Input/Output

Input Function

The function to read user input is the scanf function.

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int scanf(string format, ...);

// Common Usage
int scanf(format string, input list); // Assign to variables in list

Firstly, all user inputs are read as string. The parameter format specifies how the input string should be treated. This is done by specifying the format specifiers which are placeholders for values to be read.

Secondly the ... can take in zero or more optional parameters. If they are given, the parameters should be an address to a variable. The variable will be assigned the value on the placeholders based on its position. So, the first optional variable will be assigned the value on the first placeholder, the second optional variable will be assigned the value on the second placeholder, and so on.

The address of a variable can be obtained by using the address operator &. For instance, given a variable x, its address can be obtained using the expression &x.

Note that scanf has a return value of type int. This return value actually indicates the number of items in the ... that has successfully been assigned a value. However, in most cases, we assume that we have managed to read all and will typically ignore this value.

Lastly, the input will only be processed by scanf when the user press Enter.

Output Function

The function to display output on the monitor is the printf function. Its function prototype is

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int printf(string format, ...);

// Common Usage
int printf(format string);             // Print string
int printf(format string, input list); // Print values in list

Firstly, the return value int is often ignored as it is simply the number of actual characters being printed on the screen.

Secondly, unlike scanf, we can specify more information for the format specified in printf.

For int, we can specify the minimum width to be displayed using %nd where n is an integer. For instance, %5d displays an integer in a width of 5. If there are fewer digits than the width, the number will be right justified.

For float/double, we specify two numbers separated by a decimal point (i.e., a dot) to specify the width and the number of decimal places. In this case, using the format %n.mf, n is the width and m is the number of decimal places. For instance, %8.3f displays a real number with total width of 8 including the 3 decimal places. Similar to the case of int, the number will be right justified.

Examples

Multi Lines Input

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#include <stdio.h>
int main(void) {
  int age;
  double cap; // cumulative average point

  printf("What is your age? ");
  scanf("%d" , &age); // address of variable age
  printf("What is your CAP? ");
  scanf("%lf", &cap); // address of variable cap

  printf("You are %d years old and your CAP is %f\n", age, cap);

  return 0;
}

So how do we understand the example above?

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printf("What is your age? ");

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What is your age? 

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scanf("%d" , &age);

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What is your age? 20

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printf("What is your CAP? ");

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What is your age? 20
What is your CAP? 

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scanf("%lf" , &cap);

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What is your age? 20
What is your CAP? 5.0

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printf("You are %d years old and your CAP is %f\n", age, cap);

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What is your age? 20
What is your CAP? 5.0
You are 20 years old and your CAP is 5.0

Single Line Input

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#include <stdio.h>
int main(void) {
  int age;
  double cap; // cumulative average point

  printf("What is your age and CAP? ");
  scanf("%d %lf" , &age, &cap); // &age <--> %d and &cap <--> %lf

  printf("You are %d years old and your CAP is %f\n", age, cap);

  return 0;
}

The explanation is similar to multi lines input. However, for line 7, we should highlight that the scanf expects two values since it has two placeholders %d and %lf on the same function call. These two values have to be separated by a space. For instance, try putting in 20 and 5.0 as shown below.

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What is your age and CAP? 20 5.0

• The first value 20 match the first placeholder %d which will be assigned to the first additional argument age.
• The second value 5.0 match the second placeholder %lf which will be assigned to the second additional argument cap.

Special Characters

Like other modern languages, C uses escape sequence in printf for characters that otherwise cannot be represented properly. The common escape sequences are shown below.

Exercises

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2100

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2100 2100 2100 2100

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1

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1 1 1 1

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int user_input; // good practice to write meaningful name
scanf("%d", &user_input);
printf("%d %d %d %d\n", user_input, user_input, user_input, user_input);