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Lecture 9 —printf, declarations, preprocessor
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pointer review
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variables are labels for a location in memory where some data is stored
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variable names are never actually stored — they disappear once a program is compiled
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a pointer like int *ptr is just another variable
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what’s different is that the data it labels is the memory address of other data.
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left values vs right values (in an assignment)
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left_expression = right_expression
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left expressions get evaluated to locations (addresses)
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right expressions get evaluated to values
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values include number and pointers (addresses)
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Rule for variables
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in a left expression, a variable is a location
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in a right expression, a variable is evaluated to its location’s contents
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dangerous for a function to return a pointer to a local variable
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int* f() {
int x = 5;
int *p = &x;
return p;
}
int* g() {
int x = 10;
int *p = &x;
return p;
}
int main() {
int *p = f();
int *q = g();
}
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what will the value of *p be at the end of main
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10, it will have been overwritten since it’s pointing to memory no longer owned by f
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exercise: what size of array do we need to store the string “funtimes”
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9: [‘f’, ‘u’, ‘n’, ‘t’, ‘i’, ‘m’, ‘e’, ‘s’, ‘\0’]
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exercise: write a function to compute the length of a string
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not counting null terminator
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int strlen(char *str) {
int count = 0;
for ( ; s[count] != ‘\0’; count++) { }
return count;
}
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another way:
int strlen(char *str) {
int n;
for (n = 0 ; *s != ‘\0’; s++) {
n++;
}
return n;
}
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printf, scanf
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printf outputs to stdout, scanf reads from stdin
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both take a format string
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a string with special control sequences that indicate the type of data being output or input
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these control sequences are matched with the arguments that follow the format string
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examples
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http://www.cplusplus.com/reference/cstdio/printf/
printf ("Characters: %c %c \n", 'a', 65);
printf ("Decimals: %d %ld\n", 1977, 3000000000L);
printf ("Preceding with blanks: %10d \n", 1977);
printf ("Preceding with zeros: %010d \n", 1977);
printf ("floats: %4.2f %+.0e %E \n", 3.1416, 3.1416, 3.1416);
printf ("Width trick: %*d \n", 5, 10);
printf ("%s \n", "A string");
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line_count example
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declaration vs definition
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declaration gives the compiler the basic information about how something is used
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type
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number of parameters
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for example: int strlen(char *str); declares the strlen function without defining it
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called a function prototype
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things must be declared before they can be used
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something can be declared any number of times (though not more than once per scope/file)
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definition provides the actual value or behavior
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for example:
int strlen(char *str) {
int count = 0;
for( ; s[count] != ‘\0’; count++) { }
return count;
}
defines the strlen function
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preprocessor
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separate first step of compilation
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#include used to bring in declarations from other files
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connecting declarations with definitions in other files takes place in a step called linking
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#include <filename> searches for standard library files
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#include "filename" searches in the same directory as the source file
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#define name replacement text does simple substitution
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everywhere in the file where the token name appears, replacement text is substituted
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gcc -E stops compilation after preprocessing and outputs the results to stdout
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compilation
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gcc -Wall -std=c11 -g -o output_file input_files…
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exercise: extend to print out character count as well
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