CS201 Discussion Section 1 Notes
Week of Feb 10, 2001

Structures

There are three basic types of structures in C:

• Sequential structures
• Selection structures
• Repetition structures

Sequential structures

Sequential structures are simply a list of expressions that will be evaluated in order, for example:

printf("Hello World") 
scanf("%d", &i); 
printf("i = %d", i);

Selection structures

if example

if (i == 1) {  
    printf("one\n"); 
} else if (i == 2) { 
    printf("two\n"); 
} else if (i == 3) { 
    printf("three\n") { 
} else { 
    print("num\n"); 
}

switch example

Doing the same thing but with a switch:

switch (i) 
{ 
    case 1: 
        printf("one\n"); 
        break; 
    case 2: 
        printf("two\n"); 
        break; 
    case 3: 
        printf("three\n"); 
        break; 
    default: 
        printf("num\n"); 
        break;  
}

the purpose of break

If i is 2 then the program should print ``two''. However if the switch statement was:

switch (i) 
{ 
    case 1: 
        printf("one\n"); 
    case 2: 
        printf("two\n"); 
    case 3: 
        printf("three\n"); 
    default: 
        printf("num\n"); 
}

then the program would of printed:

two 
three 
num

because with out the break statements C will continue to execute the remaining lines of code until it finds a break statement. So....

don't forget the break statements

    

Repetition structures

for loops

int i; 
for (i = 0; i != 3; i++) { 
    printf("%d\n", i); 
}

do while loops

int i; 
scanf("%d", i); 
do { 
    printf("%d\n", i); 
    i++; 
} while (i < 3);

If the user enters 0 then this do/while loop is the same as the for loop. If the user enters 5 then the program will print ``5'' and quit.

while loops

int i; 
scanf("%d", i); 
while (i < 3) { 
    printf("%d\n", i); 
    i++; 
}

This is not the same code as the previous do/while loop because if a user enters 5 then nothing will be printed as the condition is checked before the body is executed in a while loop but after in a do/while loop.

Boolean Variables

C does not have a true boolean variable. In C any nonzero integer is considered true and a 0 is considered false. For example

int i,x,y;

x = 10;

y = 20;

i = x < y;

if (i) {

    /* this code should execute since i is non zero */

}

i = x > y;

if (i) {

    /* this code should not execute since i is zero */

}

What's Wrong with this Code

Example 1

#include <stdio.h> 

void main() 

{ 
    float i; 
 
    for (i = 0; i != 12; i++); 
    { 
        if (i % 3 = 0)  
        { 
            printf "%f" i;  
        }  
    } 
}

Example 2

include <stdio.h> 

int main()  
{ 
    int num; 
 
    printf("Please enter in a number: "); 
 
    scanf("%d", num); 
 
    if (num != 2 || num != 3)  
    { 
        printf("Your number does not equal 2 or 3\n");  
    } 
 
    return 0; 
}

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