Nested Loops

Very often it is useful to nest one loop inside another.

Say we want to print the multiplication table: every product of the integers 0 .. 9:

int row, col;

for (row = 0; row <= 9; row = row + 1) {
    for (col = 0; col <= 9; col = col + 1) {
        printf("%3i", row * col);
    }
    printf("\n");
}

This code produces the following output:

0  0  0  0  0  0  0  0  0  0
0  1  2  3  4  5  6  7  8  9
0  2  4  6  8 10 12 14 16 18
0  3  6  9 12 15 18 21 24 27
0  4  8 12 16 20 24 28 32 36
0  5 10 15 20 25 30 35 40 45
0  6 12 18 24 30 36 42 48 54
0  7 14 21 28 35 42 49 56 63
0  8 16 24 32 40 48 56 64 72
0  9 18 27 36 45 54 63 72 81

Char data type, char literals

The char data type represents a character values.

A literal char value is specified as a character in single quotes, e.g.:

‘A’

‘?’

‘ ’

Two char literals are special - the char literal for the single quote character and the char literal for the backslash character. They are written

‘\’'

‘\\’

The idea is that the first backslash “escapes” the following character. The escape is necessary in the first case because otherwise, an occurrence of a single quote within the char literal would end the char literal. In the second case, the escape is necessary because a single backslash is treated as escaping the next character.

Using scanf to read a single character

char ch;

printf("enter a character: ");
scanf(" %c", &ch);

printf("you entered '%c'\n", ch);

This can be useful for prompting the user to answer a yes or no question:

char answer;
int keep_going = 1;

while (keep_going == 1) {
    // some code...

    printf("Try again (y or n)? ");
    scanf(" %c", &answer);
    if (answer != 'y' && answer != 'Y') {
        keep_going = 0;
    }
}

Triangular loops

In the nested loops we have discussed so far, the inner loop in a pair of nested loops executes the same number of iterations no matter which iteration of the outer loop is being executed. We call such a loop pair “rectangular” because the two loop variables explore a space of values that looks like a rectangle if plotted on the x-y coordinate plane.

In a triangular loop, the number of iterations the inner loop executes depends on the value of the outer loop’s loop variable. For example:

int height, j, i;

printf("height of triangle: ");
scanf("%i", &height);

for (j = 0; j < height; j++) {
    for (i = 0; i <= j; i++) {
        printf("*");
    }
    printf("\n");
}

Example run (user input in bold):

height of triangle: 6
*
**
***
****
*****
******