Computer Systems Organization

Start Lecture #3

1.8: Arguments—Call by Value

Remark: All the online material is now associated with V22.0202-002. You should now sign up for the mailing list as shown in lecture 1.

1.9: Character Arrays

Unlike Java, C does not have a string datatype. A string in C is an array of chars. String operations like concatenate and copy (assignment) become functions in C. Indeed there are a number of standard library routines for string.

The most common implementation of strings in C is null terminated. That is, a string of length 5 has 6 characters, the 5 characters in the string and a sixth character = '\0'.

Print Longest Line

#include <stdio.h>
#define MAXLINE 1000
int getLine(char line[], int maxline);
void copy(char to[], char from[]);
int main() {
  int len;
  int max;
  char line[MAXLINE];
  char longest[MAXLINE];
  max = 0;
  while ((len=getLine(line,MAXLINE))>0)
    if (len > max) {
      max = len;
      copy(longest,line);
    }
  if (max>0)
    printf("%s", longest);
  return 0;
}
int getLine(char s[], int lim) {
  int c, i;
  for (i=0; i<lim-1 && (c=getchar())!=EOF && c!='\n'; ++i)
    s[i] = c;
  if (c=='\n') {
    s[i]= c;
    ++i;
  }
  s[i] = '\0';
  return i;
}
void copy(char to[], char from[]) {
  int i;
  i=0;
  while ((to[i] = from[i]) != '\0')
    ++i;
}

This program reads lines from the terminal, converts them to C strings by appending '\0', and prints the longest one found. Pseudo code would be

    while (more lines)
      if (longest)
        save it and its length
  

Thus we need the ability to read in a line and save it and write two functions getLine and copy for these tasks (the book uses getline, but that doesn't compile for me since there is a library routine in stdio.h with the same name and different signature). Given these two routines, main is fairly simple, needing only a few small comments.

1. Note that getLine and copy are declared before main. They are defined later in the file but C requires declare before use so either main would have to come last or the declarations are needed. Since only main uses the routines, the declarations could have been in main but it is common practice to put them outside as shown.
2. Note %s inside printf. This is used for (null-terminated) strings.
3. Note that main is declared to return an integer and it does return 0. In unix at least, this is the indication of a successful run.

The for condition in getLine is rather complex. Perhaps it would be clearer if the test were simply i<lim-1 and the rest was done with if-break statments inside the loop.

copy is declared and defined to return void.

• This means that it does not return a value.
• Similarly, a function taking no arguments should be declared and defined to have a void argument list. Leaving the argument list blank (i.e., writing fun-name()) actually means that you are not specifying the argument signature. This weird rule is for compatibility with older versions of C that played a little fast and loose with such issues.

Homework: Simplify the for condition as just indicated.

1.10: External Variables and Scope

Solving Quadratic Equations

#include <stdio.h>
#include <math.h>
#define A +1.0   // should read
#define B -3.0   // A,B,C
#define C +2.0   // using scanf()
void solve (float a, float b, float c);
int main() {
  solve(A,B,C);
  return 0;
}
void solve (float a, float b, float c) {
  float d;
  d = b*b - 4*a*c;
  if (d < 0)
    printf("No real roots\n");
  else if (d == 0)
    printf("Double root is %f\n", -b/(2*a));
  else
    printf("Roots are %f and %f\n",
           ((-b)+sqrt(d))/(2*a),
           ((-b)-sqrt(d))/(2*a));
}
#include <stdio.h>
#include <math.h>
#define A +1.0   // should read
#define B -3.0   // A,B,C
#define C +2.0   // using scanf()
void solve();
float a, b, c;   // definition
int main() {
  extern float a, b, c; // declaration
  a=A;
  b=B;
  c=C;
  solve();
  return 0;
}
void solve () {
  extern float a, b, c; // declaration
  float d;
  d = b*b - 4*a*c;
  if (d < 0)
    printf("No real roots\n");
  else if (d == 0)
    printf("Double root is %f\n", -b/(2*a));
  else
    printf("Roots are %f and %f\n",
           ((-b)+sqrt(d))/(2*a),
           ((-b)-sqrt(d))/(2*a));
}

The two programs on the right find the real roots (no complex numbers) of the quadratic equation

    ax2+bx+c
  

    d = b2-4ac
  

The first program calls a function solve() passing it as arguments the three coeficients a,b,c.

The second program communicates with solve using external (global) variables rather than arguments/parameters.

• Note the single definition, which is outside any function; a definition causes space to be set aside for this variable.
• Also note the multiple declarations, one inside each function. They include the keyword extern and indicate that a definition is provide elsewhere (externally).
• Within a single .c file (this particular file is roots-ext.c) the definition is enough providing it is before any function that uses the variables. Normally the definitions come before all functions, as done in the example, and hence the declarations are not needed.
• The functions of a single program are spread across multiple .c files, then each file must contain a declaration or definition prior to any use of the variable. Exactly one of the .c files must contain a definition.
• Typically, the definition is directly in a single .c file and the declaration is placed in a .h (header) file that is included in all the .c files (so one .c has a declaration and a definition).

Remark: I am not assigning lab 1 yet because we haven't resolved the situation with i5.nyu.edu. But when it is resolved lab1 will include some of problems 20-24 on page 34.