Addition and Subtraction in C using fscanf() - c

Question

I have a text file with n lines of m length (it doesn't matter in the long term ). In every line, there is a sequence like this:
1 + 2 + 5 - 3 =
I'm using fscanf function to catch all numbers and operators and calculate it and when '=' operator found it prints into a different file. I'm using a loop . My question is how does C recognizes a number and operator? I know the operator is a type of %c and the number is of type %d, but how my program does work properly and why it doesn't take them for example 5 as a character :
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *InFile;
FILE *OutFile;
int number=0;
int result=0;
char operator;
int foundEqualsOp=0;
InFile = fopen("dane.txt","r");
OutFile = fopen("wynik.txt","w");
while(fscanf(InFile,"%d",&result)!=EOF)
{
number = 0;
foundEqualsOp = 0;
while(!foundEqualsOP){
fscanf(InFile,"%c",&operator);
if(operator == '='){
foundEqualsOp = 1;
fprintf(OutFile,"%d\n",result);
}
else if(operator == '-'){
fscanf(InFile,"%d",&number);
result = result-number;
}
else if (operator == '+'){
fscanf(InFile,"%d",&number);
result = result + number;
}
}
}
fclose(InFile);
fclose(OutFile);
return 0;
}

Answer 1

Let's assume your input file contains the following as in your example:
1 + 2 + 5 - 3 =
The outer scanf reads with %d. This picks up 1 as the integer value 1 and puts it in result.
Now the inner scanf runs with %c. It reads the space after 1 so none of the if blocks are entered and the inner loop runs again. The inner scanf again reads with %c and reads the + character. This causes the if (operator == '+') block to be entered where a call to scanf uses %d. This format specifier skips over the space after the + and reads 2 as the integer value 2 and adds it to result.
The above happens again reading the space after 2, the + that follows, and the space and 5 that follows. And again for the following space, -, space, and 3.
After that, the scanf in the inner loop reads the space after 3 and does nothing, then on the next iteration it reads the = after which it prints the result and sets the flag to exit the inner loop.,