QUESTION 49 · Input and output

Which scanf formats read float and double values?

2 marks

Answer

Use %f with the address of a float and %lf with the address of a double.

For float f, write scanf("%f", &f);.
For double d, write scanf("%lf", &d);. Check that one value was assigned before using it.

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QUESTION 50 · Input and output

Explain printf and scanf with suitable format specifiers.

5 marks

Answer in brief

Formatted input and output functions read and display values using a format string. They are declared in the stdio.h header.

printf: formatted output

printf displays values on standard output, usually the screen. A format specifier tells it how to display each value. For example, printf("Age = %d", age); displays an integer. For price = 12.5, printf("%.2f", price); displays 12.50. Here .2 requests two digits after the decimal point. It changes the displayed form, not the stored value.

scanf: formatted input

scanf reads values from standard input, usually the keyboard. For int age;, use scanf("%d", &age);. The & operator gives the address where the value must be stored. Check that scanf returns 1 before using this one value. A different result means the required assignment did not succeed. The input must also be within the range of the destination type.

Common format rules

Integer: Use %d for int.
Floating input: Use %f with float * and %lf with double *. For example, scanf("%lf", &price); reads a double.
Floating output: Use %f for a double. A float passed to printf is converted to double automatically.
Character: Use %c for a character.
Long double: Use %Lf for both input and output with the appropriate argument.

Important points

The format specifier and argument type must match. scanf normally needs addresses; printf normally needs the values to display. stdin is standard input, stdout is normal output and stderr is error output. These streams can be redirected to files. A successful input count does not prove that the whole line is valid.

Reference tables

↔ Scroll horizontally on a small screen

Common format specifiers
Value type	printf	scanf
int	%d	%d with int *
unsigned int	%u	%u with unsigned int *
long long	%lld	%lld with long long *
float	%f (promoted to double)	%f with float *
double	%f	%lf with double *
long double	%Lf	%Lf with long double *
char	%c	%c with char *
size_t	%zu	%zu with size_t *

↔ Scroll horizontally on a small screen

Standard streams
Stream	Role	Example
stdin	Standard input, often connected to the keyboard.	scanf reads from stdin.
stdout	Normal program output.	printf writes to stdout.
stderr	Diagnostic/error output.	fprintf(stderr, "Invalid input\n");

C program and output

Download .c Open compiler

/* CodeBhavya | PPS AR25 | Learning example */
#include <stdio.h>
int main(void)
{
    double price;
    if (scanf("%lf", &price) != 1) {
        puts("Invalid input");
        return 1;
    }
    printf("Price = %.2f\n", price);
    return 0;
}

Sample input and output

Input

12.5

Output

Price = 12.50

Key points

Match every format to the actual argument type.
Check input success before reading the result.