Simple C Programs

Basic Example

Using the editor of your choice, write the following program (save it as base.c):

#include <stdio.h>

int main ()
{
    printf ("Hello World !\n");
    return 0;
}

In a terminal, type the following command:

gcc base.c

You should get no output (which means everything went fine); the compilation has generated a program named a.out (the default name). You can now run the compiled program with the command, which outputs:

Hello World !

From this first example, a few things are worth remembering. The usual workflow is:

1. write the C code in an editor
2. compile it (gcc); if there are errors, go back to step 1
3. run it (./a.out)

Assignment Operations

Integer Operations

• Given unsigned int a = 0;. What is the value of a if you decrement it?
• Given short a = 32766;. What is the value of short b = a + 3;?
• Given short b = 30000, c = 30000;. What is the value of short d = b * c;?

Sequences of Instructions

Given three integer variables x, y, z declared and initialized, write sequences of instructions to:

• Display the maximum of x and y
• Display the absolute value of z
• Display whether the integer x is even or odd
• Display the maximum of x, y, z

Given character variables c and d declared and initialized, write sequences of instructions to:

• Determine which of c and d comes first in the alphabet
• Determine whether c is a lowercase or uppercase letter
• Convert c to uppercase (without knowing in advance whether it is lowercase or uppercase)
• Determine whether c is a consonant or a vowel

Classic Boolean Expressions

Translate each of the following expressions into C:

• x=y=z
• x, y, z are all different from each other
• x<y<=z
• at least two equal numbers among x, y, z
• exactly two equal numbers among x, y, z
• at most two equal numbers among x, y, z

Simple Problems

• Given the birth dates of two people provided as day, month, year (where each value is an integer), determine which person is older (person 1 or person 2).
• Given an integer i representing a duration in seconds, display the equivalent duration in the form $h:min
  $. Write the inverse: given three integer variables h, m, s, determine the equivalent duration in seconds.

for Loops

Simple Loops

Write sequences of instructions to:

• Display the integers from 1 to 10 separated by spaces, then from 10 to 1.
• Sum the integers from 1 to 100 and display the result.
• Sum the even integers from 6 to 2048 and display the result.
• Display the list of multiples of 3 or 5 (or both) that are less than 60.

Power Computation

Write an algorithm to compute x^n using only multiplications.

Problem

If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23. Find the sum of all the multiples of 3 or 5 below 1000. —source Project Euler