Lecture 18 Last Pointers

Joseph Haugh

University of New Mexico

Free Recall

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", *foo[1]);

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", *foo[1]);

4

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", (*foo)[1]);

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", (*foo)[1]);

2

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", **foo);

Review Quiz

What is the output of the following pieces of code?

int *foo[2];
int a[3] = {1, 2, 3};
int b[4] = {4, 5, 6, 7};
foo[0] = a;
foo[1] = b;
printf("%d\n", **foo);

1

Review Quiz

What is the output of the following pieces of code?

char s[] = "Hello, World!";
int i;
int j = 0;
for (i = 0; i < 5; i++) {
if (i >= j) {
    *(s + i) = *(s + j + 1);
    j += 2;
}
}
printf("%s\n", s);

Review Quiz

What is the output of the following pieces of code?

char s[] = "Hello, World!";
int i;
int j = 0;
for (i = 0; i < 5; i++) {
  if (i >= j) {
    *(s + i) = *(s + j + 1);
    j += 2;
  }
}
printf("%s\n", s);

eell,, World!

Review Quiz

What is the output of the following pieces of code?

int x = 0x40; // sizeof(int) == 4
int *p = x;
printf("%p\n", p);
p += 4;
printf("%p\n", p);

Review Quiz

What is the output of the following pieces of code?

int x = 0x40; // sizeof(int) == 4
int *p = x;
printf("%p\n", p);
p += 4;
printf("%p\n", p);

0x40
0x50

Command Line Arguments

int main(int argc, char *argv[]) {

Call program: ./a.out Hello World

argv ->	argv[0] ->	a.out\0
	argv[1] ->	Hello\0
	argv[2] ->	World\0

argv is a pointer to an array of pointers.

Each pointer is the address of the first char in a null terminated string.

Echo Arguments: Array Style

void main(int argc, char *argv[]) { 
  int i;
  printf("Number of arguments = %d\n", argc);
  for (i = 0; i < argc; i++) { 
    printf("  argv[%d]=%s\n", i, argv[i]);
  }
}

a.out pi is 3.1415

Number of arguments = 4

argv[0]=a.out

argv[1]=pi

argv[2]=is

argv[3]=3.1415

argv[i] is the address of a null terminated string

Echo Arguments: Pointer Style

void main(int argc, char *argv[]) { 
  printf("main(): argc=%d\n", argc);
  while (argc-- > 0) { /* test first, then decrement */
    printf("argc=%d: %s\n", argc, *argv++);
  }
}

What is going on with *argv++?

Dereference argv. This is argv[0]: a pointer to the first argument.
Send that pointer to %s.
Increment argv, not *argv. Now points to argv[1].

a.out Hello World

main(): argc=3

argc=2: a.out

argc=1: Hello

argc=0: World

More About *argv++

void main(int argc, char *argv[]) {
  printf("%p: %p->%s\n", argv, *argv, *argv);
  argv++;
  printf("%p: %p->%s\n", argv, *argv, *argv);
}

> ./a.out Hello World
0x7ffd6a971038: 0x7ffd6a972be8->./a.out
0x7ffd6a971040: 0x7ffd6a972bf0->Hello

Why do the addresses differ by 8?

That is the size of char*.

Double Echo Arguments: Array Style

void main(int argc, char *argv[]) { 
  int i, k;
  char* str;
  printf("Number of arguments = %d\n", argc);
  for (i = 0; i < argc; i++) { 
    printf("argv[%d]=%s\n", i, argv[i]);
    k=0;
    str = argv[i];
    while (str[k]) { 
      printf(" %c ",str[k]);
      k++;
    }
    printf("\n");
  }
}

