CS100 Lecture 1

The First C Program

Contents

• Brief history of C
• The first C program
  • Functions (basic) and the main function
  • scanf and printf

Brief history of C

The UNIX operating system and C

• In 1969, a small group of AT&T Bell Labs led by Ken Thompson and Dennis Ritchie began to develop UNIX.
• In 1973, UNIX kernel was rewritten in C.
• From 1969 to 1973, Dennis Ritchie developed C in Bell Labs.
• In 1978, Kernighan and Ritchie published the K&R book: The C Programming Language.

Dennis Ritchie and the K&R book

• Dennis M. Ritchie (1941 - 2011)

  • The inventor of C
  • Co-inventor of UNIX
  • ACM Turing Award (1983) with Ken Thompson for UNIX

Standardization of C

• "K&R C": Informal specification (the K&R book)
• ANSI C: Known as "C89"
  • American National Standards Institute
  • Came out in 1989
• ISO C standard: ISO/IEC 9899
  • International Organization for Standardization
  • First version: "C90" in 1990, the same standard as C89 with only formatting changes
  • C99 (1999), C11 (2011), C17 (2017/2018), C23 (2023)

The first C program

Hello World

#include <stdio.h>

int main(void) {
  printf("hello world\n");
  return 0;
}

• Save the code as hello.c.
• gcc hello.c -o hello.exe generates hello.exe
• .\hello.exe prints hello world, with a newline at the end.

The main function

Every C program coded to run in a hosted execution environment contains the definition of a function named main, which is the designated start of the program.

// Other things (functions, structures, ...), if any ...

int main(void) {
  // The program starts here.
  statement_1;
  statement_2;
  // ...
  statement_n;
}

* What is a function?

A function in C

A function in mathematics: , accepts some arguments and returns some value.

Example: accepts an integer argument, and returns its square.

Write it in C:

int f(int x) {
  return x * x;
}

A function in C

Example:

Write it in C:

double f(double x, double y) { // Two arguments
  return x + y;
}

double: double-precision floating-point number will be covered in later lectures.

A function in C

Syntax: ReturnType FunctionName(Parameters) { FunctionBody }

FunctionBody can also contain more complex statements:

int max(int a, int b) {
  if (a < b)
    return b;
  else
    return a;
}

if statement will be covered in later lectures.

A function in C

Syntax: ReturnType FunctionName(Parameters) { FunctionBody }

A function can have no arguments. To define such a function, write void in Parameters:

int always42(void) {
  return 42;
}

We will introduce more on functions in later lectures.

The main function

Every C program coded to run in a hosted execution environment contains the definition of a function named main, which is the designated start of the program.

According to the standard, the main function must has one of the following signatures:

1. int main(void) { ... }
2. int main(int argc, char *argv[]) { ... }
3. /* another implementation-defined signature */

For now, we only use the first one: int main(void) { ... }.

"signature": consisting of the return-type, parameters, and some other possible information

The main function

int main(void) {
  printf("hello world\n");
  return 0;
}

The return value of main: Indicates whether the program exits successfully.

A program exits successfully if and only if its main function returns 0.

The main function

A program exits successfully if and only if its main function returns 0.

You may also see this somewhere else:

The main function

A program exits successfully if and only if its main function returns 0.

You may also see this somewhere else:

The main function

It is ok to omit return 0; in main (but not in other functions):

int main(void) {
  printf("hello world\n");
}

According to the standard:

If the return type is compatible with int and control reaches the terminating }, the value returned to the environment is the same as if executing return 0;.

printf

Declared in the standard library header file stdio.h.

• That's why we need #include <stdio.h> in the beginning.

Writes something to the standard output.

printf("hello world\n");

• Prints hello world, with a newline \n at the end.
• Try this out: printf("hello\nworld\n");

Output vs return

int main(void) {
  printf("hello world\n");
  return 0;
}

• What is the output of the program?
• What is the return value of main?

Output vs return

int main(void) {
  printf("hello world\n");
  return 0;
}

• What is the output of the program? hello world with an ending newline.
• What is the return value of main? 0.

The "A+B" problem

Reads two integers from input (separated by whitespaces), and prints the sum of them.

#include <stdio.h>

int main(void) {
  int a, b; // declares two variables of type "int", named "a" and "b".
  scanf("%d%d", &a, &b);
  printf("%d\n", a + b);
  return 0;
}

The "A+B" problem

Reads two integers from input (separated by whitespaces), and prints the sum of them.

scanf

Also declared in stdio.h.

Reads something from the standard input.

Example: Reads two integers from the standard input, separated by whitespaces.

scanf("%d%d", &a, &b);

• %d: Indicates that an integer is expected, and will be stored into an int variable.
• &: The address-of operator will be covered in later lectures.

For now, just remember to add & when passing things to scanf.

scanf

scanf("%d%d", &a, &b);

How should these two integers be separated? Try it out:

scanf

scanf("%d%d", &a, &b);

%d will skip any leading whitespaces.

• "whitespace" refers to the character that looks "blank": space ' ', newline '\n, tab '\t', etc.

More on the rules related to scanf will be covered in recitations.

printf printing an integer

Given a , b as input:

• printf("%d\n", a + b);

  prints 72, with a newline in the end.

• printf("%d + %d equals %d\n", a, b, a + b);

  What is the output?

printf printing an integer

Given a , b as input:

• printf("%d\n", a + b);

  prints 72, with a newline in the end.

• printf("%d + %d equals %d\n", a, b, a + b);

  prints 30 + 42 equals 72, with a newline in the end.

Summary

Understand the following two programs:

#include <stdio.h>

int main(void) {
  printf("hello world\n");
  return 0;
}

#include <stdio.h>

int main(void) {
  int a, b;
  scanf("%d%d", &a, &b);
  printf("%d + %d equals %d\n",
         a, b, a + b);
  return 0;
}

Summary

• Why is #include <stdio.h> needed?
  • scanf and printf are declared in the standard library header file stdio.h.
• What does int main(void) mean?
  • The main function is where the program starts. int is the return type and void indicates that this function accepts no arguments.
• What is the meaning of return 0;? Can it be omitted?
  • A program returns 0 if it exits successfully. The main function of C will execute return 0 automatically at the end if we don't write it exiplicitly.

Summary

• How do we represent a newline?
  • '\n'.
• What does %d mean?
  • Indicates that the type of the data being read or printed is int.
• How are whitespaces handled when reading integers with scanf?
  • When reading int with %d in scanf, leading whitespaces are ignored.

Exercises

1. Run the following code. Try to understand what it means.

   #include <stdio.h>
   
   int max(int a, int b) {
     if (a < b)
       return b;
     else
       return a;
   }
   
   int main(void) {
     int a, b;
     scanf("%d%d", &a, &b);
     printf("%d\n", max(a, b));
     return 0;
   }
   

Exercises

2. Try to modify the program above. Add some prompts for input and output. For example: