Foundations

Variables

Variables store typed values that later expressions can reuse.

type Each C variable has a type such as `int` or `double`.
assignment An assignment stores a new value in an existing variable.

Variables

unitPrice
variables.c
Replay: real traced execution (multi-file project)
#include <stdio.h>

int main(void) {
    int unitPrice = 12;
    int quantity = 3;
    int total = unitPrice * quantity;

    printf("unit=%d\n", unitPrice);
    printf("total=%d\n", total);
    return 0;
}

#include <stdio.h>

int main(void) {
    int unitPrice = 8;
    int quantity = 3;
    int total = unitPrice * quantity;

    printf("unit=%d\n", unitPrice);
    printf("total=%d\n", total);
    return 0;
}

#include <stdio.h>

int main(void) {
    int unitPrice = 20;
    int quantity = 3;
    int total = unitPrice * quantity;

    printf("unit=%d\n", unitPrice);
    printf("total=%d\n", total);
    return 0;
}

  1. unitPrice ← 12, quantity ← 3, total ← 36

    3int main(void) {4    int unitPrice→ 12 = 12; //@unitPrice=8, 205    int quantity→ 3 = 3;6    int total→ 36 = unitPrice12 * quantity3;78    printf("unit=%d\n", unitPrice12);9    printf("total=%d\n", total36);10    return 0;11}
    outputunit=12
total=36

  1. unitPrice ← 8, quantity ← 3, total ← 24

    3int main(void) {4    int unitPrice→ 8 = 8;5    int quantity→ 3 = 3;6    int total→ 24 = unitPrice8 * quantity3;78    printf("unit=%d\n", unitPrice8);9    printf("total=%d\n", total24);10    return 0;11}
    outputunit=8
total=24

  1. unitPrice ← 20, quantity ← 3, total ← 60

    3int main(void) {4    int unitPrice→ 20 = 20;5    int quantity→ 3 = 3;6    int total→ 60 = unitPrice20 * quantity3;78    printf("unit=%d\n", unitPrice20);9    printf("total=%d\n", total60);10    return 0;11}
    outputunit=20
total=60

Follow the Total

  1. unitPrice starts at 12.
  2. quantity starts at 3.
  3. total = unitPrice * quantity multiplies 12 * 3.
  4. total becomes 36.
  5. The program prints unit=12 and total=36. | unitPrice | quantity | total | | --- | --- | --- | | 8 | 3 | 24 | | 12 | 3 | 36 | | 20 | 3 | 60 |

Exercise: variables.c

Reproduce unit=12 and total=36, then use the pinned unitPrice values 8 and 20 to predict each total.