Why is my C program asking for one more input than it should? And the extra input doesn't even do anything

Question

The the code for that portion, if you want to compile it is below.

Basically I have to take the following data and input it into the program:

4 5
12 5 7 0 -3
9 11 2 5 4
0 -5 9 6 1
2 12 93 -15 0
5 3
7 1 31
0 0 5
-5 -3 2
9 41 11
0 13 31

The first 4 5 & 5 3 represent the dimensions of the first and second matrix, while the data after it is the data for the matrix.

The problem is, when I copy and paste that in, it asks for one more input afterward, and when I input anything (say 84) it works flawlessly (output wise), and the 84 appears to do nothing. Why's it asking for this extra one?

#include <stdio.h>

int main(int argc, char *argv[]) {
    int rows1 = 1, columns1 = 1, rows2 = 1, columns2 = 1;    // variables for number of rows and columns in each matrix 
        int i, j, k;                                             // loop variables 

        // These will affect the loop's length
        scanf("%d %d", &rows1, &columns1);
        int matrix1[rows1][columns1];
        for (i = 0; i < rows1; i++) {
            for (j = 0; j < columns1; j++) {
                scanf("%d ", &matrix1[i][j]);
            }
        }


        scanf("%d %d", &rows2, &columns2);
        int matrix2[rows2][columns2];
        for (i = 0; i < rows2; i++) {
            for (j = 0; j < columns2; j++) {
                scanf("%d ", &matrix2[i][j]);
            }
        }
}

Answer 1

Your last scanf requires 2 things

                scanf("%d ", &matrix[i][j]);
                //     112

Thing 1 is an integer
Thing 2 is zero or more of whitespace

If you provide "31" (or "31 " or "31\\n") to the scanf it will be expecting more whitespace. Once you provide "84", it know the whitespace has ended and carries on with the program.

---

Suggestion: remove all whitespace from the scanf conversion specifiers: scanf("%d%d") is good! The "%d" conversion specifier already removes whitespace before the number.

Better yet, read data with fgets() and parse with sscanf()

Answer 2

The behaviour is due to the spaces in:

scanf("%d ", &matrix1[i][j]);
scanf("%d ", &matrix2[i][j]);

Change the format specifiers to:

scanf("%d", &matrix1[i][j]);
scanf("%d", &matrix2[i][j]);

(Well, it's only the second one that matters, but you might as well keep them identical.)

A space in the format specifier matches one or more whitespaces in the input. When your code is run, the final scanf() sits there reading from stdin until it encounters a non-whitespace character (the 8 in 84 ), at which point it returns and your program ends.

Answer 3

I think that what is happening is that you specified white-space after integer when you used scanf("%d ", &matrix2[i][j]); . scanf ignores white-space by default when using %d . However you specified a space so when it gets to the end of the file and sees an integer with no white-space following it doesn't see anything that matches your formatted input. Once you enter the new integer you also put a space after the last number satisfying the input formatting. So it wasn't the number you needed but the whitespace.