aud-exercises-ws18

Solutions developed with students during exercises for the course Algorithmen und Datenstrukturen in winter term 2018/19.

Setup and running instructions:

1. Obtain a C-compiler of your choice; I use the Gnu Compiler Collection (gcc).
2. Open a shell and navigate to the directory where the source file is located (using cd). You want to use tab-completion!
3. Compile the program name.c by running gcc -ansi -Wall -Wextra -Wpedantic name.c. This will create an executable with the name a.out. The compiler flags enforce syntactic restrictions that are very close to the description of C in the official lecture material.
4. To run a.out from a unix-like system, type ./a.out.
5. Alternatively, you may add -o exename to create an executable named exename. It can be run by typing ./exename.

Fundamentals of C:

Variable declaration

int a; or char b[80]

Here int and char are types whereas a and b are names of the declared variables. The [80] indicates that instead of a single char an array of chars of size 80 is created. For each value we want to read or manipulate, we have to create a variable of an appropriate type. Often temporary/auxiliary variables are needed to carry out a comuptation.

Assignment to a variable is with =, e.g., a = 5.

Reading and writing

We print text or the content of some variable to the screen with the printf function. The first argument of printf is a format string that contains normal symbols and special symbol sequences starting with % where the content of some variable shall be printed. The symbols folling the % indicate for instance the type of the printed value. All other arguments of printf are variables (or values) that shall be printed. E.g.:

• printf("Hello!") just outputs Hello!.
• printf("%d + %d = %d", 2, 3, 5) outputs 2 + 3 = 5. The ith occurrence of %d (d for decimal) is replaced by the i+1th argument of printf.
• For new lines include \n into the format string.
• Other types: %s for strings (arrays of characters), %f for floating point values.

Reading in text works similarly with the scanf function. The first argument of scanf is just a special sequence indicating the type of the value to be read. The second argument is the address of a variable to which the value we read should be stored. E.g.:

• scanf("%d", &i) reads a decimal number and stores it in variable i.
• scanf("%d", &s) reads a string of characters and stores it in variable s.

Control structures

if then (else)

If we want to execute a statement (or a sequence of statements) only if a certain condition is satisfied, we can use the if-then-else statement. In C is has the following form:

if (condition)
  statement1;
else
  statement2;

while loop

If we want to repeat some statements multiple times, we can use a while-loop. As long as a particular condition is satisfied, the statements in the body of the loop are executed again and again. It is possible to exit the loop early by executing the break statement in the body. Also, we can jump back to the head (where the condition is checked) of the loop without executing the remainder of the body by executing continue. E.g.:

while (condition) {
  statement1;
  statement2;
  if (condition2) break;
  statement3;
  if (condition3) continue;
  statement5;
}

for loop

The for loop adds some syntactic sugar to while loops. Very often, a certain counting variable is incremented or decremented at the end of the body of a loop. Also, this counting variable needs to be initialized before the loop begins. To write this in a way that is compact and easy to read, we use a for loop:

for( initialize; condition; execute_after_each_iteration )
    statements_in_body;

For instance, we can have:

for( x = 0 ; x < 10; x = x + 1 )
    statement;

Note that both the initialize and the execute_after part can be empty.