getPositiveValues won't return the values - c

Question

What is not working:
In the code below, the values input in scanf under getPositiveValue will not return. They return as 0 no matter what the input is.
I have no clue how to get around this. Can someone show me why it is not working?
What I have tried:
I tried using return CHAN; and even return CHAN.n; and all the other members but that did not work.
My code:
#include <stdio.h>
#include <math.h>
#define TRUE 1
#define FALSE 0
#define N 25 //number of lines
typedef struct CHANNEL_ //Structure CHANNEL
{
char name[9];
double n;//roughness coefficient
double S;//channel slope
double B;//width
double D;//maxDepth
} CHANNEL;
double computeVelocity(CHANNEL, double);
int main(void)
{
CHANNEL CHAN;
void getPositiveValue(CHANNEL);
void displayTable(CHANNEL);
//Function declarations
printf("Enter the name of the channel: ");
fgets(CHAN.name, 9, stdin);
getPositiveValue(CHAN);
printf("Channel data for %s\n Coefficient of roughness: %lf\n Slope: %lf\n Width: %lf\n Maximum depth: %lf\n", CHAN.name, CHAN.n, CHAN.S, CHAN.B, CHAN.D);
printf("Depth Average Velocity\n");
displayTable(CHAN); //function call to display the table with values
}
void getPositiveValue(CHANNEL CHAN)
{
int Flag; //sentinel
do
{
Flag = FALSE;
printf("Give the coefficient for roughness, slope, width, and maxdepth: ");
scanf("%lf %lf %lf %lf", &CHAN.n, &CHAN.S, &CHAN.B, &CHAN.D);
if(CHAN.n < 0 || CHAN.S < 0 || CHAN.B < 0 || CHAN.D < 0) //sentinel checkpoint
{
Flag = TRUE;
printf("The values must be positive.\n");
}
} while(Flag == TRUE);
}
void displayTable(CHANNEL CHAN)
{
double increment = CHAN.D/N;
double H = 0; //depth
double arraydepth[N]; //N is used to avoid magic numbers when defining array size
double arrayvelocity[N]; //N is used to avoid magic numbers when defining array size
int i; //using separate integers for the two different arrays just so it looks better and less confusing
for ( i = 0; i < N; i++)
{
H += increment;
arrayvelocity[i] = computeVelocity(CHAN, H);
arraydepth[i] = H;
printf("%lf %lf\n", arraydepth[i], arrayvelocity[i]);
}
}
double computeVelocity(CHANNEL CHAN, double H)
{
double U;
U = CHAN.B / H;
U = U / (CHAN.B + (2 * H));
U = pow(U, (2 / 3));
U = U / CHAN.n;
U = U * (sqrt(CHAN.S));
return U;
}

