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Lecture 8 — C programs
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get a partner, each draw a picture, explain to your partner
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what memory looks like when swap is called
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void swap(int *a, int *b) {
int temp = *a;
*a = *b;
*b = temp;
}
int x = 17;
int y = 42;
swap(&x, &y);
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what memory looks like after these assignments
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int a = 10;
int *p = &a;
int **s = &p;
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booleans
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no built-in boolean values (no true/false)
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integers are used instead
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0 is false
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any non-zero is true (usually 1)
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arrays in C
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array syntax that you are used to
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int arr[10]; initializes an array of 10 integers
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arr[0], arr[1], …
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except arrays are actually pointers
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arr points to the location of the first element (arr[0])
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arr == &arr[0]
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parr = arr;
*parr == arr[0]
*(parr + 1) == arr[1]
arr[i] == *(arr + i)
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what’s the result of parr++
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parr now points to arr[1]
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pointer arithmetic
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adding an integer i to a pointer p gives you the address for the ith object beyond p
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C strings
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no String objects in C
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a string is represented by an array of characters
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char s[15]; // a string up to 15 characters long
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a char with value 0 is used to mark the end of a string
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called a null terminator
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written '\0'
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string literals are converted into this representation
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"howdy" is actually [‘h’, ‘o’, ‘w’, ‘d’, ‘y’, ‘\0’]
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C programs
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int main(int argc, char **argv)
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argc is the number of arguments
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argv is the argument values
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since we know arrays are points, we can see how char **argv is an array of strings
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pointer to pointer to char
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argv[0] is always the name of the executable
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so for ./gasp arg1 arg2, argv[0] == “./gasp”
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line_count example
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dealing with arguments
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read from file
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#include and #define
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compilation
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gcc -Wall -std=c11 -g -o output_file input_files…
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