> ./a.out Hello World
Number of arguments = 3
argv[0]=./a.out
 .  /  a  .  o  u  t 
argv[1]=Hello
 H  e  l  l  o 
argv[2]=World
 W  o  r  l  d

charCompCaseInsensitive()

int charCompCaseInsensitive(char c1, char c2) {
  int lowerCaseOffset = 'A' - 'a';
  if (c1 >= 'a' && c1 <= 'z') { 
    c1 += lowerCaseOffset;
  }
  if (c2 >= 'a' && c2 <= 'z') { 
    c2 += lowerCaseOffset;
  }
  return c1 == c2;
}

findSubstringCaseInsensitive()

char *findSubstringCaseInsensitive(char *haystack,
                                   char *needle)  { 
  int len = strlen(needle);
  int matchCount = 0;
  while (*haystack) { 
    if (charCmpCaseInsensitive(*(needle+matchCount),
                               *haystack)) { 
      matchCount++;
      if (matchCount == len) { 
        return (haystack - len) + 1;
      }
    } 
    else { haystack -= matchCount; matchCount = 0; }
    haystack++;
  }
  return NULL;
}

Redone with Single Exit Code Style

char *findSubstring(char *haystack, char *needle)  { 
  int len = strlen(needle);
  int matchCount = 0, done = 0;
  char *startPt = NULL;
  while (*haystack && !done) { 
    if (charCmpCaseInsensitive(*(needle+matchCount), 
                               *haystack)) { 
      matchCount++;
      if (matchCount == len) { 
        startPt = (haystack - len) + 1;
        done = 1;
      }
    } 
    else { haystack -= matchCount; matchCount = 0; }
    haystack++;
  }
  return startPt;
}

Scanf: Read from stdin

void main(void) { 
  int n, m, a;
  float x;
  scanf("%d %d %f %d", &n, &m, &x, &a);
  printf("%d %d %f %d\n", n, m, x, a);
}

Input:
1 2 3.1415 
128

Output:
1 2 3.141500 128

See Kernighan and Ritchie, 7.4 Formatted Input for more details.

Sscanf: Read from string

void main(void) {
  char sentence[] = "Rudolph is 12 years";
  char s1[20], s2[20];
  int i;
  
  sscanf (sentence,"%s %s %d",s1,s2,&i);
  printf ("[%s] [%s] [%d]\n",s1,s2,i);
}

Output:
[Rudolph] [is] [12]

fgets: Get a String From a Stream

char *fgets(char *s, int n, FILE *stream);

The fgets() function reads bytes from a stream into the array pointed to by s until n-1 bytes are read, or a newline is read and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null character.

strtol: Convert String to Long Integer

long int strtol(const char *nptr, char **endptr, int base);

The strtol() function converts the initial part of the string pointed to by nptr to a long integer value according to the given base, which must be between 2 and 36 inclusive, or be the special value 0. The first unrecognized character ends the conversion and is pointed to by endptr.

strtol: Example

void main(void) { 
  char *endPtr;
  long n = strtol("1001", &endPtr, 2);
  printf("n=%ld, char at endPtr=[%c]\n", n, *endPtr);
  n = strtol("1011a", &endPtr, 2);
  printf("n=%ld, char at endPtr=[%c]\n", n, *endPtr);
}

Output:
n=9, char at endPtr=[]
n=11, char at endPtr=[a]

Pointer and Index to the Same Place

void main(void) { 
  char data[] = "Hello World";
  data[2] = 'X';
  char *linePt = &data[3];
  *linePt = 'Z';
  printf("[%s], [%s]\n", data, linePt);
}

What is the output?

[HeXZo World], [Zo World]

Power of Pointers

Pointers if used incorrectly, lead to very difficult bugs: bugs that only sometimes manifest.
Code that uses pointers is often harder for humans to read
Code that uses pointers is much harder for compilers to optimize

Find First Zero

int array[] = {4, 5, 8, 9, 8, 1, 0, 1, 9, 3};
int *array_ptr;
       
int main()  {
  array_ptr = array;
     
  while ((*array_ptr) != 0) {
    ++array_ptr;
  }
       
  printf("# of elements before zero %d\n", array_ptr - array);
  return 0;
}