Answer 1

The input problem you are having is because of the fact that functions are call by value in C. This means that when you pass a struct to a function, it is a copy of the struct that is worked with in the function, not the original. Any changes made to the struct within the getPositiveValue() function are not visible once control returns to main().
To fix this problem, pass a pointer to the structure. Use the -> operator to dereference the pointer and access members in one shot. Here is a modified version of your code. I also took the liberty of moving your function declarations to the top of the program.
There is also an error in the call to the pow() function found in computeVelocity():
U = pow(U, (2 / 3));
should be:
U = pow(U, (2.0 / 3.0));
The expression 2 / 3 performs integer division, with the result zero, so after this call to pow(), U is always 1. This can be easily fixed by forcing floating point division, as in the second line above.
#include <stdio.h>
#include <math.h>
#define TRUE 1
#define FALSE 0
#define N 25 //number of lines
typedef struct CHANNEL_ //Structure CHANNEL
{
char name[9];
double n;//roughness coefficient
double S;//channel slope
double B;//width
double D;//maxDepth
} CHANNEL;
double computeVelocity(CHANNEL, double);
void getPositiveValue(CHANNEL *);
void displayTable(CHANNEL);
int main(void)
{
CHANNEL CHAN;
printf("Enter the name of the channel: ");
fgets(CHAN.name, 9, stdin);
getPositiveValue(&CHAN);
printf("Channel data for %s\n Coefficient of roughness: %lf\n Slope: %lf\n Width: %lf\n Maximum depth: %lf\n", CHAN.name, CHAN.n, CHAN.S, CHAN.B, CHAN.D);
printf("Depth Average Velocity\n");
displayTable(CHAN); //function call to display the table with values
}
void getPositiveValue(CHANNEL *CHAN)
{
int Flag; //sentinel
do
{
Flag = FALSE;
printf("Give the coefficient for roughness, slope, width, and maxdepth: ");
scanf("%lf %lf %lf %lf", &CHAN->n, &CHAN->S, &CHAN->B, &CHAN->D);
if(CHAN->n < 0 || CHAN->S < 0 || CHAN->B < 0 || CHAN->D < 0) //sentinel checkpoint
{
Flag = TRUE;
printf("The values must be positive.\n");
}
}while(Flag == TRUE);
}
void displayTable(CHANNEL CHAN)
{
double increment = CHAN.D/N;
double H = 0; //depth
double arraydepth[N]; //N is used to avoid magic numbers when defining array size
double arrayvelocity[N]; //N is used to avoid magic numbers when defining array size
int i; //using separate integers for the two different arrays just so it looks better and less confusing
for ( i = 0; i < N; i++)
{
H += increment;
arrayvelocity[i] = computeVelocity(CHAN, H);
arraydepth[i] = H;
printf("%lf %lf\n", arraydepth[i], arrayvelocity[i]);
}
}
double computeVelocity(CHANNEL CHAN, double H)
{
double U;
U = CHAN.B / H;
U = U / (CHAN.B + (2 * H));
U = pow(U, (2.0 / 3.0));
U = U / CHAN.n;
U = U * (sqrt(CHAN.S));
return U;
}
Sample program interaction:
Enter the name of the channel: chan
Give the coefficient for roughness, slope, width, and maxdepth: 0.035 0.0001 10 4.2
Channel data for chan
Coefficient of roughness: 0.035000
Slope: 0.000100
Width: 10.000000
Maximum depth: 4.200000
Depth Average Velocity
0.168000 0.917961
0.336000 0.566077
0.504000 0.423161
0.672000 0.342380
0.840000 0.289368
1.008000 0.251450
1.176000 0.222759
1.344000 0.200172
1.512000 0.181859
1.680000 0.166669
1.848000 0.153840
2.016000 0.142843
2.184000 0.133301
2.352000 0.124935
2.520000 0.117535
2.688000 0.110939
2.856000 0.105020
3.024000 0.099677
3.192000 0.094829
3.360000 0.090410
3.528000 0.086363
3.696000 0.082644
3.864000 0.079214
4.032000 0.076040
4.200000 0.073095

Answer 2

There are many compiler error in your code. Here is my first try to fix it
#include <stdio.h>
#include <math.h>
#define TRUE 1
#define FALSE 0
#define N 25 //number of lines
typedef struct CHANNEL_ {
char name[50];
double n;//roughness coefficient
double S;//channel slope
double B;//width
double D;//maxDepth
} CHANNEL;
double computeVelocity(CHANNEL, double);
void getPositiveValue(CHANNEL);
void displayTable(CHANNEL);
int main(void) {
CHANNEL CHAN;
printf("Enter the name of the channel: ");
fgets(CHAN.name, 50, stdin);
getPositiveValue(CHAN);
printf("Channel data for %s\n Coefficient of roughness: %lf\n Slope: %lf\n Width: %lf\n Maximum depth: %lf\n", CHAN.name, CHAN.n, CHAN.S, CHAN.B, CHAN.D);
printf("Depth Average Velocity\n");
displayTable(CHAN); //function call to display the table with values
}
void getPositiveValue(CHANNEL CHAN) {
int Flag; //sentinel
do {
Flag = FALSE;
printf("Give the coefficient for roughness: \n Give the slope: \n Give the channel width: \n Give the maximum depth of the channel: ");
scanf("%lf %lf %lf %lf", &CHAN.n, &CHAN.S, &CHAN.B, &CHAN.D);
if(CHAN.n < 0 || CHAN.S < 0 || CHAN.B < 0 || CHAN.D < 0) {
Flag = TRUE;
printf("The values must be positive.\n");
}
} while(Flag == TRUE);
}
void displayTable(CHANNEL CHAN) {
double increment = CHAN.D/N;
double H = 0; //depth
double arraydepth[N];
double arrayvelocity[N];
int i;
for ( i = 0; i < N; i++) {
H += increment;
arrayvelocity[i] = computeVelocity(CHAN, H);
arraydepth[i] = H;
printf("%lf %lf\n", arraydepth[i], arrayvelocity[i]);
}
}
double computeVelocity(CHANNEL CHAN, double H)
{
double U;
U = CHAN.B / H;
U = U / (CHAN.B + (2 * H));
U = pow(U, (2 / 3));
U = U / CHAN.n;
U = U * (sqrt(CHAN.S));
return U;
}
The first error would be struct definition. In C, you can define the struct and at the same time define a variable. But you should not use the same name to confuse yourself and the compiler. Also you need to understand void function does not return a value and cannot be on the right side of an = expression.
Use typedef can save you to type struct keyword each time you need it. You also need to use %s to output a string. Also typos here and there.