CIS241

System-Level Programming and Utilities

C Types (and Printing)

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Data types

Non-floating point

• int - at least 16 bits
• long - at least 32 bits
• long long - at least 64 bits

Floating point

• float - typically 32 bits
• double - typically 64 bits

Character

• char

Boolean?

true/false?

https://stackoverflow.com/questions/1921539/using-boolean-values-in-c

Pre C-99 - no boolean type!

• Use a char or int with an 'accepted' value for what is true and false
  • Note that for ints - 0 is false and non-zero is true!

C version?

https://stackoverflow.com/questions/36662063/how-can-i-know-the-version-of-c

gcc -dM -E - < /dev/null | grep __STDC_VERSION__ | awk '{ print $2 " --> " $3 }'

• My output: __STDC_VERSION__ --> 201710L
  • C17 (might see c89, c99, c11)

Booleans (c17 or c99)

#include <stdio.h>
#include <stdbool.h> // need the boolean header!

int main(void)
{
  bool var;
  var = true;

  printf("Boolean value: %d\n", var);
  printf("Boolean value as str: %s\n", var ? "true" : "false");

  return 0;
}

Printing output

printf(controlstring [, data])

• controlstring indicates surrounding text to print and how to format variable pritning
• data is optional - print variable values
  • i.e., what variable to print!
  • controlstring contains format specifiers for each data printed

Format specifiers

integer: %d (you may see %i)

• Can add additional formatting information
• Add number before d - specify minimum width %3d
• Specify 0-fill: %03d
• Specify left justify: %-3d

Format specifiers

float/double: %f, %e, %g

• %f: fixed point notation
• %e: exponential notation
• %g: chooses between normal and exponential (drops trailing)

Number before decimal - total width to use

Number after decimal - places after decimal point

Use 0 for 0-fill, use - for left justify

e.g.

We can still further customize:
printf("PI = %08.3f", 3.14);

Here:

• 0 is for zero pad
• 8 is for 8 total characters (including the decimal point)
• 3 is for places after decimal point

Format specifiers

Character: %c

String: %s

• Same with int/float you can add:
  • Number to specify width
  • - to specify left-justify

Signedness

These are different:

• int x = 5;
• unsigned int y = 5;

Why would we ever use unsigned ints?

• Range!
• Unsigned ints can hold numbers twice as large.
• Technically, ASCII chars are unsigned chars

Type conversions (implicit)

C will automatically convert types in certain scenarios:

• Operating on mismatched types

  • 3.0 / 2

• Assigning values

  • double x = 5;
  • int y = 4.0;

• Calling functions

  • float_exp(2, 3);

Type conversions (explicit)

We can also typecast!

• int x = (int) 5.0;
• (unsigned int) -1;

Format: (type) expression;