Input

We're now going to discuss how to take user input from the standard input (stdin) in C.

scanf

scanf() allows you to read different data types from the user's input and store them in variables, making it a versatile function for interactive applications. It enables you to control the input format, extract data from the input stream, and store it in the appropriate variables.

Signature

The general signature of scanf() is similar to printf(), it takes a format string indicating the format of the input sequence and a variadic list of addresses (pointers)where values are stores. scanf()'s format string uses a similar format specification to printf() however, the variable specifiers (denoted with %) indicate values that are to be read in and stored in program variables. As we are reading values in with scanf(), when we wish to store a variable we must pass the address of the variable we want to store the value in. This is because variables have copy semantics in C meaning we cannot pass a variable to a function and get the modified value with returning it or using a pointer. scanf() uses pointers.

scanf(char* fmtstring, ...);

Example

This simple program is similar to the printf() example we saw earlier in the book except, instead of hard coding values we are able to take input from the user in the form x + y and the program will return the result.

#include <stdio.h>

int main()
{
    double a = 0.0;
    double b = 0.0;
    double sum = 0.0;

    printf(">> ")
    scanf("%f + %f", &a, &b);
    sum = a + b;
    printf("%f", sum);

    return 0;
}

Formatting Specification

The format specification is a mirror to that used by printf() (there are many _f() functions in the C standard library that all use similar format specifications) as values being read in must conform to a given C type. This allows us to easily convert the incoming string data as int, double or even substrings.

The general format specification is as below:

_%\[width\]\[.precision\]\[length\]type-specifier_

There are a variety of different options for each part of the specification. Below is a series of tables breaking down the various options for each sub-specifier but note that only type-specifier is needed, the others are optional.

Type Specifiers

Width

Precision

Length