Introduction to C

:class: tip

This lecture will cover contents from Chapter 1 - Appendix version of the book.

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What is C?

Overview

• Developed by Dennis Ritchie at Bell Labs
• First public released in 1972.
• The book: *The C programming languange” by Dennis M. Ritchie and Brian W. Kernighan. Prentice Hall 1988.

:::{dropdown} How to learn C (now that you already know Java)?

• C for Java programmers
• C programming vs. Java programming

:::

Scary stuff ahead …

• C is much less supportive for programmers than Java.
• (Much) easier to make mistake, and (much) harder to fix.

But it is exciting …

• C requires less memory resources than Java.
• C, in many instances, runs faster than Java.
• Knowing C will make you a better programmer overall.

C and Java

:::::{tab-set} ::::{tab-item} hello.c

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#include <math.h>
#include <stdio.h>

/* main function definition: */
int main() {
    printf("Hello World\n");
    printf("sqrt(4) is %f\n", sqrt(4));
    return 0;  // main returns value 0
}
 

::::

::::{tab-item} hello.java

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import java.lang.Math;

/* define a HelloWorld class */
class Hello {

  /* main method definition: */
  public static void main(String[] args){
    System.out.println("Hello World");
    System.out.println("sqrt(4) is " + Math.sqrt(4));
  }
}
 

:::: :::::

Similarities

• Values, types, literals, expressions
• Variables
• Control flow (if, switch, while, for, do-while)
• Call-return: parameters, arguments, return values
• Arrays (mostly)
• Primitive and reference types
• Type casting.
• Library usage.

Differences

• C has no classes or objects (but something similar)
• C is not object-oriented.
• C arrays are simpler:
  • No boundary checking.
  • No knowledge of array’s own size.
• String operations are limited.
• No collections, exceptions, or generics.
• No automatic memory management.
• Pointers!!!

:::::{tab-set} ::::{tab-item} Running Java

:::{image} fig/02-c/java.png :alt: Java compilation and execution :class: bg-primary mb-1 :align: center :::

:::: ::::{tab-item} Running C

:::{image} fig/02-c/c.png :alt: C compilation and execution :class: bg-primary mb-1 :align: center :::

:::: :::::

Hands-on: Getting started

• SSH to molly. Refer to the Setup page if you need a refresher on how to do so.
• Change into csc231 from inside your home directory.
  • Your home directory is represented by the ~ sign.
• Create a directory named intro-c inside csc231, then change into that directory.

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cd ~/csc231
pwd
mkdir intro-c
ls
cd intro-c
pwd
 

:::{image} fig/02-c/02.png :alt: create intro-c directory :class: bg-primary mb-1 :align: center :::

Create hello.c

• Inside the terminal, make sure that you are still inside intro-c, then use nano to create hello.cwith the source code below.

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pwd
nano -l hello.c
 

• Once finished editing in nano:
  • first hit Ctrl-X (same for both Windows and Mac).
  • Next hit Y to save modified buffer (new contents).
  • Hit Enter to save to the same file name as what you opened with.
• Memorize your key-combos!.

:::{image} fig/02-c/03.png :alt: Create hello.c using nano :class: bg-primary mb-1 :align: center :::

What’s in the code?

• Line 1: Standard C library for I/O, similar to Java’s import.
• Line 2-4: Function declaration/definition for main:
  • Line 2:
    • return type: int
    • function name: main
    • parameter list:
      • argc: number of command line arguments.
      • *argv[]: pointers to array of command line argument strings.
  • Line 3: Invoke builtin function printf to print out string Hello world! with an end-of-line character \n. This is similar to System.out.printf.
  • Line 4: Exit a successfully executed program with a return value of 0.

Hands-on: Simple compile and run

• Similar to javac, we use gcc to compile C code.
• Before compile, make sure that you are still inside intro-c in the terminal.

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ls
pwd
gcc -o hello hello.c
ls
./hello
 

:::{image} fig/02-c/04.png :alt: Creating hello.c using nano :class: bg-primary mb-1 :align: center :::

Hands-on: Compile and show everything

• There are a number of steps from C codes to executable binaries.

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ls -l
gcc -save-temps -o hello2 hello.c
ls -l
./hello2
 

:::{image} fig/02-c/05.png :alt: compile and run hello.c, keeping intermediate files :class: bg-primary mb-1 :align: center :::

What are those?

• hello.i: generated by pre-processor
• hello.s: generated by compiler.
• hello.o: generated by assembler.
• hello (or hello2): executable, generated by linker.

:::{image} fig/02-c/06.png :alt: hello.c compilation process :class: bg-primary mb-1 :align: center :::

Hands-on: View files

• For hello.i and hello.s, they can be view on the editor.
• Run the following command to view hello.i

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cat -n hello.i
 

• Run the following command to view hello.s

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cat -n hello.s
 

• For hello.o and hello, we need to dump the binary contents first.

