Learn to Program with C_ Learn to Program using the Popular C Programming Language ( PDFDrive )

7.12 Fetch the Next Integer
Chapter 7 ■ Functions
Previously, we wrote Program P6.13, which read the data character by character, constructed and
stored the next integer found in a variable, and finally printed the integer.
Let us now write a function, getInt, which reads the data character by character and returns
the next integer found. The function does not take any arguments but the brackets must still
be written after the name. The code is essentially the same as in P6.13, except that we use the
predefined function isdigit. Here is getInt:
int getInt() {
char ch = getchar();
// as long as the character is not a digit, keep reading
while (!isdigit(ch)) ch = getchar() ;
// at this point, ch contains the first digit of the number
int num = 0;
while (isdigit(ch)) { // as long as we get a digit
num = num * 10 + ch - '0'; // update num
ch = getchar();
}
return num;
} //end getInt
Note that
while (ch < '0' || ch > '9')
of program P6.13 is replaced by
while (!isdigit(ch))
and
while (ch >= '0' && ch <= '9')
is replaced by
while (isdigit(ch))
We believe this makes the program a little more readable.
The function needs the variables ch and num to do its job; ch holds the next character in the
data and num holds the number constructed so far. We declare them within the function, making
them local variables. This way, they will not conflict with any variables with the same names
declared anywhere else in the program. This makes the function self-contained – it does not
depend on variables declared elsewhere.
The function can be used as in
id = getInt();
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