added sum code to measure CPU time

source_codes/complexity/.gitignore

+sum

source_codes/complexity/Makefile

+CC:=gcc
+# SAN:=-fsanitize=address
+CFLAGS:=-Wall -Wextra -pedantic -g $(SAN)
+LDFLAGS:=-lm $(SAN)
+
+EXECS := $(shell find . -type f -iname '*.c' | sed 's/\.c//g')
+
+all: $(EXECS)
+
+$(EXECS): %: %.c
+	$(CC) $(CFLAGS) -o $@ $< $(LDFLAGS)
+	@echo $@ >> .gitignore
+
+.PHONY: clean all
+
+clean:
+	rm -f *.o $(EXECS) .gitignore
+

source_codes/complexity/sum.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+#define SIZE 1000000
+#define NUM_TIMES 10
+
+void init(int n, double tab[]) {
+    for (int i = 0; i < n; ++i) {
+        tab[i] = (double)rand() / (double)RAND_MAX;
+    }
+}
+
+double sum(int n, double tab[]) {
+    double s = tab[0];
+    for (int i = 1; i < n; ++i) {
+        s += tab[i] * tab[i] * tab[i] * tab[i];
+    }
+    return s;
+}
+
+int main() {
+    double tab[SIZE];
+    init(SIZE, tab);
+
+    struct timespec tstart = {0, 0}, tend = {0, 0};
+    clock_gettime(CLOCK_MONOTONIC, &tstart);
+    double s = 0;
+    for (int i = 0; i < NUM_TIMES; ++i) {
+        s += sum(SIZE, tab);
+    }
+    clock_gettime(CLOCK_MONOTONIC, &tend);
+    printf("the computation of %f took about %.5f seconds\n", s,
+        (((double)tend.tv_sec + 1e-9 * tend.tv_nsec) -
+            ((double)tstart.tv_sec + 1e-9 * tstart.tv_nsec)) /
+            NUM_TIMES);
+
+    return 0;
+}