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xxd -b hello.o hello.o.txt
xxd -b hello hello.txt
ls -l
 

:::{image} fig/02-c/07.png :alt: hexdumping hello.c and hello :class: bg-primary mb-1 :align: center :::

• Run the following command to view hello.o.txt

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cat -n hello.o.txt
 

• Run the following command to view hello.txt

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cat -n hello.txt
 

Challenge: output

The usage of C’s printf is similar to Java’s System.out.printf. Find out how to modify hello.c so that the program prints out Hello Golden Rams! with each word on a single line. The program should use exactly one printf statement.

:::{dropdown} Answer

:::

Input: scanf

C formatting placeholders: (similar to Java)

• %g: placeholder for a float (or double) value
• %d: placeholder for a decimal value (int, short, char)
• %s: placeholder for a string value
• %c: placeholder for a char value

:::::{tab-set} ::::{tab-item} readInput.c

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#include <stdio.h>

int main() {
  int num1, num2;
  printf("Enter a number: ");
  scanf("%d", &num1);
  printf("Enter another: ");
  scanf("%d", &num2);
  printf("%d + %d = %d\n", num1, num2, (num1+num2) );
  return 0;
}
 

::::

::::{tab-item} InputReader.java

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import java.util.Scanner;

class InputReader {
  public static void main(String[] args) {
    int num1, num2;
    Scanner in = new Scanner(System.in);
    System.out.print("Enter a number: ");
    num1 = in.nextInt();
    System.out.print("Enter another: ");
    num2 = in.nextInt();
    System.out.printf( "%d + %d = %d\n",
          num1, num2, (num1+num2) );
  }
}
 

:::: :::::

Challenge

Create a file called mixedInput.c that is modified from readInput.c. The file should do the followings:

• Ask user to provide three input of type float
• Print out the multiplication statement with all the multiplicants and results as done in readInput.c.

:::{dropdown} Answer

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#include <stdio.h>

int main() {
  float num1, num2, num3;
  printf("Enter a number: ");
  scanf("%g", &num1);
  printf("Enter another: ");
  scanf("%g", &num2);
  printf("Enter another: ");
  scanf("%g", &num3);
  printf("%g * %g * %g = %g\n", num1, num2, num3, (num1*num2*num3) );
  return 0;
}
 

:::

Conditionals and Loops

Similar to Java.

• Conditionals: compare.c

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#include <stdio.h>

int main(void) {
    int num1, num2;

    printf("Enter the 1st number: ");
    scanf("%d", &num1);
    printf("Enter the 2nd number: ");
    scanf("%d", &num2);

    if (num1 > num2) {
        printf("%d is biggest\n", num1);
    } else if (num1 == num2) {
        printf("%d and %d are the same\n", num1, num2);
    } else {
        printf("%d is biggest\n", num2);
    }

    return 0;
}
 

• Loops

:::::{tab-set} ::::{tab-item} forLoop.c

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#include <stdio.h>

int main(void) {
    int num, i;

    printf("Enter a value: ");
    scanf("%d", &num);
    // make sure num is not negative
    if (num < 0) {
        num = -num;
    }

    for (i = 0; i < num; i++) {
        printf("%d\n", i);
    }

    return 0;
}
 

::::

::::{tab-item} whileLoop.c

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#include <stdio.h>

int main(void) {
    int num, val;

    printf("Enter a value: ");
    scanf("%d", &num);
    // make sure num is not negative
    if (num < 0) {
        num = -num;
    }
    val = 1;
    while (val < num) {
        printf("%d\n", val);
        val = val * 2;
    }

    return 0;
}
 

:::: :::::

Function in C

• Almost the same as methods in Java, except for one small difference.
• They need to either be declared, or must be defined prior to being called (relative to written code position).

::::{dropdown} Hands-on: Functions in C - definition and declaration

• Create three C files, function-1.c, function-2.c, and function-3.c, with the source codes below:

• Compile and run these files.

:::{image} fig/02-c/23.png :alt: Compile and run function-1.c, function-2.c, function-3.c :class: bg-primary mb-1 :height: 250px :align: center ::: ::::

Arrays

• Statically declared arrays
• Multiple data values of the same types
• Adjancent locations in memory (we will talk about this in the future)
• Inside the terminal, make suse that you are still inside intro-c, then use nano to create array-1.cwith the source code below.

• What is the distance between addresses? Why?
• Compile and run array-1.c

:::::{dropdown} Exercise

• Create a copy of array-1.c called array-2.c.
• Change the type of numbers to double.
• What is the address step now?

::::{dropdown} Answer

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::::{dropdown} An array variable

• … is in fact pointing to an address containing a value.
• … without the bracket notation and an index points to the corresponding address of the value at the index.
• … is quite similar to a pointer! ::::