CS 223 - Spring 2017. 1/25

Welcome to CS 223!

Logical problem of the day

int main(){
  int a[10] = {0};
  for (int i = 0; i < 10; i++){
    a[i] = i++;
    printf("a[%d]: %d\n", i, a[i]);
  }
}

What is interesting (or disturbing) about the above code?

Lecture 4: Introduction to C: strings

Administrivia

Office hours:

We have added the public tests for assignment 1. In addition, we have met ULAs and scheduled office hours. The office hours are Sun/Tue/Thu 8-11 PM at Hillhouse 17 Rm 111.

Correction from previous class. Gauss's formuala is n(n+1)/2.

Assignment 2

Problem set 2

Lecture Starts Here

See /c/cs223/hw1. Specifically, check out the sample file /c/cs223/hw1/Total.c

Strings

Aspnes: Strings

Strings are a sequence or array of characters. Strings may be delimited (using a specific termination character) or counted (specifying a numeric length). C strings are delimited by the null character, '\0'. This allows simple pointer manipulation of strings.
This convention is not without problems: The Most Expensive One-byte Mistake
String constants: \n, \r, \a. Two string constants separated by whitespace are combined: "foo" "bar" == "foobar"
String buffers: hi.c
String buffers and the perils of gets: getLine.c

Operations on strings.

void
strcpy2(char *dest, const char *src)
{
    /* This line copies characters one at a time from *src to *dest. */
    /* The postincrements increment the pointers (++ binds tighter than *) */
    /*  to get to the next locations on the next iteration through the loop. */
    /* The loop terminates when *src == '\0' == 0. */
    /* There is no loop body because there is nothing to do there. */
    while(*dest++ = *src++);
}

Because C pointers act exactly like array names, you can also write strcpy2 using explicit array indices. The result is longer but may be more readable if you aren't a C fanatic.

char *
strcpy2a(char *dest, const char *src)
{
    int ;

    i = 0;
    for(i = 0; src[i] != '\0'; i++) {
        dest[i] = src[i];
    }

    /* note that the final null in src is not copied by the loop */
    dest[i] = '\0';

    return dest;
}

A similar operation to strcpy is strcat. The difference is that strcat concatenates src on to the end of dest; so that if dest previous pointed to "abc" and src to "def", dest will now point to "abcdef". Like strcpy, strcat returns its first argument. A no-return-value version of strcat is given below.

void
strcat2(char *dest, const char *src)
{
    while(*dest) dest++;
    while(*dest++ = *src++);
}

Length of a string:

int
strlen(const char *s)
{
    int i;

    for(i = 0; *s; i++, s++);

    return i;
}

Note issue of strlen tarpit.

Comparing strings:

int
strcmp(const char *s1, const char *s2)
{
    while(*s1 && *s2 && *s1 == *s2) {
        s1++;
        s2++;
    }

    return *s1 - *s2;

if(strcmp(s1, s2) == 0) or if(!strcmp(s1, s2))

Dynamic string allocation:

char *
strdup(const char *s)
{
    char *s2;

    s2 = malloc(strlen(s)+1);

    if(s2 != 0) {
        strcpy(s2, s);
    }

    return s2;
}

Command line arguments: printArgs.c

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