C语言：为什么在int声明的变量中输入浮点数时结果会变化

Question

using a simple code:

#include <stdio.h>

int main(int argc, int **argv)
{
    int A, B, C, D, E, F; 
    printf ("input 1  : ");
    scanf ("%d", &A);
    printf ("input 2 : ");
    scanf ("%d", &B);
    C = A + B;
    D = A - B;
    E = A * B;
    F = A / B;
    printf ("sum  : %d\n", C);
    printf ("difference : %d\n", D);
    printf ("product  : %d\n", E);
    printf ("quotient : %d\n", F);
    return 0;
}

My question is as such, in the first scanf [ps I know I can use other input methods its for a project] if you input a float/double number such as 1.3 or 20.5 the sum and difference are quite random for me,anyone can explain to me why the results are what they are?

Answer 1

Continuing from the comments, you must always validate all input (especially user input). All input functions provide a return. The scanf family returns the match count , the number of successful conversions processed based on the number of format specifiers in the format string . You use that to validate your input. Eg, at minimum:

#include <stdio.h>

int main(int argc, int **argv)
{
    int A, B, C, D, E, F; 
    printf ("input 1  : ");
    if (scanf ("%d", &A) != 1) {
        fprintf (stderr, "error: invalid input - A.\n");
        return 1;
    }
    printf ("input 2 : ");
    if (scanf ("%d", &B) != 1 || B == 0) {
        fprintf (stderr, "error: invalid input - B.\n");
        return 1;
    }
    C = A + B;
    D = A - B;
    E = A * B;
    F = A / B;
    printf ("sum  : %d\n", C);
    printf ("difference : %d\n", D);
    printf ("product  : %d\n", E);
    printf ("quotient : %d\n", F);
    return 0;
}

note: your "quotient" will always be the result of integer division and truncated accordingly.

Answer 2

The first scanf() , if the input has a decimal point will stop at the decimal point and leave it to be read on the next operation. For input of 1.3 , this means A will get the value 1 , and the .3 will remain in the input stream to be read. This is because of the %d format - which tells scanf() to expect an integral value, and to stop on any characters that do appear in any representation of an integral value. A decimal point is one such character that is not used in the representation of an integral value.

The next operation ( scanf("%d", &B) ) immediately encounters the decimal point and returns, without changing B .

Since B is not initialised at all (before, during, or after the scanf("%d", &B) ) in the program, any subsequent attempt to access its value gives undefined behaviour. Among other things, this can mean the value of B is indeterminate. From what you describe, for your setup, that results in "random" input.

If you're expecting to read input that looks like floating point values (eg that contains a decimal point) either read as floating point (eg %f format, and variables of a floating point type) or read a whole line (eg using fgets() ) and check the contents of the line BEFORE trying to read integral values from it.

If you insist on using scanf() , check its return value. On the second call of scanf() in your scenario, scanf() will return 0 rather than 1 (since it has read no values, rather than the one value the format string specified). Which is an indication that something has gone wrong reading the input.