C Academy · Lesson

scanf and Input

Read user input.

scanf and Input is a free C Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

What scanf Does

scanf reads formatted input typed by the user. It uses the same specifiers as printf.

#include <stdio.h>
int main(void) {
    int x = 0;
    /* scanf("%d", &x); */
    x = 5;
    printf("x = %d\n", x);
    return 0;
}

The Address-Of Operator

scanf needs the address of a variable so it can store the value there. That is why you write &x.

#include <stdio.h>
int main(void) {
    int x = 42;
    int *p = &x;
    printf("value via address = %d\n", *p);
    return 0;
}

Reading an Integer

To read an integer, pass %d and the address of an int.

#include <stdio.h>
int main(void) {
    int n;
    /* scanf("%d", &n); */
    n = 7;
    printf("You entered %d\n", n);
    return 0;
}

Reading a Float

To read a float use %f; for a double use %lf.

#include <stdio.h>
int main(void) {
    double d;
    /* scanf("%lf", &d); */
    d = 3.14;
    printf("d = %.2f\n", d);
    return 0;
}

Reading Multiple Values

One scanf can read several values separated by whitespace.

#include <stdio.h>
int main(void) {
    int a, b;
    /* scanf("%d %d", &a, &b); */
    a = 3; b = 4;
    printf("sum = %d\n", a + b);
    return 0;
}

Reading a Character

Use %c to read one character. Beware: it also reads spaces and newlines.

#include <stdio.h>
int main(void) {
    char c;
    /* scanf(" %c", &c); */
    c = 'X';
    printf("char = %c\n", c);
    return 0;
}

Reading a Word

%s reads a whitespace-delimited word into a char array. No & is needed because an array name is already an address.

#include <stdio.h>
int main(void) {
    char word[20];
    /* scanf("%s", word); */
    word[0] = 'H'; word[1] = 'i'; word[2] = 0;
    printf("word = %s\n", word);
    return 0;
}

scanf Return Value

scanf returns how many items it successfully read. Check it to detect bad input.

#include <stdio.h>
int main(void) {
    int n = 5;
    int read = 1; /* pretend scanf returned 1 */
    if (read == 1)
        printf("got %d\n", n);
    return 0;
}

Skipping Leading Spaces

Numeric specifiers skip leading whitespace automatically, but %c does not. A space before %c fixes this.

#include <stdio.h>
int main(void) {
    char c = 'Y';
    printf("Reading with \" %%c\" skips spaces\n");
    printf("c = %c\n", c);
    return 0;
}

Validating Input

Always verify the return value before trusting the data.

#include <stdio.h>
int main(void) {
    int value = 10;
    int ok = 1;
    if (ok)
        printf("valid: %d\n", value);
    else
        printf("invalid input\n");
    return 0;
}

Putting It Together

A typical program prompts, reads, then uses the value.

#include <stdio.h>
int main(void) {
    int age;
    printf("Enter age: ");
    /* scanf("%d", &age); */
    age = 20;
    printf("Next year you will be %d\n", age + 1);
    return 0;
}

Quick Check

Test your understanding of scanf.

Recap: scanf and Input

You learned how to read user input.

scanf uses printf-style specifiers
Pass &var so it knows where to store
Use %lf for double, %s for words
Check the return value to validate input
Put a space before %c to skip whitespace

Frequently asked questions

Is the “scanf and Input” lesson free?

Yes — the full text of “scanf and Input” is free to read here on the web, and the C Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the C Academy course, upgrade to CoddyKit PRO.

What will I learn in “scanf and Input”?

Read user input. You practise C Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start C Academy?

No prior experience is required. C Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “scanf and Input” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this C Academy lesson?

Yes. Every C Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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