Input

So far, all the values in our programs were hardcoded. Time to let the user type something. In C, we read input with scanf

#include <stdio.h>

int main(void) {
    int age;

    printf("Your age: ");
    scanf("%d", &age);

    printf("Next year you will be %d\n", age + 1);
    return 0;
}

Run the program, type a number, press Enter

Two things to notice

• scanf is the partner of printf: instead of writing output, it reads input
• The first argument is a format specifier, exactly like in printf. %d for an int, %f for a float/double, %s for a string, %c for a single char
• The second argument is &age with a strange & in front

Why the &? In C, &variable means "the address of this variable". When we call scanf, we're telling it: "here's where in memory you should write the value the user types". Without the &, scanf would receive a copy of the variable's value (which makes no sense before reading), and your program might crash or silently corrupt memory

Don't worry too much about it for now. The rule is simple

• printf uses just the variable
• scanf uses &variable (almost always)

There is one exception: when reading a string (a char[]), you don't use &. We'll cover that in the strings lesson

To read a double

#include <stdio.h>

int main(void) {
    double height;
    scanf("%lf", &height);
    printf("%f\n", height);
    return 0;
}

Important: for scanf, doubles use %lf (l for "long", f for "float"), not %f. %f in scanf would read a regular float instead. This is a small C quirk that catches many beginners. (For printf, both %f and %lf work the same on doubles. Inconsistent? Yes)

Reading multiple values in one scanf call

#include <stdio.h>

int main(void) {
    int a, b;
    scanf("%d %d", &a, &b);
    printf("Sum: %d\n", a + b);
    return 0;
}

The user types two numbers separated by space (or Enter), scanf reads both. Notice we still use & in front of each variable

But what if we want to read a full line with spaces? scanf("%s") stops at the first space, so if the user types Dennis Ritchie, scanf only reads Dennis. For full lines, we use fgets

#include <stdio.h>

int main(void) {
    char name[64];

    printf("Full name: ");
    fgets(name, sizeof(name), stdin);

    printf("Hello %s", name);
    return 0;
}

fgets takes three arguments:

• name: where to store the text
• sizeof(name): the maximum number of characters to read (prevents a buffer overflow!)
• stdin: read from standard input (the keyboard)

Unlike scanf, fgets is safe: it will never write more characters than the buffer can hold. This is why fgets is preferred over scanf for reading strings in real C programs

One small catch: fgets keeps the newline character (\n) at the end. To remove it

#include <stdio.h>
#include <string.h>

int main(void) {
    char name[64];

    printf("Full name: ");
    fgets(name, sizeof(name), stdin);
    name[strcspn(name, "\n")] = '\0';

    printf("Hello %s!\n", name);
    return 0;
}

strcspn(name, "\n") finds the position of the newline, and we replace it with \0 (the string terminator). This is a common C pattern you'll see everywhere

Mission: The Computing Center's Access Log

Every operator arriving at the computing center must register at the entrance terminal. The system reads their name and age, then displays a welcome message with their age next year.

Read a name (single word, no spaces) and an age using scanf, then print the welcome line.

Example

Input

Ritchie
60

Output

Hello Ritchie, you are 60 years old. Next year you will be 61

Example

Input

Thompson
33

Output

Hello Thompson, you are 33 years old. Next year you will be 34