User Input and Variable Scope in C Programming

CS& 131, Autumn 2022 Lecture 7: user input; parameters Thank you to Marty Stepp and Stuart Reges for parts of these slides 1

scanf scanf: a built in function that reads input from the console. Reading a number using scanf: scanf(format, &variable); Example: int count; scanf("%d", &count); printf("There were %d cats\n", count); 2

scanf example 2 int main() { printf("Please type two numbers: "); int num1; int num2; scanf("%d %d", &num1, &num2); int product = num1 * num2; printf("The product is %d", product); } Output (user input underlined): Please type two numbers: 8 6 The product is 48 scanf can read multiple values from one line. 3

Format string Can contain any pattern Uses similar place holders for variables as printf %d - int %lf - double %s - string When a token is not the type you ask for, it doesn't match and doesn't read anything 4

Limitations of variables Idea: store the user input size in a variable. Use the variable's value in the functions. Problem: A variable in one function can't be seen in others. void top_half() { for (int i = 1; i <= size; i++) { // ERROR: size not found ... } } void bottom_half() { for (int i = size; i >= 1; i--) { // ERROR: size not found ... } } int main() { int size; scanf("%d", &size); top_half(); print_bottom(); } 5

Scope scope: The part of a program where a variable exists. From its declaration to the end of the {} braces A variable declared in a for loop exists only in that loop. A variable declared in a function exists only in that function. void example() { int x = 3; for (int i = 1; i <= 10; i++) { printf("%d\n", x); } // i no longer exists here } // x ceases to exist here i's scope x's scope 6

Scope implications Variables without overlapping scope can have same name. for (int i = 1; i <= 100; i++) { printf("/"); } for (int i = 1; i <= 100; i++) { // OK printf("\\"); } int i = 5; // OK: outside of loop's scope A variable can't be declared twice or used out of its scope. for (int i = 1; i <= 100 * line; i++) { int i = 2; // ERROR: overlapping scope printf("/"); } i = 4; // ERROR: outside scope 7

Parameterization parameter: A value passed to a function by its caller. Instead of line_of_4, line_of_3, write line to draw any length. When declaring the function, we will state that it requires a parameter for the number of equals. When calling the function, we will specify how many equals signs to draw. 4 #================# line main 3 line #============# 10

Declaring a parameter Stating that a function requires a parameter in order to run void <name>(<type><name>) { <statement>(s); } Example: void say_password(int code) { printf("The password is: %d\n", code); } When say_password is called, the caller must specify the integer code to print. 11

Passing a parameter Calling a function and specifying values for its parameters <name>(<expression>); Example: void main() { say_password(42); say_password(12345); } Output: The password is 42 The password is 12345 12

Parameters and loops A parameter can guide the number of repetitions of a loop. void main() { chant(3); } void chant(int times) { for (int i = 1; i <= times; i++) { printf("Just a salad...\n"); } } Output: Just a salad... Just a salad... Just a salad... 13

How parameters are passed When the function is called: The value is stored into the parameter variable. The function's code executes using that value. void main() { chant(3); chant(7); } 3 7 void chant(int times) { for (int i = 1; i <= times; i++) { printf("Just a salad...\n"); } } 14

Common errors If a function accepts a parameter, it is illegal to call it without passing any value for that parameter. chant(); // ERROR: parameter value required The value passed to a function must be of the correct type. chant(3.7); // ERROR: must be of type int 15

Complex figure that scales Finish modifying the mirror code to use parameters so it can be scaled successfully. A mirror of size 3: #============# | <><> | | <>....<> | |<>........<>| |<>........<>| | <>....<> | | <><> | #============# A mirror of size 4: #================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================# 16

Loop tables and parameters Let's modify our loop table to use a user input size This can change the amount added in the loop expression spaces dots size line 4 1,2,3,4 6,4,2,0 0,4,8,12 3 1,2,3 4,2,0 0,4,8 #================# | <><> | | <><> | | <>....<> | | <>....<> | | <>........<> | |<>........<>| |<>............<>| |<>........<>| |<>............<>| | <>....<> | | <>........<> | | <><> | | <>....<> | #============# | <><> | #================# #============# 17

Partial solution // Prints the expanding pattern of <> for the top half of the figure. void top_half(int size) { for (int line = 1; line <= size; line++) { printf("|"); for (int space = 1; space <= (line * -2 + (2*size)); space++) { printf(" "); } printf("<>"); for (int dot = 1; dot <= (line * 4 - 4); dot++) { printf("."); } printf("<>"); for (int space = 1; space <= (line * -2 + (2*size)); space++) { printf(" "); } printf("|\n"); } } 18

Observations about scale factors The size can change the "intercept" in an expression. Usually the "slope" is unchanged. void example(int size) { for (int space = 1; space <= (line * -2 + (2 * SIZE)); space++) { printf(" "); } ... It doesn't replace every occurrence of the original value. for (int dot = 1; dot <= (line * 4 - 4); dot++) { printf("."); } 19

Double border Alter the mirror code so that a double border can be drawn at the top and bottom #++++++++++++++++# #================# | <><> | | <>....<> | | <>........<> | |<>............<>| |<>............<>| | <>........<> | | <>....<> | | <><> | #================# #++++++++++++++++# Does this new border row remind you of another part of our program? How can we keep this new code from being redundant? 20

Multiple parameters A function can accept multiple parameters. (separate by , ) When calling it, you must pass values for each parameter. Declaration: void <name>(<type><name>, ...,<type><name>) { <statement>(s); } Call: <name>(<exp>, <exp>, ..., <exp>); 21

Multiple parameters example void print_number(int number, int count) { for (int i = 1; i <= count; i++) { printf("%d", number); } printf("\n"); } void main() { print_number(4, 9); print_number(17, 6); print_number(8, 0); print_number(0, 8); } Output: 444444444 171717171717 00000000 22

Strings string: A sequence of text characters. char <name> [] = "<text>"; Examples: char name[] = "Marla Singer"; char point[] = "(1, 2)"; Note: You will sometimes see strings declared as char* This will work too but is slightly different More later on 23

Strings as parameters void say_hello(char name[]) { printf("Welcome, %s\n", name); } int main() { say_hello("Marty"); char teacher[] = "Bictolia"; say_hello(teacher); } Output: Welcome, Marty Welcome, Bictolia 24

Value semantics value semantics: by default when variables (int, double) are passed as parameters, their values are copied. Modifying the parameter will not affect the variable passed in. void strange(int x) { x = x + 1; printf("1. x = %d\n", x); } int main() { int x = 23; strange(x); printf("2. x = %d\n", x); ... } Output: 1. x = 24 2. x = 23 25

A "Parameter Mystery" problem void mystery(int x, int z, int y) { printf("%d and %d", z, (y - x)); } int main() { int x = 9; int y = 2; int z = 5; mystery(z, y, x); mystery(y, x, z); } 26