }

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    After you enter your operator and another operand, you hit the enter key. It gets stored in the input buffer (as a '\n'), so the next time through the loop, your scanf call reads the '\n' and thinks it's an invalid operator. Read this for some possible solutions: Cprogramming.com FAQ > Flush the input buffer.
    Last edited by anduril462; 10-03-2011 at 10:02 AM.

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    Registered User camel-man's Avatar
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    is a '\n' key always stored in the buffer or is it only stored after you have entered a character first. example you enter in a integer, then a char, will the char see the \n or is that only if you enter in a char then another char?

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    Quote Originally Posted by camel-man View Post
    is a '\n' key always stored in the buffer or is it only stored after you have entered a character first. example you enter in a integer, then a char, will the char see the \n or is that only if you enter in a char then another char?
    The newline char is always stored in the buffer, however since scanf is formatted input you will only see it when you try to input a char. To tell scanf to ignore whitespace characters, you can put a leading space in the format string.
    Quote Originally Posted by anduril462 View Post
    Now, please, for the love of all things good and holy, think about what you're doing! Don't just run around willy-nilly, coding like a drunk two-year-old....
    Quote Originally Posted by quzah View Post
    ..... Just don't be surprised when I say you aren't using standard C anymore, and as such,are off in your own little universe that I will completely disregard.
    Warning: Some or all of my posted code may be non-standard and as such should not be used and in no case looked at.

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    spurious conceit MK27's Avatar
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    Quote Originally Posted by camel-man View Post
    is a '\n' key always stored in the buffer or is it only stored after you have entered a character first. example you enter in a integer, then a char, will the char see the \n or is that only if you enter in a char then another char?
    The best thing to do with scanf() and questions like this is to write a short program and experiment to see what happens.

    '\n' is always in the buffer in sequence with everything else, eg, if you type "3 abc" and hit enter, the buffer will contain "3 abc\n". However, some scanf formats (such as %d) will "intelligently" skip it. Again, thoughtful experimentation should resolve any ambiguities you find in your documentation.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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    Quote Originally Posted by camel-man View Post
    is a '\n' key always stored in the buffer or is it only stored after you have entered a character first. example you enter in a integer, then a char, will the char see the \n or is that only if you enter in a char then another char?
    Question: Did you press the <enter> key?
    Answer: Yes.
    Result: It's in the buffer.

    Even if you enter nothing and hit <enter> ....

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    Thanks Andrew! Putting a space between opening quotes and the %c did the trick! That was really annoying. I thought C ignored white space characters...I guess this rule does not apply 100% of the time or it only doesn't apply inside quotations? This is a strange issue.

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    Quote Originally Posted by Charlie Lesoine View Post
    Thanks Andrew! Putting a space between opening quotes and the %c did the trick! That was really annoying. I thought C ignored white space characters...I guess this rule does not apply 100% of the time or it only doesn't apply inside quotations? This is a strange issue.
    C does what you tell it to do... and you weren't telling it to ignore whitespace.

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    what do you meant by this program?is it like this?
    1+1
    2 then you want to use 2 as your 1st operand for the 2nd time?
    if that is the case,then this program will not generate the result

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    Quote Originally Posted by vikasvijayan View Post
    what do you meant by this program?is it like this?
    1+1
    2 then you want to use 2 as your 1st operand for the 2nd time?
    if that is the case,then this program will not generate the result
    Please don't post nonsense.

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    Algorithm Dissector iMalc's Avatar
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    Quote Originally Posted by CommonTater View Post
    Please don't post nonsense.
    I actually kinda followed what that poster wrote, but at the same time I'm not impressed with most of the posts from that poster.
    My homepage
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