C-使用scanf（）时额外输入新行

Question

I have the below 'C' code for a Console Win32 application using Visual Studio. It checks for arguments and directly calls a function or gets inputs using scanf() and then calls the function. The program works perfectly if I give command line arguments, but when I give inputs using scanf() , it probably adds an extra new line character which I want to avoid.

I know it does this because when I give arguments, after showing 'Press ENTER to quit...', it waits for the ENTER input, but doesn't do so when using scanf() . This problem occurs if I use scanf_s() too.

Appreciate your help on this.

Regards,

Shahid

    unsigned long int ipv;
    unsigned int yrs;
    float acr, acl, asp, irr, psp;

        if(argc<=1) {
            //get inputs
            printf("Initial value of portfolio: "); scanf("%lu", &ipv);
            printf("Average cash rate: "); scanf("%f", &acr);
            printf("Average cost of living: "); scanf("%f", &acl);
            printf("Average return for share portfolio: "); scanf("%f", &asp);
            printf("Initial removal for living expenses: "); scanf("%f", &irr);
            printf("Percentage of shares in initial portfolio: "); scanf("%f", &psp);
            printf("Years for the analysis: "); scanf("%u", &yrs);

            //simulate the investment with given inputs
            simulate(ipv, acr, acl, asp, irr, psp, yrs);
        } else {
            if (argc == 8) {
                //invsim 1000000 0.05 0.036 0.12 0.08 0.5 20

                //simulate the investment with given inputs
                simulate(atol(argv[1]), (float)atof(argv[2]), (float)atof(argv[3]), (float)atof(argv[4]), (float)atof(argv[5]), (float)atof(argv[6]), atoi(argv[7]));
            } else {
                printf("Please enter arguments as...\ninvsim.exe ipv acr acl asp irr psp yrs");
            }
        }

printf("\n\nPress ENTER to quit...");
getchar();

Answer 1

I know it does this because when I give arguments, after showing 'Press ENTER to quit...', it waits for the ENTER input, but doesn't do so when using scanf().

That is the right behaviour, isn't it? When you supply arguments via command line, you're not pressing any ENTER key. So, there is no \\n in the input buffer. So, getchar() will wait for the input from stdin .

However, when you intake values using scanf() , you press the ENTER key after the inputs. For numerical values, the numeric value is scanned but the \\n (produced by hitting ENTER )is remained in the input buffer to be read by next getchar() . So, your code doesn't wait .

To resolve: add one getchar(); at the end of the the scanf() part of code, ie, at the end of if block.

Answer 2

To improve your "pause" feature use the following update:

 printf("\n\nPress ENTER to quit...");
 while( getchar() != '\n' ); // clear the input buffer after reading numbers with scanf
 getchar();  // wait for new input

EDIT:

Taking into account the if-else of your program, move suggested while loop above:

 printf("Years for the analysis: "); 
 scanf("%u", &yrs);
 while( getchar() != '\n' ); // clear the input buffer

Answer 3

I ran into this issue as well, it's one of the more frustrating parts of C. I can't remember exactly what I did, but I remember that I ended up writing a small library to handle each type of input, ie, string, number, etc.

IIRC, I just used getline() and parsed the string into the type that I needed using atoi() and the other conversion functions found in ctype.h; I checked the length of the string and if it was 1(?) the only way that happens is if there was a new line already in the buffer, so I just called getline() again.

In this way, I could write the input validation ONCE, and use it a million times. So my code ended up looking like:

do {
      get_int("Enter an integer:", &myInt);//returns -123456789 for invalid input, or such
 } while ( myInt <= 0 )//assuming you only wanted positive input 

This also has the non-trivial benefit of not seeing your program crash when you entered a string and scanf() is expecting an integer.